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The proper term is "storm
chaser" not "tornado chaser." An experienced storm
chaser observes a tornado
only about one out of every seven chases. It's even rarer to see a tornado
up close. Many of my best chases included a tornado encounter, but they
were usually associated with an equally impressive supercell
thunderstorm.
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versus |
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| Oklahoma City tornadic supercell from June 13,
1998. Courtesy R.J. Evans. |
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Oklahoma tornado from the movie
"TWISTER". Copyright Warner Bros. and
Universal Pictures, 1996. |
The movie TWISTER
could have been even more "thrilling" and "chilling"
if the creators had the tornadoes descend from realistic looking storms.
Real tornadoes are produced by supercell thunderstorms which often resemble
the Mother
Ship from the movie "Close
Encounters of the Third Kind." Not from flat, featureless, stratus
clouds. We know the creators based their digitally created tornadoes on
video taken of real tornadoes. They should have known what a real tornadic
supercell was supposed to look like. Perhaps the they thought the public
would balk at such a scene? "That's an overblown fake! The real thing
couldn't possibly look THAT spectacular!"
WHEN AND WHERE TO CHASE
INTRODUCTION
Anybody can be a storm
chaser. There is no licensing or certification process. Anybody with
a pair of eyes and a vehicle can traverse highways looking for storms
and tornadoes. But routine successful requires education, training and
experience. It helps to know where to start.
One of the most revealing conversations I have ever had occurred with a TV
reporter from Washington, DC. She wanted to film tornadoes and didn't understand
why she couldn't just chase them from her station. "We just had one in
Virginia a few weeks ago!"
A chaser can greatly increase his odds and enhance his chase experience
by knowing such basic factors as WHEN is the best time and WHERE are the
best places to chase storms. To answer these questions, I wrote "When
to Chase" for Storm
Track magazine back in 1992 (revised slightly in 1997). In this article,
I suggest the best place to chase was the southern and central Great
Plains. The best time was late April through mid-June with the heart
of the season extending from the last three weeks of May and the first
nine days of June.
Eric Becusla wrote a comprehensive follow-up article (not available online) in Storm
Track Magazine. He suggested the best time to chase storms in the southern
and central Great Plains was during late May and early June with the peak of the
season around May 31st.
Allow me to document reasons why the southern and central Great Plains of the US
is the best chase territory in the world and why I call it "Chase
Alley."
TORNADO (and supercell)
FREQUENCY
click to enlarge
Courtesy National Severe Storms Laboratory (NSSL)
The above map is provides an analysis of all United States tornado reports
between 1955 to 1989 by Manually Digitized Radar (MDR) box. A MDR box is 48 km x
48 km or 30 statue miles x 30 statue miles. Many storm chasers and tornado
climatologists believe the axis of highest tornado (and supercell) frequency is
actually located a little further west across the high plains. Tornadoes across
the high plains are under-reported because of sparse population and lack of
objects for tornadoes to hit and leave their mark. The graph below covers the
time period 1980-1994 and is probably more representative of the true tornado
threat.
Note that neither graphs distinguish between tornado size, path length or
intensity. A small, brief, weak touchdown is counted the same as a mile-wide, long-track, violent
tornado. Also, both graphs are biased due
to population and reporting differences.
Tornado frequency across the state of Kansas is likely underestimated for
both of the above graphs. The 1970's and 1980's were extraordinarily quiet when
compared to other decades since Kansas was settled.
Notice the increased tornado threat across northeast Colorado in the bottom
graph (1980-1994). The advent of storm chasing and increased storm spotter
awareness has led to a dramatic increase in the number of reports the
past few decades. Most of these tornadoes are weak "landspouts,"
but many strong and violent tornadoes have been reported too.
Harold Brooks
of the The
National Severe Storms Laboratory (NSSL) has created an excellent
Severe Thunderstorm
Climatology web site that details the risks from tornado, wind, and
hail for the United States. Below and left, I have created a map (based
on his research) that highlights were "Tornado Alley" is located.
The outlined areas are based on the combination of high tornado frequency
and high repeatability of the annual cycle. Please refer to www.nssl.noaa.gov/hazard/svrwxmisc.html
for details on how Harold subjectively determined these areas. Below and
right is my subjective interpretation of where "Chase Alley"
is located.
 
TREES
For most Americans, the Great
Plains is simply a desolate "nothingness" to hurry through
between the comforting forests to the east and the mountain ski resorts
to the west. For storm chasers during Spring, the Great Plains is a wonderland
of fascination and beauty where Mother Nature paints bold, towering storms
with breathtaking colors and other magical atmospheric forms on the canvas
of the sky.
Alan Moller once said "My definition of the Great Plains is where the sky
becomes big, and becomes an integral part of the landscape." Storm chasers
have a love affair with the sky and become claustrophobic when boxed-in by a
dense canopy of trees. An awesome sunset is a routine occurrence when chasing
across the treeless Great Plains.
The Great
Plains is relatively free of trees
and the sky comprises almost the entire 180 degrees of the horizon. It can be
very difficult or impossible to see the sky from roads in the Eastern US where
the forests act like a canopy blocking all but perhaps a narrow cone 10 to 50
degrees directly overhead. Many a chaser has given up when his storm moved into
the national forests of Missouri, Arkansas, eastern Oklahoma and east
Texas...never to be seen again.
Click to enlarge
Source: USDA Forest Service, Forest Inventory and Analysis Program, http://fiatools.fs.fed.us
ROADS
The biggest, meanest tornadic storm in the world is useless to a storm chaser
unless he has enough good roads to follow the storm and obtain a view of the
storm's updraft base. Paved roads are much preferred, though chasers will drive
on gravel roads if conditions are not too bad. Dirt roads are generally avoided
since they can turn into slippery mud during a rainstorm. I do not have a map of
road density per county for the US (If you know of one, please tell me).
The best rural road network across "Chase
Alley" exists in west central Texas in the agricultural areas
around Lubbock. This
area is the ultimate in "chaser nirvana" with grid of paved
section line roads just about every mile. Texas has the best rural road
network in the plains. Roads which are gravel in Kansas, Nebraska and
Colorado would be paved if they were in Texas.
Another region with a very good rural road network is the "Corn
Belt." (eastern Nebraska/Dakotas to western Ohio). Although most
of the "Corn Belt" is not technically located in my definition
of "Chase Alley,"
it is relatively flat terrain and has a high tornado frequency. It is
the second best storm chase region after the Great Plains.
The region with the worst road network in or near "Chase
Alley" is the Sand
Hills region of Nebraska. There are very few roads and many great
storms have been missed in this region. Other areas in or near "Chase
Alley" with a poor road network include: southwest Texas, eastern
New Mexico (except for the Clovis/Portales area), parts of eastern Colorado,
eastern Wyoming and the western Dakotas. Many of these areas are still
chaseable, however, due to flat terrain and/or great visibility and lighting.
VISIBILITY AND LIGHTING
To chase and photograph storms it is essential to have good visibility and
lighting. Once again, the biggest, meanest storm in the world is useless to a
chaser unless he can actually see and film it. Showers, rain, low clouds, fog,
haze, smoke and smog all prevent a chaser from seeing his storm.
The optimal visibility and lighting conditions occur when relative humidity
is low and obstructions to vision are absent. "Bone dry" air
allows for vivid colors and great visibility which is one reason why so
many landscape painters and photographers live in New Mexico and the desert
southwest. Chasers prefer that their storm develop as an isolated Cumulonimbus
(Cb) cloud in a "clear atmosphere." The way to accomplish
this is by capping layer of warm, moist (but unsaturated) air beneath
a strong, but breakable capping
inversion (or "cap").
The cap is a layer of warm air (thermal temperature inversion), that suppresses
the formation of thunderstorms. The cap is typically located between 5
to 10 thousand feet (1.5 to 3 thousand meters) Above Ground Level. Sometimes
you can see it where air pollution and haze stagnates in stable, stratiform
layers. This pollution and haze is typically not as bad across the sparsely
populated Great Plains as it is across the industrial northeast.
A cap suppresses the formation of showers and thunderstorms and allows
large quantities of heat and moisture to build up. This produces an extremely
unstable environment (see diagram below) meteorologist call this Conditional
Instability. Conditional Instability is a form of atmospheric stability
in which parcels of air initially resist vertical displacement, but will
rise up spontaneously if lifted to their Level of Free Convection (at
the top of the cap). The source of lift is usually due to fronts,
troughs,
drylines,
upslope
flow, and outflow
boundaries left behind from prior thunderstorms.
Copyright
Tim Marshall, 1988
Almost all thunderstorms initiate in a conditionally unstable atmosphere
(i.e., have a cap), but across the Great Plains the cap can be especially
strong. The source of a strong cap across "Chase Alley" is hot,
dry air from the elevated desert terrain of the southwest US and northern
Mexico. As this air is advected (blown) northeastward over the lower terrain
across the Plains, where it caps warm, moist air near the surface that
originated from the Gulf of Mexico and Caribbean.
The cap can allow tremendous amounts of heat and moisture to build up
and create an extremely unstable atmosphere. When rising air parcels finally
penetrate the cap, very powerful thunderstorms are the result. The cap
has difficulty extending into the eastern US which is why extreme instability
is unusual there.
LAND FORMS (HILLS AND
MOUNTAINS)
Chasers generally desire flat country...The more skyline they can see
the better. The Great
Plains fits this description well. The "Chase
Alley" region contains some minor geographical problem areas
such as the Smokey
Hills (northcentral Kansas), the Wichita
Mountains (northwest of Lawton, OK), the Arbuckle
Mountains (southcentral Oklahoma) and the Palo
Duro Canyon (southeast of Amarillo, Texas). However, these features
are usually considered photographic "opportunities" rather than
real barriers to chasers.
Some of the flattest land in the world is the Llano
Estacado located in west Texas and far eastern New Mexico. Here chasers
can see the full 180 degrees of skyline and don't have to worry about
missing a tornado or some other storm feature because they were caught
driving through a valley or their view was blocked by a mountain.
Click on image to choose larger and/or more detailed versions
Courtesy John Hopkins University Applied Physics Laboratory
SPEED OF STORM MOVEMENT
During the early spring, tornadic supercells are most common with across the
southeast US and typically move very fast. Storm speeds of up to 60 mph (100
km/hr) are common. This makes it very difficult if not impossible for a chaser
to keep pace with a storm.
By late spring, tornadic supercell activity shifts northwest into the Great
Plains and storms move much slower. Storm speeds between 15 to 25 mph (25 to 40
km/hr) are typical. This allows a chaser to spend more time filming and enjoying
the storm and less time driving to keep pace with it.
Tornado "cluster outbreaks" have produced some of the best storm
chases of all-time. During these events, tornadic supercells are stationary
or continue to redevelop over the same few counties for multiple hours.
Chasers dream about experiencing this type of event. In his Master's Thesis,
Al Moller suggested tornado "cluster outbreaks" are most common
during late spring (mid-May through June) along an axis from near Littlefield,
Texas (just northwest of Lubbock) north-northeastward to near Wakeeny,
Kansas.
TIME OF DAY
Most tornadic supercells occur in the daytime during the late afternoon or
evening hours. This is especially true across "Chase
Alley." Storm chasers know quite reliably that their storm will
initiate during the warmest part of the day, between 3 - 6 pm local time.
Across the southeast US a large percentage of tornadic supercells occur at night
when chasers cannot see them. This is because southeast US events are most
common during late fall, late winter and early spring when days are short (click
here to view animated loops that show the annual cycle of US severe weather
threats through the year). Also, the storms are usually associated with
intense, large-scale, weather systems and not so closely tied to heating by the
sun. Thus, tornadic supercells often occur during odd hours and a tornado could
strike just as easily at 6 am as 6 pm.
FREQUENTLY ASKED QUESTIONS
(FAQ)
I often receive questions about storm chasing, storms and tornadoes. I
am a professional meteorologist instructor and enjoy answering questions
about my favorite subjects. However, my time is limited due to work and
family. Before asking a frequently asked question, please refer to the
links below first. Then, if they do not answer your question, I would
be happy to do so.
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