acus01 kwns 211959
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.
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
acus11 kwns 212018
sels mesoscale discussion
Storm Prediction Center mesoscale discussion 212018
Mesoscale discussion 0756
National Weather Service Storm Prediction Center Norman OK
0318 PM CDT Tuesday may 21 2013
Areas affected...central into the arklatex
Concerning...Tornado Watch 201...205...
Valid 212018z - 212145z
The severe weather threat for Tornado Watch 201...205...continues.
Summary...a line of severe storms should continue to become better
organized with widespread damaging winds likely across northestern Texas.
Other storms should increase in coverage farther S along or just
ahead of the cold front...with large hail and damaging winds and
perhaps an isolated tornado.
Discussion...severe storms currently extend from near the Red River
southwestward toward the Dallas area...with a general upswing in intensity
noted. With ample instability ahead of these storms...this trend
should continue. Given linear storm Mode...damaging winds are likely
To the S...supercells were rapidly developing near the southern border of
Tornado Watch 201...and should continue into the watch. These cells
will have a distinct large hail threat...and could produce a tornado
as well before the cold front overtakes them. If a southward trend in
development persists...northern portions of the Houston County warning area may need to
be included in a watch.
..please see www.Spc.NOAA.Gov for graphic product...
Latitude...Lon 31569202 31449527 31779528 30360002 30890004 31129942
31559846 32179746 32709688 33239668 34179658 34549521
34219522 34329201 31569202