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Storm Surveys »
April 7, 2006 Severe Outbreak »
Weather Summary |
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Weather
Summary/Overview
Early on Friday the 7th, a classic spring severe weather
setup began to unfold across the area. A strong upper level storm
system (see an upper-air map) was positioned
across the central Plains with an associated band of very strong winds
aloft extending into Arkansas and Missouri. This system was forecast
to shift southeast with time and approach the mid Mississippi and
Tennessee Valleys Friday evening. At the same time, a strong surface
low pressure system was located across Kansas (see a
surface map from 7am 4/7/06) with a trailing dryline extending
through the Southern Plains. This surface low was forecast to develop
east southeast and to eventually track across northwest Tennessee
by the late night hours. The combination of these two features helped
to bring a surge of gulf moisture northward into the Tennessee Valley
and also create favorable wind shear conditions for supercell thunderstorms
and tornadoes.
Supercell thunderstorms began to erupt rapidly across western Tennessee
and Kentucky during the late morning hours on Friday. Several of these
supercell thunderstorms continued to gain strength and wind energy
as they tracked eastward toward the Nashville Metropolitan area by
the early afternoon hours (see a radar image from
4:30pm Friday afternoon). However, storms were slower to develop
across the Tennessee Valley. The primary reason was that a pocket
of warm air about 5000 feet above the ground, commonly referred to
as a “cap” or thermal inversion, was inhibiting the growth of thunderstorms.
This cap began to erode by late in the afternoon as moisture began
to increase across the region and the upper level trough began to
bring in cooler temperatures above the surface.
Thunderstorms began to erupt across northern Mississippi shortly after
4 PM CDT Friday afternoon, and many of these became severe very quickly
(see a radar image from 5pm Friday afternoon).
The first tornadic supercell moved into northwest Alabama around 5
PM, but it would not be the last. All totaled, eight severe storms
moved across the Tennessee Valley region between 5 PM and midnight,
several of which were long lived supercells.
Related
Images
Click the thumbnail to view a larger version. |
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| A map of the conditions around 15-20,000 feet
(500mb) at 7pm on 4/7/06 shows the strong upper-level system
over Kansas, and a band of higher winds extending from western
Texas into the Ozarks of Missouri and Arkansas. (Map from the
Storm Prediction
Center.) |
Infrared satellite imagery from 3:30pm CDT
on April 7. At this point, the coldest cloud tops (the shades
of blue indicating -50 to -60 degree Celsius) are over Tennessee,
where numerous tornadic supercells are already in progress.
However, the "bumpy" clouds over northeast Mississippi
indicate that thunderstorms are beginning to develop in thar
area. |
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| A surface map from 7am CDT on the morning
of April 7th. The system centered north of Topeka, Kansas would
eventually move east to impact the Tennessee Valley. (Map from
the Hydrometeorological
Prediction Center.) |
A surface map from 7am CDT on the morning
of April 8th. By this time, the main cold front had moved east
of the area, and the warm, muggy airmass partly responsible
for the storms had been replaced with cooler, drier air. (Map
from the Hydrometeorological
Prediction Center.) |
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| A view of several supercell thunderstorms
raking across middle Tennessee, from the Hytop, AL radar at
4:30pm on the 7th. Note the lack of storms across north Alabama,
thanks to a pocket of warm air above the ground commonly referred
to as a "cap". |
By 5pm CDT, supercells had been firing across
northern Mississippi for nearly an hour. This radar image, taken
from the Columbus, MS radar at that time, shows the first round
of supercells starting to move into northwest Alabama. |
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| Infrared satellite imagery from 7:15pm CDT
on April 7. Note the swirl in the clouds over Kansas indicating
the upper-level low, and the cold cloud tops (-60 to -65 degrees
Celsius!) indicating thunderstorms over Mississippi, Alabama,
and Tennessee. |
Visible satellite imagery from around 5pm
CDT on April 7. The swirl indicating the upper-level low is
even more apparent in this image. You can also see tiny "bubbles"
in the centers of the clouds; these are often called "overshooting
tops", indicating very strong thunderstorms. |
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