PDF

COVER: Cloud-to-ground lightning illuminates a shelf cloud north of Vernon, Texas on April 19, 1992. Photograph by
Tim Marshall.
Storm Track is a non-profit publication intended for the scientist and amateur alike who share an avid interest in the acquisition
and advancement of knowledge concerning severe or unusual weather phenomena. It is published bi-monthly in Lewisville,
Texas. David Hoadley founded the publication in 1977 and STORMTRACK has continued to grow and improve ever since.
Gene Rhoden designed the current cover. David Hoadley still contributes drawns and sketches. Current, we have about 675
subscribers!
Anyone can submit an article or letter to STORMTRACK. Articles should be single-spaced and contain proper english. Right
justified margins are preferred or the editor can retype the text. High contrast photographs reproduce best. Diagrams should be
clear and legible, subject to photo-reduction. All articles will be edited.
Subscription rates are: U.S. First Class mail $14/year. For Canadians, it’s also $14/year in U.S. currency. Overseas is $20/year
in U.S. currency. Individual issues are $2.35/copy. Back issues are available by year, or the complete 19 year set (1978-1995)
can be purchased for $135. Please note that issues up through 1993 are photocopies. To subscribe or renew, send a check or
money order PAYABLE ONLY to Tim Marshall, 4041 Bordeaux Circle, Flower Mound, Texas 75028.
STORM TRACK CLASSIFIEDS
Sell or swap your wares. Only 30c per word with a 20 word minimum = $6. Quarter page ads are $70, half page ads are $130,
and whole page ads are $250. Send your request for the next issue by October 15, 1997.
"TWISTER" MUSEUM, Wakita, Oklahoma, open 1 to 5, Wednesday through Sunday. 405-594-2312. This Museum is a
fascinating glimpse into the film-making art, and the extent to which the crew must go to insure the greatest possible effect. A visit
provides a first-hand "feel" for the adventures that happened in this small town. On a slow day, the "TWISTER" Museum is worth
a look. Open house Saturday, September 13th for 75th Cherokee Strip Celebration. Storm chasers are invited to drive vehicles
through the 4pm parade!!
For Sale: "TWISTER DESTROYER". Full color 24" x 36" poster size photograph of a rare tornado in N.W. Minnesota. Only
$10. Price includes shipping and handling. Ken Kostrzewski, RR 1, Box 109, Viking, MN 56760.
AN UNFORGETTABLE MOMENT by David Hoadley
The many scenes of storm damage I have seen tend to blur together over the years into one long mosaic of shattered glass walls,
insulation in trees, and grass hanging from power lines. Stunned survivors wander about, picking through rubble and seeking
direction. However, one unforgettable scene was burned into the memory early in my chase experience and remains vivid to this
day.
While driving through central Ohio on the way west, I passed through an area that
had a severe storm several days before. It was sunny now and there were few signs
of damage until I came upon something that took my breath. On the left and over a
fence was a solitary young tree, maybe 30 feet tall, laying on its side. It had a full,
dense perfectly rounded crown of bright, very green leaves, blowing gently in the
light morning wind. No broken branches were nearby. It just lay there in a neat
green field, still surreally alive and, at that moment, completely unaware that it
would never grow any more. It wasn't like seeing bruised and bloody survivors of
shattered homes, which are almost unrecognizable. Here was an in-tact, beautiful young tree that would have graced any estate
on the Hudson. It seemed to be symbolic of the impact sudden human loss must have on a family. Whether from accident, crime or
wind, the loss of anyone young, energenic, and full of life must be truly awful. I am fortunate never to have had that experience but now felt, for the first time, like I could understand it in others. Many more scenes of human loss and destruction may be seen
in future years. But I will never forget that solitary, eloquent tree, still filled with radiant green leaves, in a field in Ohio, that was
so beautiful and doomed at the same time. Although an irrational response to an rational event, it left me repeating silently as I
continued down the highway, "How could it be? How could it be?..."
STORMTRACK
http://www.telepath.com/www/storm/
$14/year
HURRICANE DENNY CIRCUS
I. COMMENTARY
Step right up. Get ready for some excitement. I'm sure us boys and girls were impressed with the three ring circus on television
about Hurricane Danny. I just loved those weather clowns! Their assignment was to get to the ocean, stand on the windward side
of the building, hold on to something sturdy, and act as if they were being blown around by hurricane force winds. Wear
something loose, O.K.?, so we can see it blow around. What really got to me was a story explaining why the camera crew hasn't
eaten correctly in days since all the power was out. I found this hard to believe. Hmm, the crew brought along plenty of extra
batteries to keep their cameras going but they didn't have enough sense to bring any extra food? That's what you get for missing
the buffet at the better hotels -who usually have generator power. Keep in mind this is news, right -so hype it up? But, what if a
piece of sheet metal flew off a building and struck the person holding the microphone? Would that be a great shot -something for
REAL TV?
For years, the media have put themselves in harm's way to try to report a story -firsthand. Many journalists have lost their lives in
Vietnam and Central America trying to do their job. Why? Was it so their bosses could sell more newspapers or increase
television ratings? If you want ratings, try MONDAY NIGHT FOOTBALL -or better yet, WEATHERBALL. Imagine Frank, Al
and Don narrating the game. John Holliman steps back and throws a pass to Bernie Shaw -but it's intercepted by Jim Cantore
TOUCHDOWN! This would be fun to watch and be less dangerous.
Some of you may say that I am a hypocrite since I chase tornadoes. But, you won't find me trying to ride out a tornado "up
underneath the girders". The fact is, most of the media do not have a clue what they are getting into in chasing storms. This may
make their adventure more exciting, but it is not funny when you end up in a barr ditch with a mesocyclone bearing down on you.
Fortunately, some of the media have hired us chasers to take them around and show them the ropes. Not one of them has regretted
this decision.
II. CHASER NEWS
The next issue of STORMTRACK will feature articles on the May 25, 26, and 27th chase events. Please send your articles and any photographs or
diagrams to us. You can e-mail articles to the editor at 105566.1564C@C,'ompuserve.com Also, check your mailing label and make sure it is
correct. The postal service would appreciate it if you would let us know your entire 9 digit zip code. Mine is 75028-7050.
The First Annual High Plains Conference will be held on September 15-17, 1997 in Dodge City, Kansas. hosted by the local National Weather
Service Forecast Office. The primary theme of the conference is National Weather Service Modernization Issues. Conference registration fee is $15
and no conference preprint will be available. For further information contact Jim Johnson, National Weather Service Office, 104 Airport Rd.
Dodge City, KS 67801 (tel: 316- 227-7140; fax: 316-227-2288; e-mail: [email protected]) or vice-president, Matt Gerard, National Weather
Service Office, 104 Airport Rd. Dodge City, KS 67801 (tel: 316-227-7140; fax: 316-227-2288; e-mail: [email protected]).
The 28th Conference on Radar Meteorology will be held on September 7-12, 1997 at the Hyatt Regency Hotel in Austin, Texas. For more
information, contact Mike Biggerstaff, Texas A and M University, College Station, Texas 77843- 3150. (tel: 409-847-9090, fax: 409-862-4466).
Planning is now underway to celebrate the 50th anniversary of the first tornado forecast. Events will be held Monday, March 23 - Wednesday,
March 25, 1998. There will be an open house for NOAA's facilities in Norman, OK on March 23rd, a Tornado Symposium with invited speakers
on March 24th (location to be arranged), and a monument unveiling at Tinker AFB on March 25th. Other special events will include a special
canceled post card from the US Postal Service, participation from local schools through essay and poster contests on tornado safety, special fly-over
by aircraft at Tinker AFB, a special edition of the technical journal Weather and Forecasting, and more. Further information and updates can be
found at: http://www.nssl.noaa.gov/GoldenAnniversary
Clarification: The Funnel Funny in the last issue of STORMTRACK which depicts the Chuck Doswell Tornado Chaser Questionnaire was written
by the editor not Chuck Doswell. The editor regrets any confusion regarding the origin of the humorous questions.
III. LETTERS TO THE EDITOR
Tim Vasquez summarizes his May 1, 1997 chase: "Gene Rhoden and I headed out to Shamrock TX and got there around 1430
local time just as two towers went up, one to our west and one to the southwest. The northern storm looked interesting but very
high based, so we moved down to the other storm, which was closing in on Wellington. From there, we basically followed the
storm. We saw tremendous inflow dust around Dodson TX, which made for fantastic video. We had many good views of the
storm, which was high-based and very LPish but had tremendous cloud motion and a base that was simply -big-. The precipitation
shaft remained some distance downstream, often about 10-20 miles away, so I think this was a big factor in the lack of any
tornadoes in spite of the shear. After Dodson, and moving towards Mangum, OK we picked up the chaser "zoo", along with the
NSSL Doppler on Wheels truck. Near Granite, OK we and a lot of other chasers got into some larger hail (quarter to golf-ball
sized), so we moved south to intercept an eastbound farm road (Robert Willis and us were pretty much caravaning at this point).
South of Hobart we stopped for a break. Chaser convergence drew in Richard Bedard/Mark Herndon and their newly cracked
windshield, along with Chuck Robertson and Tim Marshall. We let the storm go and turned our attention to a new dryline storm
west of Hobart, which was pretty much the same except a little more intense. We worked up to l-40 to follow it, and stopped near
Hinton to see a spectacular coffee-can updraft in the lightning flashes. The storm looked great but it quickly withered and headed
northeast. We called it quits and headed to a Denny's west of Oklahoma City. A BIG thumbs down to the Weather Channel. Their
coverage of the west Oklahoma storms was simply pathetic and we didn't get any interesting information from them out in the
field, not even a regional radar frame during their radar segments."
Kevin Scharfenburg writes about his chase on May 7, 1997: "We decided to wait it out in Panhandle, TX, right where this cell
developed! We were banking on thunderstorms developing along old outflow boundaries. In Panhandle, before the storm began
developing, winds were actually calm! The storm began very multicellular, with 4-5 very narrow updrafts. As the boundaries
finally converged southwest of the storm, the southwestern-most updraft widened rapidly, and became one huge, monstrous
updraft just north of Panhandle. This storm was actually shooting up through a mammatus-filled anvil being produced by the
Dumas storm. By the time we got in position to the south of the storm along Highway 293, we saw a wall cloud develop very
rapidly. We were impressed by the scud developing below the wall cloud halfway to the ground and being pulled rapidly up.
Within minutes, the wall was spinning rapidly, with dust being pulled up underneath. The bad news was our winds, due south of
the wall cloud about 1-2 miles, were only around 10 mph. About this time, we started noticing other chasers reaching the storm as
it became briefly tornadic. All in all, an awesome storm!
Mark Herndon writes about his chase on May 7, 1997: "Dave Robertson and I were heading north to intercept the Lefors storm
from Memphis. We were too late to catch the tornado that the College of Dupage guys saw...very poor contrast in our view to the
north. The storm started rapidly backbuilding a new base to the southwest. Near the junction of TX 70 and l-40, we saw a rapidly
rotating wall cloud to our west, with a lot of dust beneath it. Small scud tags were really cranking around, but there was never a
well developed condensation funnel. We were probably 10 miles east of it, but it was nicely backlit and plainly visible. A few
brief funnels were visible at times...none more than 1/4 way to ground; small "needles" lasting only a few seconds. This was right
around 8pm. This "feature" was relatively long lived...at least 15 minutes. Charles Edwards later said they watched the same
thing, but we weren't sure enough to actually call it a tornado. This storm eventually gusted out into a beautiful shelf cloud, with
some of the most incredible CGs that I have seen. Feeling that the show was over for us, we returned south to Clarendon and
parked beneath an awning to let the gust front and hail pass. After we got in the car we heard of a large tornado on the ground just
to our southeast. How that evolved so rapidly, I will never know but just before dark storms were erupting and evolving incredibly
fast. The drive back on I-40 was a feast, with anvil crawlers lighting up the night the whole way back.
Mike Umscheid writes about his chase on May 7, 1997: "This day was setting up to be a pretty active severe weather day from
the northern midwest all the way through the central plains to the Texas Panhandle. It wasn't a perfect day for tornadic activity,
but the intense low level instability for early May proved to be enough for sporadic tornadic activity. With surface temperatures in
the lower 80s and dewpoints reaching the mid-60s, surface based CAPE values approached 3000 in Central KS. With this and a
well defined surface wind field convergence, it wouldn't take much to initiate severe weather activity from eastern Nebraska
through central Kansas. Our chase started after school and after a good brief look at surface data and SPC (Storm Prediction
Center) guidance's. We decided to go west to Emporia. Shortly before 400pm (CDT), a red box was put out for much of central
Kansas south and west of Emporia. We left Jon's house, my chase partner, around 415pm and headed southwest along I-35. We
were frustrated with the strato-cu inhibiting further surface instability and our view of the sky to our west. As we approached
Emporia, the strato-cu broke up somewhat. The Topeka office was issuing severe thunderstorm warnings for activity near the
Nebraska border, and severe activity was just starting around the Salina area. We gave up on NOAA Weather Radio and tuned to
a.m. radio.
The Central Kansas Severe Weather Network was activated in central Kansas and they had numerous hail reports and wall clouds
northwest of the Abilene, KS area. Around 540pm, the Topeka National Weather Service issued a Tornado Warning for northern
Dickinson County. At Emporia we headed west on US-50 toward the developing activity to our northwest. Around 5:45pm, we
jogged north on K-177 to Council Grove and the view to the northwest was increasingly dark. Reports of rotating wall clouds near
Abilene were still coming in. We reached US-56 at Council Grove and continued west. With the breaks in the strato-cu, we could
see the sharp knuckles on the storm tower continuing to develop southwest. We finally arrived at Herington and headed north on
US-77... The chase was on.
The view to the north was very dark, and we soon came to the intense updraft region just ahead of the wall of precipitation. The
motion in the low clouds ahead of the downdraft region was rather intense and stretched for at least a mile east and west. We then
headed back south to K-4 where we jogged west for a mile. We could see other updraft bases that looked suspicious to tornadic
activity. We stopped along K-4 to see what would happen. We had three main areas we were watching for possible formation.
After about two or three minutes, a small funnel rapidly formed in the updraft region of the first, most eastern cell, that we were
origionally after. About four to five miles away, this very thin funnel appeared to reach completely to the ground, but the hills in
the distance troubled my vision to the immediate east. Just as soon as it possibly reached the ground, it quickly retreated back to
the cloud base, and in the meantime there continued to be rapid motion in the updraft base.
The time was shortly before 7:00pm, and most of the tornadic activity in our area had ceased. We had to get back to Overland
Park by 1000pm, so we had to get back home. No sooner than fifteen minutes later, the Topeka National Weather Service issued a
Tornado Warning for eastern Morris County. We were stunned to say the least, for we were in the core of that storm. We knew
the tornadic activity associated with this storm was several miles to the north, but just as we passed Council Grove, we saw some
very strong updraft motion in the clouds. We were on the lookout for rapidly changing surface winds as we were very near the
bear's cage. After that brief encounter, we were in between cells on the way home, and witnessed some outstanding electrical
activity all around on the way back home in Overland Park. It was an interesting chase day to say the least!
Robert Prentice writes about his chase on May 7, 1997: "I too saw the Western Gray Co, TX supercell, but did not see the
tornado that the College of DuPage folks or Rich Thompson/Roger Edwards observed. The storm developed and behaved very
much as predicted. It developed very rapidly (few tens of minutes) in an extremely unstable environment, and rapidly developed
into a supercell with low-level rotation. According to Rich, it produced one (or more?) brief tornadoes during the first hour of its
life and then developed into a "gusting-out" HP supercell. I got ofT from work and drove to Erik, OK arriving around 5:30pm. My
target area was the east-central Texas Panhandle (around Carson and Donley Co's.). With no development yet in sight, I called
Jim Leonard/Casey Crosbie/Marty Feely (-6pm) and discovered that they were in the middle of major "chaser convergence" ("30+
chasers") at Childress. After eating Taco Bell, I stopped at a nearby motel to watch The Weather Channel (at 6:30pm) and saw
the classic supercell southeast of Dalhart. I thought I could catch this storm before sunset, but I still hoped for new development in
my target area as opposed to a storm moving into my target area.
My wish was quickly granted with the supercell bomb in Carson Co, TX. I drove north on 70 towards Pampa and near the 293
intersection observed a rain free base with a menacing wall cloud to my west-northwest. Contrast was poor with haze and lots of
precipitation to the W-N-ENE of the storm. Even to the east of the storm light rain was falling which made photography difficult.
The only way to get a good view was from the east. Unfortunately, east road options were poor and the storm was approaching
hwy 70. I thought (hoped?) the storm would last awhile longer and move east- southeast. So I made the quick decision to beat the
giant hail to Lefors where I had good road options to the east- southeast and south. There I could film the storm the last 45 minutes
before sunset. Unfortunately the storm back-built a few times (with a brief tornado or two reported) before gusting out into a
squall line. I did film some of the intense lightning from McLean. I drove the entire way home to Norman in rain and lightning. As
far as Rich having his camcorder in pause mode, this is now the second time he has done this! The other time was 6/9/95 at
Farmer's Valley/Vernon, TX. That spectacular 100yd wide multi-vortex tornado (with a roar!) passed within a few hundred yards
of us! Rich, please, you're killing yourself! When in doubt, leave it in record mode!
Jim Leonard caught two landspouts near Hutchinson, KS on May 18, 1997: "Richard Pasch, Casey Crosbie and I camped out in
Wichita Saturday night after chasing some storms as far east as Topeka. Sunday we decided instead of blowing the day off and
driving back to Norman we drove west on hwy. 54 between Chancy and Midway and sat in the shade of an overpass for several
hours until we saw a line of flat cu form to our west and northwest. Around 4pm, we decided to move north toward Hutchinson
then hung out north of there while the towers bubbled to the west. Around 5 to 5:30pm, we went west on Hwy. 50 then dropped
south on Hwy. 14 to Hwy. 61 at about 5:45pm, we were heading down Hwy. 61 Richard was looking to his right and said there a
debris cloud. So, then of course I slammed on the breaks then we filmed which looked like a large tight dust devil probably 5 or 6
miles to our north near Abbyville, the funnel was well to the east of the dust tube which was being pushed out by the outflow. We
continued on down Hwy. 61 toward Pratt when we encountered the core of the south storm near Preston which was a good hail
show, heavy marble to golfball stones. After going through Pratt we went east on Hwy. 54 to get a look at the structure of the
storm as we were heading east I looked to my west north west I spotted our second landspout I spotted our second dust tube which
appeared to be in the vicinity of Pratt. After that we went west and went through Pratt then went south on Hwy. 281 and watch the
storm start to gust out when a county sheriff pulled up and warned us impending danger from a giant white scud bomb moving
toward us from the northeast. ( These are trained storm spotters). To end an exciting day, we ended up running into Mike Morgan
and Jeff Piotrowski in Wellington at the Braum's restaurant."
Top Ten Rejected Titles for the Movie "TWISTER" (Humor)
10. "Vorticity Park"
9. "I, Cumulus"
8. "One House Flew Over the Cuckoo's Nest"
7. "Terms of Backshearment"
6. "Killer Genuine Draft"
5. "Four Weddings & A Funnel"
4. "Indiana Jones and the Trailer Park of Doom"
3. "Romancing the Storm"
2. "Roofless in Seattle"
l. "Ace Ventura: When Nature Hauls"
FOTO FUNNIES
An amusement park in Roanoke, Texas
FUNNEL FUNNIES: Quicksand
Philbius Ferd went to retrieve his hat when it blew away while looking out the window during a rainstorm. He soon realized the
saturated ground was like quicksand.
"UNDER THE WHIRLWIND" BOOK REVIEW By David Hoadley
Under the Whirlwind is an illustrated book about last year's rare, April tornadoes in Ontario, Canada, the human impact of
sudden, devastating loss and the unexpected but healing generosity of neighbors and strangers. The authors, Arjen and Jerrine
Verkaik, have been lifelong sky and cloud enthusiasts, who have written and lectured extensively about their hobby and
photographic trips over much of North America. All the pictures and graphics are high quality color reproductions, including
many of the Ontario tornadoes. The book also includes a thorough and well illustrated but non-technical explanation about the
causes and structure of severe weather.
A refreshing and original approach is the emphasis on the human impact of tornadic disasters and how individual people and
families react to imminent danger, the destruction of homes, the sudden loss of hope, and then picking up the pieces --physically
and emotionally to start again. The reactions of people vary widely from terror and numbness to anxiety and loss of memory.
Some are overwhelmed with too much help and need to be left alone. Others only need someone to sit down beside them and talk
awhile.
Hundreds of interviews produced many brief but memorable accounts from people in a part of
the country that rarely see tornadoes and still have a "Gee whiz, golly!" reaction to what their
American cousins take for granted. At one farm: "All the birds came out of the trees and laid on
the ground just before it hit. Every kind...there were hundreds or thousands. The whole back
[lawn] was just covered." One little girl looked out the window as her daddy's barn blew to
pieces and said, "It was taking the barn apart just like Lego." Afterwards, a stunned housewife
looked at the damage around her home for the first time (barn gone, cattle dead in the fields,
vehicles in a scrap heap, a still-live calf hanging from a tree), when she was approached by a
police officer who asked, 'What did you lose?" "I couldn't answer him. I was-- what a ridiculous
question!" The authors discuss the special needs and reactions of children and include some of
their drawings and remembrances: "When the tornado came the cows were okay. The hay thing's
broken... Lots of people are helping us. Daddy said it was good no one was hurt... Tippy is okay
too but he bit the insurance man. We're in our trailer now. I like it."
At least two impressions were left with some survivors emerging from basements. After the
terrifying roar, shattering glass and cracking of timbers had passed, almost more striking was the
pervasive silence. "...all of a sudden it stopped- very quickly." "Dead silence!
...It's the quietist I've ever heard it!" "I'll bet you could hear people whispering ten miles away! There wasn't a bird- you couldn't
hear anything." The other impression was the strange smell of damp earth mixed with broken trees. For those downwind from a
marsh or swamp, the air had a stagnant smell.
This book also has excellent advice and information on preparing for tornadoes, such as documenting household effects and
property; kinds of insurance available and specific questions to ask your agent; dealing with the adjuster and preapprovals for
work to begin; whether to replace or repair; and documenting a wide range of expenses from immediate damage and debris
removal to spoiled food and temporary living quarters. For this alone, it is as essential read for homeowners living in "tornado
alley."
Most compelling, however, were the many accounts of neighbors and strangers rushing to help devastated people, even before
emerging from their cellars. Local Mennonites showed up unasked with willing hands and tools to pick up and set things right.
"This one man just showed up the very first day. He came walking up the driveway --no bus, no ride, nothing-- ...with a little
wooden ladder and a hammer and a bag of nails. He just looked around, and then he was on the roof of the chicken coop, nailing
away for four or five hours. He just started working. And the same way he came, he left."
For more information about this book write or call Ed or Jerrine Verkaik at RR 3, Elmwood, Ontario, Canada NOG 1S0 or call
519-363-5785.
III. DISCUSSION: WHY WERE THE MAY 1, 1997 SUPERCELLS NON-TORNADIC?
Brian Curran wrote: "Yesterday morning, I noticed an area of potentially cool theta-e air parked over southern Arkansas/northern
Louisiana. I didn't think anything of it; my vision would have this air being pulled into central Oklahoma as the wind field
adjusted to the lee cyclogenesis over eastern Colorado. Sure enough, there was a narrow slot of quality theta-e air along the warm
front; to the east was the air with its source over southern Arkansas/northern Louisiana. Once the supercells went into this lower
CAPE (convective available potential energy) air, they died. The MCS (mesoscale convective system) which churned through
Oklahoma Friday morning laid out a nice outflow boundary...rather, a ducted low level gravity wave...that really hosed the wind
field across north Texas Friday morning. The balance between shear and buoyancy (in a 2D sense, anyway) was tilted toward
shear as the storms moved into the potentially cooler air. This in turn caused the outflow to race ahead of the MCS (Mesoscale
Convective System), which in turn was ducted underneath a rather wicked cap across northwest Texas Friday morning. I don't
think I could have envisioned this scenario 36 hours ago; for every paradigm, there's an exception. Such is life in a nonlinear
environment. What lessons does this teach the chase forecaster? ALWAYS look upstream and envision the source region for
parcels arriving in your target area. Sometimes modification will help; other times it will hurt. Furthermore, consider the balance
between shear and buoyancy with regards to the expected storm type. A decrease in buoyancy holding shear constant can cause a
linear system to "gust out". Increasing buoyancy while holding shear constant can result in interesting things, such as a squall line
evolving into supercells on April 26, 1984."
Rich Thompson: "I don't have a good explanation, either. It's probably a combination of several factors: When the storm was most
intense and in the axis of decent instability (near Tx/OK border), there was no evidence of a RFD (rear flank downdraft). This
was probably due to all the rain falling well northeast of the mesocyclone. By the time the storm developed an RFD, it was
moving into a more stable environment, thus the RFD disrupted the low-level structure (went from solid updraft to a narrower
"horse shoe") instead of a strong low-level occlusion. There was some half decent cloud base rotation during the first RFD
attempts, but nothing persisted more than a few minutes and the majority of the updraft remained along the trailing gust front. We
were probably just the "victims" of poor timing. The second storm had even more going for it shear-wise since began along the
outflow boundary from the first storm (and the low level jet increased to 85 kt at 2 kft!), but again low-level theta-e was a
problem. Both storms (especially the second) probably would have produced tornadoes with about 1000-1500 j/kg more surface
CAPE when they developed RFDs. The first storm was indeed a supercell with a flared base, 45kt surface inflow, striations, sort
of a beaver's tail, etc. As soon as it began to develop decent cloud base rotation and RFD attempts, it began to move east of the
best instability. From Greer Co. eastward, the shear dominated the storm and eventually killed it. Farther west, we chased another
supercell that developed along the trailing boundary from the first. It quickly developed a similar barber pole appearance, but
there just seemed to be too much shear (for the instability) to expect much of a hose threat. The storm stayed quite organized for a
couple of hours, and we drove right into the southeast side of the hook with the only tornado warning of the evening. Again, lots of
nice structure, but no confirmed tornadoes. Seems to me that the storms formed a little too early, or that we just didn't have a wide
enough instability axis. It probably all comes back to the last minute return flow. A nice overall chase, but I'm still surprised there
weren't any tornadoes. Blame this one on Roger Edwards!"
Al Moller: "I agree with Rich - the storms simply outran the good low-level air by moving east of the warm front. Thus, a bad
balance of shear and instability likely shut the tornado potential down. The inflow was significantly cooler in (non-tornadic)
Oklahoma than across the border. We (Jesse Moore, Martin Lisius, and myself) also chased the Memphis to Hobart storm and the
secondary supercell that blew up to the first storm's west-northwest. Mike Foster had an interesting observation from his nonchasing desk yesterday, in reference to some of us, who on Wednesday thought there might be warm-frontal storms south of
Childress on Thursday. When one looks at Thursday's visible satellite loop, there is no southern New Mexico high-based
mountain cumulus and/or wave clouds during the day (a rare event), mainly south of a 35 degree north latitude. If you run a
straight line east-northeast from the southeast edge of the New Mexico mountain convection, it pretty much runs into the southernmost storm (the Palo Duro/Memphis TX storm). Nothing formed south of that complex. Thus, the cap (and probably the
subsidence area south of the polar jet) in New Mexico more or less "pointed" to the southern end of potential convective
development further east-northeast. This is food for thought, especially with tight, strongly dynamic systems, as to how far south
"chaseable" weather may occur. Of course, like everything else this is not a hard and fast rule, for sooner than later it will be
broken! It turns out that Bob Johns slight risk early Thursday (which ended near the Red River) was a good one. The later, further
southward extending, moderate risk (which I would have agreed with, from what I saw of the morning data) didn't pan out. One
thing that chasing has taught me, is that I am never satisfied with my expertise in severe weather forecasting. About all that I can
do is make a forecast, and (hopefully) have the sense to make acute observations (e.g., M. Foster's observation from above) and
thereafter adjust my forecast. Because of my perceived mediocrity, I prefer to forecast a "zone" of potential (rather than a point),
or maybe even several "zones". So, if I get to my forecast area and nothing happens (imagine that!), I will start thinking of other
zones of severe storm potential, and whether or not I can reach the alternate area, should something develop."
Robert Prentice: "1 intercepted the SW OK supercell at Mangum. At this point is was a classic/LP supercell hybrid with a
striated, barrel-shaped updraft and a significant (but not overwhelming) precipitation core well displaced to the northeast by about
5-15 miles. The updraft resembled 5/31/90 Waka/Spearman, Texas tornadic supercell, though not as physically large. As the
northeast side of the updraft approached the northwest side of Mangum, I observed persistent moderate rotation at cloud base.
Then the storm occluded and became disorganized. When it finally reorganized in Kiowa County, it was a struggling, high-based
supercell with a laminar base and flanking line and a strongly sheared- over updraft. What I speculate happened was this. Keep in
mind I don't know much about the storm before Mangum, except what I have heard and seen from the ADAS (OK Mesonet
objective Analysis) Homepage. At 3pm, the dryline extended from Dumas-Amarillo-Hall Co-Lubbock. The dryline was bowing
eastward into the southeast Panhandle around Hall Co. Meanwhile a warm front extended northwest/southeast from a triple point
near Amarillo. These boundaries were well-defined by cumulus/towering cumulus (as seen on visible satellite images). Apparently
the storm formed around 4-5pm in the warm sector along the dryline bulge in Hall Co. Even though the storm had great low-level
inflow and sufficient instability, it never produced a tornado. Its best attempt at producing a tornado was when it crossed the warm
front (baroclinic zone) at Mangum around 6:30pm. You would think that even in the strong shear environment, the supercell
would have had enough time to organize (-2 hours?) (see Brooks, et al.) and produce a tornado. Erik Rassmussen (VORTEX) has
emphasized just how quickly strong low-level rotation can occur within an updraft around these baroclinic (temperature) zones.
Storm chaser's have know this for many years and (mainly) informal scientific literature has discussed the importance of
baroclinic boundaries. However, mesocyclonic tornado formation seems to be tied to the Rear Flank downdraft (RFD). VORTEX
cases have show that strong vorticity generation along the storm-generated Forward Flank Downdraft (FFD) is really not
occurring. The latest thinking from Erik is we must learn more about the Rear Flank Downdraft, before we can unlock all the
secrets of tornado formation. This is going to be tough to do since, ground-based probe observation are insufficient (may occur
just above the surface) and with virtually no scatterers, radar is insufficient. Erik has said it may take near-storm LIDAR
observations to study the RFD. Brooks and Doswell wrote a great paper about atmospheric parameters used to distinguish
between tornadic and non-tornadic supercells. It is probably available through their homepages. To summarize, they think there is
a delicate balance between storm-relative helicity, the max mixing ratio ("dew point") in the low-levels. and the minimum stormrelative wind (at any point) in the mid-levels.
Rich Thompson has done a great deal of study, particularly on the last parameter (storm relative winds) and its ability to predict
tornadic versus non-tornadic supercells. He even has PC Gridds macros to utilize Brooks and Doswell's technique in an
operational SPC (Storm Prediction Center) environment. A few of the SPC forecasters will now try to distinguish tornadic versus
non-tornadic supercells in their discussions/outlooks. A lot of this goes back to trying to predict supercells with strong RFD
potential. Many scientists and chasers think this is tied to the strength of the storm- relative, mid-level wind, but without hard
scientific observations about how the RFD forms and evolves, we don't know for sure. The next major goal for VORTEX may be
RFD studies. So a possible reason the supercells did not produce tornadoes could have been related to RFD generation problems. I
did not observe a well-defined clear slot with the Mangum supercell until it was a high-based storm in Kiowa County. Why did
the storm slowly die after Mangum? It crossed the warm front and moved into cooler, drier, more stable air. That would also
explain the high cloud base. At least I got some pretty video (for later time lapse) and stills from an abandoned cobblestone farm
house in central Kiowa County. It died to my east-northeast in southern Caddo county as the updraft got completely sheared over
around sunset. I never knew about the "new supercell" which moved east-northeast from Hobart (unless that was it and I got
completely turned around!)."
Dave Gold wrote: "Yes, I was afraid for a while there that the narrowness of the theta-e ridge was going to be the quick death of
the nice Wellington barber pole. Around 22Z, it seemed to lose a lot of intensity judging by Frederick 1 and 2.4 km reflectivity,
but soon afterward really intensified near Dodson, TX, and became a flared flying eagle with WER (weak echo region), notch,
and mid-level meso. The storm never did show any signs of a good low-level meso, though... ...and that explains it. In addition to
the Edwards Factor, it seemed that there was just a bit too much shear for the given instability. Vici wind profiler showed
horrendous low-level shear and 50 kt mid-levels while the 00z OUN (Norman, OU) sounding showed garbage low-level theta-e.
A bit more time for moist return to broaden the theta-e axis and you likely would have had a real tornado show."
Tim Vasquez: "Before we intercepted the storm (near Dodson TX), the Wellington TX flanking line towers were well- separated
(I have time lapse of this from my VX-3). For the rest of the evening, it appeared to me that the storm's behavior and features
were going through cycles on the order of 8-15 min. (hard tops, soft tops, hard tops, soft tops, for instance). It suggests linemulticell characteristics and this "pulsing" would have to be detrimental to continued evolution into a tornado producer, in spite of
the great shears. Shouldn't a supercell have at least a quasi-steady updraft? I definitely agree with the statement about the poorer
theta-e's over Oklahoma. The moisture axis was very narrow and was locked over San Angelo to the eastern Panhandle most of
the day. Anyone notice how the storm bases kept rising as you went east?"
IOWA CHASE SUMMARY: MAY 18, 1997 By Gilbert Sebenste
A. PRE-CHASE MODEL ANALYSIS
Friday afternoon, May 16th, the forecast soundings from the AVN model and the 48 hour NGM/ETA models confirm that
something might happen on Sunday. But as the 12Z models for Saturday, May 17th roll in, it looks good. Not super, but it goes
like this: A surface low coming down from southwest Canada is forecast to move across southern Minnesota and Wisconsin by
OZ Monday. At that point, in the warm sector across Illinois, models are agreeing about how much moisture will be present, and
how warm it will get into the warm sector. But first things first: will an overnight MCS (Mesoscale Convective System) destroy
the chances for surface heating the next day? And will the best activity be along the warm front? The latter scenario is dismissed.
Clouds will make it way too cool, as lots of isentropic lift will take the available moisture and give widespread general rain and
weak thunderstorms ahead of the warm front. So, I turn my attention to the warm sector.
With the surface low continuing to deepen as it spins up, pressure falls should back the winds more. The NGM says it will only be
in the 70's in the warm sector, while the ETA heads for a much more robust upper 80's. But the NGM goes for upper 60
dewpoints and the ETA goes for right around 60. This will be critical to figure out. As I pull up the 182 Saturday surface map, I
notice a classic low level jet situation setting up, with dewpoints to the Iowa/Missouri border already in the low 60's, with upper
60's not far behind. And already the NGM is out to lunch on temperatures. Unless a huge MCS occurs, which is not likely,
moisture advection will be plenty and dewpoints should make it into the upper 60's in Illinois, with temps in the upper 80's. That
brings CAPE (Convective Available Potential Energy) to 4000 J/KG with 500 MB temps at -13C. With 10C 700 MB temps
advecting in from the south, things will be capped until the short- wave trough helps kick things off late in the afternoon. Speed
shear is good, directional shear is adequate but not great, and it appears any supercells that do develop will be borderline
classic/HPs. This forecast, as it turns out, will be dead on except for one thing. My call is northern IL for a chase. Thatll change.
B. THE CHASE
At 11am, the pre-frontal trough is still west of Des Moines, and the cold front is just getting to Omaha. Also, overnight convection
from the warm front has left a distinct outflow boundary from Sterling, IL (SQI) to Moline to Iowa City (IOW) and then westsouthwest north of Lamoni (30I). Eastern Iowa has cleared out and my jaw is dropping. Already temps are in the low 80's, with
mid-60 dewpoints and 70 dewpoints not far away. Then the phone starts ringing off the hook. George Sreckov, a recent graduate,
and Ryan Williams want to go chasing. Get in here fast, boys! Ryan Towell calls and offers to hold down the NIU weather office
and watch satellite, radar and surface data while we chase. That's nice, and then people start coming in. George can't make it until
2 PM (arrgh), but that's OK. I figure the cap will hold until 3 PM before things start breaking loose. Iowa City is my target town.
That too would verify nicely, but not for us.
By 19Z (2 PM CDT) everyone is ready to go. I hold off for just a few minutes to get a quick look at the 192 surface data from the
Iowa mesonet. Amazing. In the warm sector, cu's are popping along the outflow boundary, and the short-wave is clearly visible
west of Des Moines near Omaha. A tornado watch is issued for central and eastern lA until 22 Monday (9PM Sunday), and we
are GONE! I savor the 90 degree (32C) temperatures and 70 degree (21C) dewpoints in eastern and southern IA with south winds
at 20 knots gusting 30!
After packing our two-way radios, road maps and such, at 2:15 pm we leave NIU and head for 1-88 west. We learn that David
Paul, another recent graduate who lives in Carroll County about 60 miles west near l-88 wants to join us. He has a cell phone (we
have 4!), so we'll meet him at State Road (SR) 78 west of Sterling. We meet Dave around 3 PM and he's talking with Ryan back
at NIU. Since he gets free weekend calling on his cell phone, he can take his time getting all the information. A severe storm has
now popped up west of Cedar Rapids, moving east. That's what we want to hear! The cap has broken and the storm is growing
explosively, feeding on the high theta-e air. We get on 1-88 and the caravan of three cars speeds towards IA, with the remake of "I
can see clearly now, it's going to be a bright sunshiny day!" blaring on our radio.
We get on l-80 west in the Quad Cities and hit the Mississippi river at 3:49pm. We tune our radios to WMT-AM 600 out of
Cedar Rapids, which has in the past offered me good severe weather coverage. But before this day is over, I will hear one of the
finest radio broadcasts of severe weather coverage I've ever heard. An old-time music program ("big band") is swinging to Jimmy
Dorsey and Frank Sinatra, and our chase team who like to thrive on Megadeth, Aerosmith, Hootie and the Blowfish and Garth
Brooks don't seemed to be to thrilled about listening to this. But then they break in and announce a severe thunderstorm warning
for Benton and Lynn counties, with even a few funnel clouds reported in central Benton county! Suddenly, Paul Anka doesn't
sound so bad!
We decide to pull over at the rest stop west of Davenport to check out the radar, and make a quick pit stop. As we pull in at
3:52pm, KGAN-TV's meteorologist break in with a tornado warning for Lynn county, with a tornado on the ground 11 miles
northwest of Cedar Rapids! Still going 20 MPH in the rest area, getting close to the main building, I grab the radio and simply say
"Abort pitstop!". Paul doesn't have a radio in his car, so I motion to him to keep going. I decide that US 61 at Davenport north to
Grand Mound will be the way to go. At 4:05pm, we get off l-88 and head north on US- 61. We make a quick pit stop at an Amoco
just down the road from the weather office, with the WSR-88D radome looming over the gas station and the trees. With full gas
tanks and certain body parts empty, we head back north on 61 as other touchdowns come in to the radio station!
At 4:20pm, we decide to head west on US 30, hoping the storms will turn right. We now hear of ANOTHER supercell south of
the main one at Cedar Rapids, heading towards our location. That's what we want! We continue west on US 30, and at 4:32pm,
we are 4 miles east of Wheatland. A few minutes later, we hear of a tornado on the ground 20 miles west near Mechanicsville
right on US 30 as we cross the Wapsipinicon river! At 4:42pm, Dave advises us he has a graduation party to go to, and he has to
terminate the chase. Oh, man. That stinks. We say good-bye to him via cell phone as we watch him turn around. As we look west,
the sky is black. Frequent lightning is now flashing off to our west and northwest. At 4:43pm, we hit Lowden, and at 4:45pm, we
are 2 miles west of Lowden. As we break into an open are with a clear view to our west and northwest, we can see a low contrast
but HUGE rotating wall cloud off to our northwest!
This thing is a beast. It looks like a barrel in the sky. The base of the wall cloud is very low. It appears something is trying to
happen, but the contrast isn't good, with hazy skies. Still, it is adequate to see al) the features we need...including an oncoming
train which parallels the west-east road to our north, and blocks our view!!! It is one of the longer trains I have seen (it went all
the way back to Lowden, so it's almost two miles long), and he's only doing 40 mph or so. We hear of more touchdowns, and
through the breaks in each car a few people think they see a funnel develop. Great. If we miss this one because of a train...4 long
minutes later, at 4:52pm, the train passes and the wall cloud is becoming detached. That's not good, and we watch it for a few
more minutes before 1 decide we better get east of the storm to watch it. At 4:56 pm, we head one mile east (which is one mile
west of Lowden) on US 30 and turn north on a gravel road to get closer. at 5pm, we stop on this road one mile north of US 30. At
5:Olpm, a new wall cloud envelops...near the core of the storm. Dang. This is evolving into an HP. But we can't see very well
because a hill is in the way. We drive to the crest of the hill and find a FANTASTIC high point view of the entire storm.
Excellent! Then, a startling sight: A news helicopter is flying near the HP core, fading a little from time to time in the rain. We
look incredulously at this nut, and the comments fly: "Hey guys, keep your cameras on the helicopter...when it gets hit by
lightning, you'll have great video for CNN!". Good gravy, this pilot was a few french fries short of a Happy Meal! As we watch,
we seem to see something in the storm where the mesocyclone is, but we aren't sure of what. Video shows a white cone to the
ground, faint, but it's there. The problem is that I can't tell if that's a weird hail shaft or a nice looking tornado. I don't count it as
such, but then we hear numerous spotters northeast of it saying they have a cone tornado all the way to the ground. It was
confirmed, but I still won't call it such for us until I have others look at our tape.
At 5:04pm, we head back to U.S. 30 to head east and keep up with it. At 5:07 pm, a fairly low contrast, but definitely visible HP
wall cloud reveals itself as we continue east just west of Lowden. In-cloud and cloud to ground lightning is becoming very
frequent with powerful bolts in and around the mesocyclone. At 5:08pm, we're in Lowden, and one minute later after we pass
through a no-doubt-about-it funnel cloud descends from the north side of the mesocyclone. Here we go! But, it does not touch
down, and as we continue east at 5:13pm, the mesocyclone once again becomes completely wrapped in rain.
At 5:16pm, we cross the Wapsipinicon river again, losing sight of the storm. But at 5:18pm, we reach Calamus, and head north on
county road Y44, where the meso slowly re-appears to our northeast! We fly north when suddenly, the cell phone rings. It's Dave,
and as he was heading back, he heard about the additional tornado touchdowns and huge inflow jets of dust racing into the low
levels of the storm and decided "the heck with it!". He called his folks and told them he will be a little late for the party. He's just
ahead of us, and we head east on county road E-63 at 5:22 pm.. As we watch, I notice something that disturbs me: A shower has
formed just southeast of the storm, and a line of showers extends southward from there. I grimly announce on the two-way radio
that "If it doesn't go in the next 15 minutes, it won't".
Sure enough, no more than ten minutes later, at 5:32pm, the storm clearly is becoming outflow dominated, and it joins the
developing squall line. After hearing WMT's coverage, there are now two squall lines, with now little hope of a supercell
developing that we could catch. At 5:43pm, we reach Charlotte, and as we pass through the town, I say to myself "Man, this town
has a weed problem; look at all those white flowers" (yeah, I don't know the name of them, after dandelions seed). But as we slow
down, I gasp: Golfball hail! We pull over and get some classic shots of everyone holding the stones in their hand. The
KGAN/WMT radio crew tells us that the NWS has a warning for this and since they know about it, a report from us wasn't
necessary.
We head south into Clinton for dinner. Suddenly, a severe storm forms right on top of us in a matter of minutes. Marble hail starts
falling and our drivers nearly panic. They race through the town trying to find a gas station with a cover. My attempts to calm the
drivers are futile as visions of their mothers killing them for driving their new cars into large hail completely block out any mental
input from me. We screech around corners in the blinding rain and hail. Finally, with no gas stations to be found and in
desperation, we drive under roadside trees which provide sufficient coverage to stop the stones from damaging the cars. After
about five harrowing minutes, the storm moves east.
At 6:30pm, the frazzled chase crew pulls into a Hardees to have dinner in Morrison, IL. We hear about the supercell at Iowa city
dropping golfball hail on the hapless town for 15 minutes, and the tornado west of town a> well. It produces funnels, rotating wall
clouds and a tornado at Bettendorf around 7:30pm, but it is too far away to chase it...and it's also getting too dark. We head home
after eating, hitting NIU at about 9pm. All in all, a very good chase! Maybe we did get a tornado on camera, but I may never
know. Still, everyone went away very happy, except for poor Ryan, manning the consoles back at NIU, who suddenly got the itch
to chase when we saw the funnel clouds back in Iowa!
CHASE SUMMARY: MAY 18, 1997 By Don Lloyd
Driving through eastern Iowa, it became obvious we were in a prime area for severe storms. There were lots of instability clues:
atlocumulus and building altocumulus with virga. I stopped in the Quad Cities and check surface data at 19z. Dewpoints were in
the upper 60s, pressures were falling rapidly, and there was a triple point to my west. I decided it was going to happen in Iowa
rather than further east. Meanwhile, the Storm Prediction Center (SPC) sticks with their Moderate Risk over Missouri, Illinois,
and Indiana and issued a mesoscale discussion regarding the severe threat in Missouri. However, SPC later issued a red box for
Iowa and Wisconsin.
The first towers went up west of us in a linear west-to-east configuration. Storm structure was difficult to define with inflow bands
and garbage convection obscuring the updrafts. We encountered first cell near Stanwood, IA on US30 (5:05pm). There was a dark
and turbulent rain free base with modest core to the northeast. It produced several lowerings but rotation was weak. The best
feature was the vivid blues and greens in and around the rain free base though review of my video shows a short-lived funnel
(5:14pm). Then, a heavy curtain of rain wrapped around the rain free base and the storm became outflow dominant with a nice
shelf cloud. Heading west along HWY. 30 to the next cell, we could see even a more pronounced rain free base with heavy
pendant lowerings and scud (6pm). A Tnrnado Warning was issued on the storm though visible low-level rotation was weak to
non-existent. Again, the rear flank downdraft swept heavy rain around the rain free base evolving into a shelf cloud, and the storm
went HP (high precipitation). At this point, near Clinton, I met a large contingent of Wisconsin chasers including the UWMilwaukee team, Kinney Adams, and Scott Wittchow from Stevens Point. We convoyed down US HWY. 61 and out I-80 to
Walcott, IA to observe the next cell which was showing a pronounced hook-echo east of Iowa City.
We could see a heavy core to the north with strong inflow/flanking line and well-defined wall cloud which again showed only
weak rotation (7:20pm). In a seemingly inevitable fashion, the rear flank downdraft ripped around the south side of the
mesocyclone producing an impressive shelf cloud gusting out to the southeast (7:55pm). The storm took on more of an HP
configuration, with core to the north, and outflow/shelf cloud wrapping around the south side of mesocyclone, with a curved rain
free base in the middle. We eventually were forced to break off the chase due to failing light, not to mention being surrounded on
three sides by storms around 8:30pm. We had an excellent view of this storm from the southeast as it passed through Bettendorf;
we looked right into the slot. While we saw lots of pendulous scud and tornado wannabees, we saw no funnels beneath the wall
cloud. The reported touchdown in Bettendorf was inaccurate according to local papers the following day. Spent the night in
Galesburg after a rather hairy ride through heavy rain.
MORE NEWS: Dr. Joe Friday was removed from being Director of the National Weather Service after nine years of service amid
budget problems. To meet financial constraints imposed by the Clinton administration and Congress, the service had $41.5 million
less to spend this year than it did in 1996 and it was Dr. Fridays job to cut where he saw fit. Among the cutbacks (temporarily on
hold) was to eliminate 185 positions and close the administrative office of the Southern Region Headquarters located in Fort Worth,
Texas. This comes on the heals of a modernization program that included the deployment of NEXRAD radar's across the country,
adding specialized training personnel to teach students, and installing automated weather observation stations. STORMTRACK
subscribers are encouraged to write their congressman and senators (you can e-mail them too!).
"The Trucker's Friend" - The National Truck Stop Directory 1997 is available from TR Information Publishers PO BOX 476
Clearwater, FL 34617, Telephone 800-338-6317, FAX (813) 443-4921 for $9.95 plus $3 postage. Many truck stops have table
phones where you can hook-up your laptop to download weather information. They have an internet site at:
http://www.truckstops.com . Also, many KINKOS COPY places have table phones for rent.
Iowa area surface map, 15Z 5/18/97
19Z convective outlook, 5/18/97
Iowa area surface map, 22Z 5/18/97
Upper air maps at 12Z on 5/18/97
500 mb
300 mb
850 mb
700 mb
1997 STORM CHASER PARTY
The 1997 chase party was held at Robert Prentice's house in
Norman, Oklahoma. Chasers packed the place along with
three television news crews. There was plenty of food and
drink and fun was had by all. Lots of tornado videos were
shown. Among the highlights was Charles Edwards debut of
the wedge footage caught by the "dillocam" on May 25th in
southern Kansas. A special thanks to Robert and wife Tracy
for having us use and abuse their house.
The 1997 chaser picnic photograph. Where are you?
Chasers overflowed into the back yard.
Chasers are mesmerized by all the tornado videos.
Vince Miller chats about his Hollywood experience.
.
Tim Vasquez listens to a story told by Roger Jensen
Matt Biddle's race -- I mean -- chasemobile
Jim "the chef" Leonard, barbees those chickens