U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 201940 
Storm Prediction Center ac 201939 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0139 PM CST Tue Feb 20 2018 

Valid 202000z - 211200z 

..there is a marginal risk of severe thunderstorms from central 
Texas across the Ozarks... 

Thunderstorms are expected across a broad part of the south-central 
U.S. Into the Midwest, with the possibility of a few strong to 
locally severe storms through this evening from central Texas into 
parts of the Ozarks. 

..central Texas into Arkansas and southern MO... 
Changes this outlook include an expansion of the marginal risk area 
farther southwestward across central TX, and a bit farther east from 
East Texas into south central Arkansas. In addition, added a small 2% 
tornado risk area for the possibility of a brief/weak tornado over 
central Texas. 

A moist and sufficiently unstable air mass remains in place ahead of 
the cold front, with strong heating leading to isolated severe 
storms capable of hail across central Texas. Other storms may form 
along the I-35 corridor in Texas as deep moisture convergence persists, 
with some supercell potential. Extensive clouds persist in much of 
the moist sector but pockets of heating may yield further isolated 
storm activity developing across central Texas where effective srh 
values are currently in excess of 300 m2/s2. In addition, locally 
damaging wind gusts cannot be ruled out especially later this 
evening and overnight when linear structures form along the 
advancing cold front, from northeast Texas across southeast OK and into 

.Jewell.. 02/20/2018 

Previous discussion... /issued 1017 am CST Tue Feb 20 2018/ 

A broad long wave trough is expected to persist over the western 
Continental U.S. As an initial short wave trough near The Four-Corners areas 
lifts northeastward and deamplifies, and an upstream short wave 
trough off the Pacific northwest coast moves southward toward the central 
California coast. A strong cold front at the surface, currently 
located from extreme northwest Illinois into southwest Missouri, 
southwest Oklahoma into east central New Mexico, will continue 
moving southeastward into across the middle Mississippi Valley and 
the Southern Plains. 

..Southern Plains into the Ozarks... 
Multiple bands of convection are occurring within a broad region of 
moist southwest flow aloft from central Texas into the Midwest. 12z 
regional soundings ahead of the cold front indicate generally weak 
deep layer lapse rates but forward and oun soundings exhibited layers of 
7-8 c/km lapse rates that are enhancing zones of buoyancy. 
Thunderstorms have organized into several lines over north central 
Texas with radar depicting occasional Bow echo signatures moving 
toward the northeast. Latest VAD wind profiles show winds of 40-50 
kt in the lowest 1 km above ground level suggesting potential for a few strong to 
locally severe wind gusts to reach the surface as convective 
downdrafts Transfer the strong horizontal momentum downward. In 
addition, marginal hail may occur with a few of the stronger 
updrafts, especially over parts of Texas, where steeper lapse rates 
are likely to persist. 

Although mid-level height changes over the Southern Plains will 
remain neutral or are slightly positive as the long wave trough 
remains over the west, water vapor imagery indicates additional 
perturbations will move northeast from The Big Bend region of Texas 
over the Southern Plains toward the Ozarks. This suggests potential 
for additional convection to develop this afternoon. 12z href 
guidance and latest hrrr hourly runs are consistent with this 
scenario, showing several strong organized convective bands/clusters 
spreading from Texas into Arkansas later this afternoon and evening. 
Cam simulated reflectivity forecasts indicate transient qlcs 
structures with embedded Bow echoes, suggesting a continued 
potential for a few strong to locally severe wind gusts in 
association with stronger storms through the evening hours. 


Mesoscale Discussion

acus11 kwns 201712 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 201711 

Mesoscale discussion 0077 
National Weather Service Storm Prediction Center Norman OK 
1111 am CST Tue Feb 20 2018 

Areas affected...northwest Texas...north-central Texas...far south-central 

Concerning...severe potential...watch unlikely 

Valid 201711z - 201845z 

Probability of watch issuance...20 percent 

Summary...isolated small hail and damaging wind gusts are possible 
across portions of northwest TX, north-central TX, and far 
south-central OK over the next few hours. 

Discussion...a line of thunderstorms has developed along a dryline 
extending southwestward from just west of Special Weather Statement to sww. Atmospheric 
profiles are moist and largely void of steep lapse rates but warm 
and moist conditions just ahead of the dryline (i.E. Temperatures in 
the upper 60s/lows and dewpoints in the mid 60s) still support 
modest instability. Latest mesoanalysis estimates MLCAPE around 500 
to 800 j/kg. Kinematics are a bit more favorable for a few stronger 
storms with vertical veering wind profiles shown on regional vads 
sampling 20-35 kt of 0-1 km shear and around 40 kt of 0-6 km shear. 
Given these wind field, some updraft rotation is possible with the 
more persistent updraft. 

Marginal thermodynamics and favorable kinematics will likely keep 
strong updrafts predominantly short-lived. Additionally, storm 
persistence will be limited by the southward progressing cold front 
currently intersecting the dryline just northwest of Special Weather Statement. Some 
brief/localized enhancement of updrafts is possible as frontal 
circulations augment lift, but northeastward storm motions will then 
result in an undercutting of updrafts. Overall expectation is for 
isolated small hail and occasional damaging wind gust across the 
region for the next few hours. 

.Mosier/Weiss.. 02/20/2018 

..please see www.Spc.NOAA.Gov for graphic product... 


Latitude...Lon 32730008 33939892 34339809 34379709 33379713 32279895 
32109997 32730008