Transcript Slide 1

Matthew S. Bloemer
Florida State University, Department of Meteorology
What Happened
(Below) 300mb geopotential height (contoured) 850mb wind vectors (arrows) and 300mb
wind speed (shaded colors) for March 1st 12Z
(Courtesy Plymouth State University’s online product generator website)
Sounding Analysis
•12Z sounding shows a large surface inversion, a moist
adiabatic lapse rate above, near saturation throughout,
strong vertical shear (~80kts over 6km) and greatly curved
hodograph. A profile primed for development of rotating
Super-cells, provided the right amount of lift is supplied.
Figure 1 (Right)
Severe storm reports
provided by The
Storm Prediction
Center
(Courtesy: spc.noaa.gov)
•00Z sounding continues to be moist adiabatic, near
saturation throughout, with increased CAPE values
(~2300J/kg) continued strong vertical wind shear (70kts
over 6km), but a more linear hodograph. A still very
unstable situation, but primed for the development of quasilinear bow echoes and squall lines
•Parent cyclone was centered over the Iowa and Missouri and delivered Blizzard
conditions to the Northern plains states, Snow and Ice to the Northern Midwest,
flooding rains to the Mid-Ohio Valley, and severe weather to the deep south, with
a second area of severe weather over Missouri.
•49 confirmed tornadoes in the southern states, Three EF3 and Three EF4
tornadoes reported (first categorized as EF4 since the enhancement of the Fujita
Scale)
•20 fatalities due to thunderstorms in AL(10), GA(9), and MO (1), 19 due to the
Winter precipitation and cold Canada (4), MN(1), MA(1), NE(1), MI(3), ND(4), and
WI(5)
•Enterprise, AL hardest hit when one of the EF4’s hit the local high school, killing 9
students. (This storm will be studied further to the right)
•February 28th 2:00pm CST, SPC Day 1 convective outlook for March 1st, shows a
moderate risk for severe for the central plains states, accompanied by a slight risk
for severe weather for portions of MS, AL and FL. By midnight CST on the 1st this
is revised to put all of the deep south, as well as portions of MO, IL, IN, and KY in
moderate risk, and a High risk Bulls-eye over MS and AL. Later Outlooks
expanded the High Risk area to include portions of GA as well as a larger portion
of the panhandle of FL.
•The underestimation of the potential severe weather has been attributed to the
underestimation of development the Low Level Jet over the Deep South on the 1 st.
Figure 2 Storm Prediction Center Convective outlook maps with daily 12Z-12Z lightning
strike data from 20:00 UTC Feb. 28th (Below left) and 6:00 UTC March 1st (Below right)
•As of 12Z on the 1st, The parent cyclone was partially
occluded with two associated warm boundaries, one
that helped encourage stratiform precipitation around
the Ohio valley, and one that extended from Northern
Arkansas to the deep south which served to amplify
severe weather over the deep south.
A
•300mb wind patterns showed a strong area of
diffluence in the exit region of a 120kt jet streak over
the region of high risk.
(A-B) Radar returns from Fort Rucker, AL (EOX)
from around 2pm CST, March 1st 2007 and (CE) Tallahassee, FL(TLH) around 1am EST March
2nd 2007
•A low-level jet (850mb) was set up over the south,
which provided an ample supply of moisture and warm
advection over the deep south, and when coupled with
the upper level jet streak, Set up a region of very
strong vertical wind shear.
B
D
C
Conclusions
•The System that produced the severe weather on
March 1st and 2nd in the deep south did so not by
utilizing an enormous amount of CAPE, but made use
of large scale dynamic forcing attributed to the upper
level jet streak to create lift for parcels. Ample amounts
of Shear allowed for severe storms to form very easily
•The early phase of the system showed a preference
for the formation of Supercells due to the dramatic shift
in winds associated with the a curved hodograph
whereas the later phase showed a preference for the
development of quasi-linear systems due to a
hodograph with a linear profile.
• The presence of a warm boundary over the deep
south served to intensify pre-existing meso-vortexes
that crossed the boundary to a point that they might be
able to produce tornadoes. This tendency for Supercells to produce tornadoes more frequently when
crossing into or forming in a cool sector behind a warm
boundary was well documented by Rasmussen Et. Al
(2000). Below is shown the warm boundary over the
deep south constituted by a 3 degree temperature rise,
and a 3 degree dew point rise over a span of about 50
km, and the tornado tracks that were produced by the
same parent Super-cell that spawned the Enterprise
Tornado.
E
(Courtesy: spc.noaa.gov)
•The cloud shield over the south prevented an
extensive amount of surface heating, however large
scale upward forcing attributed to the diffluence in the
exit region of the jet allowed for CIN values to easily be
overcome, and thunderstorms to develop even with
only moderate CAPE values (500-1000 J/kg).
Atmospheric
Soundings for
Birmingham, AL
Mesoscale Analysis
(Left) 12Z on the 1st
Synoptic Background
Note that the surface
inversion sufficiently capped
the atmosphere so there is
no CAPE based on the
surface level
Significant values:
6km shear = 78kt
1km helicity = 722 m2/s2 3km
helicity = 847 m2/s2
(Courtesy: spc.noaa.gov)
(Right) 00Z on the 2nd
Note the erosion of nearly all
CIN and and the significant
increase in CAPE
Significant values:
6km shear = 68kt
1km helicity = 52 m2/s2
helicity = 153 m2/s2
(Courtesy: spc.noaa.gov)
(Above) US Satellite imagery and surface analysis composite for March 1st 12Z
(Courtesy of the Hydrometeorological Prediction Center’s Website)
3km
•A: A reflectivity image from EOX at 19:15 UTC Mar 1 st
(1:15pm local time) showing the a Super-cell that was
characteristic of the early phase of severe weather for this
outbreak. This particular storm was the one that ravaged
Enterprise Alabama
•B: A SRV Image from 19:05 UTC, (1:05pm Local time)
showing the velocity couplet to the SW of enterprise. At this
stage the mesovortex is just beginning to intensify to the
point of producing a tornado.
•C: A reflectivity image from TLH at 549 UTC, (12:49am
local time) showing a Quasi-linear complex of
thunderstorms with a bow echo just to the west of
Tallahassee.
•D: A SRV image from TLH at 4:40 UTC showing an
embedded mesovortex within the same Quasi-linear
complex mentioned above. This mesovortex was
responsible for producing a tornado in the vicinity of Baker
County, GA
•E: A SRV image from TLH at 5:54 UTC showing the
velocity pattern of the bow echo just to the west of
Tallahassee with associated bookend vorticies, and strong
straight-line winds just to the North of Attapulgus, GA.
(Above) Enterprise Tornado track and Analyzed surface warm boundary
(Surface data Courtesy NWS Forecast office Tallahassee)
References
•Rasmussen, E. N., S. Richardson, J. M. Straka, P. M. Markowski,
and D. O. Blanchard, 2000: The association of significant tornadoes
with a baroclinic boundary on 2 June 1995. Mon. Wea.
Rev., 128, 174–191.
•“Severe Thunderstorm Event: March 1, 2007”; NOAA’s Storm prediction center online
archive, referenced last as of 4-13-2007;
http://www.spc.noaa.gov/exper/archive/events/070301/index.html
•“HPC’s Surface Analysis Archive”; National Weather Service Hydrometeorolgical
Prediction center website, referenced last as of 4-13-2007;
http://www.hpc.ncep.noaa.gov/html/sfc_archive.shtml
•“Plymouth State Weather Center…product generator for Archived data”; Plymouth
State University’s Weather Center website, referenced last as of 4-13-2007
http://vortex.plymouth.edu/u-make.html
•Special thanks to Irv Watson (NWS-Tallahassee Science
Operations Officer) for data and insight into the case, and
to Kelly Godsey (NWS-Tallahassee Forecaster) for his
insight into the case as well.