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acus01 kwns 211959
swody1
Storm Prediction Center ac 211958
Day 1 convective outlook
National Weather Service Storm Prediction Center Norman OK
0258 PM CDT Tuesday may 21 2013
Valid 212000z - 221200z
..there is a MDT risk of severe thunderstorms over north central and northestern
Texas...southeastern OK...southern Arkansas...and northern la...
..there is a slight risk of severe thunderstorms from the Southern Plains across the
Tennessee Valley...lower Great Lakes...and the northestern states...
..srn plains into the Tennessee Valley...
A series of linear convective systems are moving eastward from southeastern
OK/north central Texas into Arkansas and KY/TN...in advance of an upper
vorticity maximum/short wave trough located over the Texas Panhandle.
Strong diabatic heating has occurred south of a convective boundary
arcing from east central Arkansas across southern Arkansas into north central
Texas...and with steep lapse rates /ref 18z forward and 19z shv special
radiosonde observations/ in place coupled with surface dew points around 70f...the
environment has become very unstable with SBCAPE near 3500 j/kg.
Visible imagery shows cloud streets over eastern Texas and la feeding northward
toward the thermal boundary indicating a favorable moisture
transport into the area. Low-level convergence is focused in three
areas: 1) near a surface low over the metroplex...2) along the cold
front extending southwestward into The Hill Country...and 3) along the
east/west convective boundary across northestern Texas/southwestern Arkansas. Storms are
expected to intensify the remainder of this afternoon into the
evening as the upper system and surface low move eastward...with
potential for supercells and bowing line segments. 12z NSSL WRF-arw
and recent hrrr models indicate development of long-lived bowing
qlcs/S moving eastward across northestern Texas into southern Arkansas and northern la...and
environment appears supportive of this event. Thus...will maintain
potential for very large hail and several tornadoes /including one
or two strong tornadoes/ associated primarily with any supercells
that form...along with widespread significant wind damage given the
potential for a progressive bowing convective system to form within
the next 1-2 hours.
Elsewhere from the middle-south into the Tennessee Valley...additional linear
convective systems over eastern Arkansas and southern Kentucky/middle Tennessee are likely to
progress eastward/northeastward over the next several hours...while new
strong/severe storms have developed along the trailing outflow
boundary over far northern MS. Given the degree of instability with cape
of 2000-3000 j/kg and deep layer shear of 30-40 knots...a threat for
damaging winds and hail will continue with these storms.
.Weiss.. 05/21/2013
Previous discussion... /issued 1128 am CDT Tuesday may 21 2013/
..substantial severe weather expected across north-central Texas to the
arklatex this afternoon and evening...
.Srn plains to lower MS valley...
Significant severe weather episode initially unfolding across the
Red River with supercells producing large hail along and north of the
composite outflow/cold front pushing southward from the Big Country to far
southeastern OK ahead of a well-defined shortwave trough over the southern High
Plains. Air mass S of the boundary is becoming strongly unstable as
gradual surface heating continues amidst lowest 100 mb mean mixing
ratios of 14-17 g/kg and 700-500 mb lapse rates of 8 to 9.5 degree c/km
sampled in all regional 12z radiosonde observations. As mlcin is minimized along the
dryline mixing eastward across the Edwards Plateau...additional supercell
development is anticipated with southward extent.
A belt of 50-60 knots middle-level swlys attendant to the compact
shortwave trough will promote risks for all severe types /some of
which will be significant/. Morning convection-allowing WRF and hrrr
simulations are insistent that upscale growth will occur throughout
the afternoon. Given the degree of instability...buoyancy and
shear...setup may yield a derecho accelerating eastward towards the
arklatex region by early evening. Here too...the strongest low-level
S/swlys will remain...promoting embedded mesovortices capable of
producing a couple of significant tornadoes. Otherwise...potential
does exist for widespread/destructive damaging wind gusts.
..TN valley to the lower Great Lakes/northeast...
A broad area of organized severe potential is expected along the eastern
extent of the elevated mixed layer plume sampled in 12z radiosonde observations. With
robust heating...moderate to strong instability is anticipated...but
most of the region will remain on the fringe of sufficient
deep-layer shear for supercells. Three zones of focus for sustained
convection are apparent. One will be downstream of an ongoing
cluster across western Tennessee. The others should be along a quasi-stationary
front bisecting New York into southern New England and perhaps in association
with a weak mesoscale convective vortex moving into lower Michigan. Multicell clusters and a few
supercells will be primarily capable of isolated damaging winds and
severe hail.
Mesoscale Discussion
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sels mesoscale discussion
Storm Prediction Center mesoscale discussion 212323
tnz000-alz000-msz000-arz000-220030-
Mesoscale discussion 0762
National Weather Service Storm Prediction Center Norman OK
0623 PM CDT Tuesday may 21 2013
Areas affected...wrn-middle Tennessee / far northwestern Alabama / northern MS / far southeastern Arkansas
Concerning...Tornado Watch 207...
Valid 212323z - 220030z
The severe weather threat for Tornado Watch 207 continues.
Summary...isolated pockets of wind damage appear to be the main threat.
Tornado potential will likely remain limited based on linear
convective Mode.
Discussion...radar mosaic shows an organized qlcs from far southern Illinois
southward to east of the greater Memphis metropolitan southwestward into southeastern Arkansas. The northern
portion of the qlcs from near mem northward into western Kentucky remains largely
sub-severe and surface observation over western Tennessee support this notion with ASOS
gusts at time of qlcs passage remaining below 30 knots. Surface
analysis shows a largely west-east oriented outflow boundary
extending from the trailing portion of the southern Appalachians qlcs
westward and intersecting the qlcs 25 Michigan east-southeast mem. Earlier in the
day substantial convective overturning occurred north of this boundary
across western and middle Tennessee -- limiting the magnitude of
destabilization occurring in its wake. Current thinking is the Tennessee
portion of the qlcs may only pose a low damaging wind threat.
Farther S over northern MS...a warm/unstable airmass ahead of this
convective line is more favorable for isolated pockets of wind damage
given the steeper low level lapse rates. It is uncertain how far
east this qlcs may pose an isolated damaging wind threat...especially
since the boundary layer will begin to cool by the time the qlcs
approaches the eastern edge of the watch near the northestern MS/northwestern Alabama
border. Although a brief tornado is possible near the outflow
boundary-qlcs intersection...this threat will likely remain limited
based on the linear convective Mode.
.Smith.. 05/21/2013
..please see www.Spc.NOAA.Gov for graphic product...
Attention...WFO...ohx...bmx...hun...Meg...Jan...lzk...
Latitude...Lon 36478874 35098975 33489168 33019107 33018934 34268807
35638754 36378760 36478874
