April 7, 2006
Severe Weather Event
Full
Text of Storm Surveys
Preliminary
storm reports
I. Overview and
Background Information
A series of supercell thunderstorms swept across the Tennessee Valley during the late afternoon and evening hours on Friday, April 7th.
The storms resulted in numerous tornado touchdowns, large hail up the size of
softballs, and straight-line wind gusts in excess of 60 mph. Although
additional storm surveys are still possible, it appears there were at least 18
brief tornado touchdowns across the Huntsville County Warning Area. The
tornadoes were all in the F0 to F1 range on the Fujita Scale.
Several days in advance of the storms, local National
Weather Service offices and media partners began to mention the potential for
significant weather across the region. In addition, early Thursday afternoon,
the Storm Prediction Center began to forecast a high risk of severe weather
across the Tennessee Valley Region for Friday afternoon and evening.
Early on Friday the 7th, a classic spring severe
weather setup began to unfold across the area. A strong upper level storm
system 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 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.
II. Weather Summary
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. 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. 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. Based on radar interrogation and storm surveys, it appears that
there were 18 separate tornado touchdowns. Of the 17 tornadoes, there were 5 of
F1 intensity and 13 of F0 intensity.
Based on local storm reports received in real-time during
the event, post event reports, and storm surveys conducted, there were
approximately 66 severe weather reports received. These included many reports
of large, damaging hail in excess of 2 inches in diameter. In addition, outside
of tornado damage, there was damage cause by straight line winds (downed trees
and power lines) due to estimated and measured gusts approaching 70 mph.
Thankfully, there were no reported injuries or fatalities across the Huntsville
County Warning Area. Several folks interviewed during storm surveys commented
they received the warnings via NOAA Weather Radio or local media channels and
took appropriate safety precautions before the storm hit.
III. Brief Radar and Damage Summary
Although not inclusive, we will review briefly a few of the
storms that caused damage and produced tornadoes across the Tennessee Valley Friday evening. This review will include storm survey photos, radar imagery, and
additional comments. For an exhaustive list of damage reports and survey
information, please reference the following links:
a.
Tornado Damage
Tuscumbia/Muscle Shoals Tornado
A multicell cluster of thunderstorms developed across northeast Mississippi
in the late afternoon hours and moved into northwest Alabama.
These cells eventually split into two distinct cells that both
had supercellular characteristics. The northern of the two supercells
was responsible for this particular tornado. The F1 tornado with
estimated maximum winds of 90 MPH occurred about 4 miles southwest
of Tuscumbia. Initially, the tornado had a width of 100 yards
and produced damage along a 200 yard path length before weakening
to F0 intensity. A gas station suffered extensive damage (pictured).
The tornado finally dissipated just southeast of Muscle Shoals.
Approximate time of the tornado touchdown was 625 PM CDT. Also
of note was a 66 MPH wind gusts reported at the Northwest Alabama
Regional Airport (KMSL) at 633 PM CDT. A tornado warning was issued
for Colbert County at 605 PM CDT.
The Storm Relative Motion (SRM) image from KGWX radar
showing the velocity couplet near the time of the tornado touchdown in
Tuscumbia.
Damage at a Tuscumbia Gas Station
Tornado damage in Tuscumbia
Northern Madison/Southern Lincoln County Tornado
This tornado was the result of a supercell thunderstorm that
tracked northeast out of Colbert County, across northern Limestone County, and into northern Madison and southern Lincoln Counties. Prior to moving into Madison County, this same storm spawned a brief F1 tornado (maximum winds estimated at 80
mph) in the Sugar Creek Estates Community falling numerous large trees.
As the storm moved into Madison County, this same supercell produced
two brief F0 tornadoes. One of those touched down just east of Hazel Green in
the Sulphur Springs Community. A roof was peeled from a mobile home and
insulation tossed into trees. The resident of the home was inside at the time
but was fortunately unharmed. The tornado touched down at approximately 745 PM CDT.
Maximum winds were estimated at 60 MPH. A Tornado Warning had been issued for Northern Madison County at 721 PM CDT.
Radar reflectivity image of the storm at the approximate time of the tornado touchdown.
Extensive damage to a mobile home near Hazel Green.
Caddo/Lawrence County Tornado
The supercell thunderstorm that spawned this tornado
actually began to get well organized along the Mississippi/Alabama border just
south of Tremont MS around 835 PM CDT. The storm strengthened rapidly as it
moved northeast and showed intense rotation as it tracked west of Hackleburg in
Marion County Alabama (where it produced damage).
The tornado touched down near Caddo Alabama around 1000 PM CDT. Maximum winds were estimated at around 70 MPH making it an F0 tornado. The tornado produced
minor tree damage and roof damage to a manufactured home. The path length was
100 yards and the width was 60 yards. A tornado warning was issued for Lawrence County at 916 PM CDT.
Radar reflectivity image of the storm showing the intense
hook echo evident at the approximate time of the tornado touchdown.
The Storm Relative Motion (SRM) image from KGWX radar
showing the velocity couplet near the time of the tornado touchdown in Caddo.
Damage to a manufactured home in Caddo.
Danville Tornado
This particular tornado was spawned by a supercell
thunderstorm that initially produced extensive damage in the Haleyville area
around 1005 CDT. In addition, this same supercell would eventually move across
the Huntsville metro area producing a brief F0 tornado near the Greenbrier exit
of Interstate 565, measured wind gusts to near 60 MPH on Redstone Arsenal, and
drop huge amounts of damaging hail across the southern part of the city.
One of the interesting things about the Danville tornado is
that it formed not in the classic hook echo region of the storm, but in what is
commonly referred to as the cyclonic bookend vortex or “comma head” portion of
the storm. This F1 tornado touched down initially 3 miles southwest of Danville at approximately 1037 CDT with maximum winds around 90 MPH. A large barn was
demolished with additional F0 type damage to Danville High School. The
intensity increased once again to F1 east of the high school as it uprooted trees
and damage roofs, awnings, and destroyed a barn. A tornado warning had been
issued for Lawrence County at 1001 PM CDT. A tornado warning was issued for Morgan County at 1026 PM CDT.
Radar reflectivity image of the storm that produced the F1
tornado near Danville. The tornado actually occurred in the “comma head” region
of the storm.
The Storm Relative Motion (SRM) image from KHTX radar
showing the velocity couplet near the time of the tornado touchdown in Danville. Note: the bright orange and pink colors are bad velocity values knows as
“dealiased data”.
Barn destroyed 2 miles northeast of Danville.
Damage near Danville.
Cullman County Tornado
An F1 tornado struck a chicken farm 5 miles northeast of
Cullman near the Pleasant Grove Community. Two chicken housed were collapsed
and stripped of their roofing with another chicken house damaged. A small shed
was destroyed as was an attached feed bin. A branch was thrown as a projectile
into the window of a nearby residence, but the residence had taken shelter in
the basement after receiving the Tornado Warning on NOAA Weather Radio. The
path length was 200 yards with a width of 30 yards. Maximum wind speeds were
around 80 MPH. The tornado touched down at approximately 1053 PM CDT. A tornado warning had been issued for Cullman County at 1036 PM CDT.
Radar reflectivity image of the storm that produced the
F1 tornado near Cullman.
Chicken houses destroyed near the Pleasant Grove Community.
Third chicken house heavily damaged.
b.
Hail Damage
There were a large number of reports of large hail with the
severe thunderstorms that rumbled across the region on April 7th.
The combination of an unstable airmass, cold temperatures in the upper levels
of the atmosphere, and vertical wind shear (rotating updrafts), set the stage
for the production of large hail with the more intense storms. In fact, there
were 28 separate reports of hail the size of golfballs (1.75”) or greater and 5
reports of baseball diameter (2.75”) hail or greater. Damage was reported to
cars, homes, and businesses due to hail damage across many areas of the Tennessee Valley.
The hardest hit areas where the northwest corner of Morgan County through southern Madison County into Marshall County.
Hail covering the ground near Airport Road in Huntsville.
Hail piled 4-5” deep in Decatur (photo courtesy of WHNT and Samuel Rhodes)
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