U.S. Severe Weather Forecast (Convective Outlook)

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Day Three

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acus02 kwns 210541 
swody2 
Storm Prediction Center ac 210540 


Day 2 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1240 am CDT Tuesday Apr 21 2015 


Valid 221200z - 231200z 


..there is an enh risk of severe thunderstorms across parts of southern OK and 
north to northeast Texas... 


..there is a slight risk of severe thunderstorms across parts of OK...SW 
Arkansas...TX...la and MS... 


..there is a marginal risk of severe thunderstorms across parts of the Southern 
Plains...arklatex...lower MS valley and central Gulf Coast states... 


..there is a marginal risk of severe thunderstorms across parts of South Florida... 


... 
thunderstorms with wind damage...large hail and possibly a few 
tornadoes are expected to develop from the Southern Plains 
southeastward across the arklatex and into the lower Mississippi 
Valley. Other storms with hail and wind damage may develop in 
central Texas and the Texas coastal plains. Marginally severe 
thunderstorms may develop across parts of the southern High Plains 
and in south Texas. Thunderstorm development may also occur across 
parts of the central to northern rockies and Sierra Nevada. 


..srn plains/arklatex/lower MS valley... 
An upper-level ridge is forecast to build north-northwestward across the southern High 
Plains into the central rockies on Wednesday. At the surface...a low is 
forecast to deepen in NE nm with a warm front extending eastward across 
scntrl OK into central Arkansas. South of the boundary...surface dewpoints 
should be in the lower to middle 60s f across North Texas and southern OK where 
moderate instability should be in place by afternoon. Convection 
appears likely to initiate by late afternoon along the warm front in 
SW OK with an mesoscale convective system organizing and moving east-southeastward across southern OK into NE 
Texas. There is still some question about timing but the GFS solution 
seems to be reasonable with a late afternoon and early evening mesoscale convective system. 


NAM forecast soundings for 00z/Thursday at Frederick OK and Vernon Texas 
show 1500 to 2000 j/kg of MLCAPE with very steep middle-level lapse 
rates approaching 9.0 c/km. In addition...an impressive wind-shear 
environment is forecast with substantial veering with height below 
850 and 0-6 km shear near 60 knots. This would favor the development of 
supercells with large hail and wind damage early in the event. 
Hailstones of greater than 2 inches in diameter will be possible 
with the more dominant cells. A few tornadoes may occur especially 
with persistent rotating storms. A relatively fast-moving mesoscale convective system may 
develop a linear structure and cold pool early Wednesday evening 
which could result in an enhanced wind-damage threat. The severe 
threat from this convective system could extend as far southeast as 
East Texas and northwest la during the evening and possibly during the early 
overnight over parts of the lower MS valley. 


..TX Hill Country/Texas coastal plains... 
An upper-level ridge is forecast to be maintained across the Texas Hill 
country and southern High Plains on Wednesday as a warm sector remains 
located over much of the southern two-thirds of Texas. The airmass across 
the Texas Hill country and the Texas coastal plains...away from the 
immediate coast...is forecast to become moderately to strongly 
unstable by Wednesday afternoon. Forecast soundings for 00z/Thursday at 
Austin and College Station show MLCAPE above 3000 j/kg with lower 
70s f dewpoints. In addition to the impressive 
thermodynamics...deep-layer shear should be in the 50 to 60 knots 
range. This should support severe thunderstorm development with 
supercells the favored storm Mode. Convective coverage is the main 
uncertainty with the upper-level ridge to the west. Even though 
storm development should remain isolated...supercells should be 
capable of producing large hail...wind damage and possibly a 
tornado. 


..srn Florida... 
An upper-level trough is forecast to move slowly eastward across the East 
Coast as a belt of stronger middle-level flow moves eastward across the northern 
Gulf of Mexico. A moist airmass is forecast across South Florida where 
moderate instability should develop by midday. Cells that can 
initiate across South Florida during the early afternoon may obtain a 
marginal wind damage threat due to moderate deep-layer shear and 
unidirectional wind profiles. 


.Broyles.. 04/21/2015