U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 211252 
Storm Prediction Center ac 211251 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0751 am CDT Sat Apr 21 2018 

Valid 211300z - 221200z 

..there is a marginal risk of severe thunderstorms from parts of 
northwest and north-central Texas to western Louisiana... 

Isolated severe thunderstorms are possible from 
northwest/north-central Texas this afternoon to southeast Texas and 
southwest Louisiana tonight. 

A progressive, split-flow pattern will persist in mid/upper levels 
this period. The main feature for this forecast is a synoptic-scale 
cyclone initially centered over south-central/southeastern co, with 
trough south-southwestward over far West Texas to Chihuahua. By 00z, 
the associated 500-mb low should move to the OK/Kansas border region 
between ict-gag, with trough southward across west-central/southwest 
Texas. The mid/upper low should reach northeastern OK overnight, with 
the trough becoming somewhat negatively tilted, extending across the 
arklatex and Texas/la border area by 12z. 

At the surface, 11z analysis showed a frontal-wave low near pvw, 
with cold front arching southwestward across the Llano Estacado and 
over extreme southeastern nm. A warm front was drawn southeastward 
across the Sat area then eastward over the Upper Texas coast and Gulf 
waters south of la. The low should move east-southeastward across 
northwest/north-central Texas today as the warm front shifts/redevelops 
northward across The Hill Country and into parts of north-central 
Texas. By 00z, the cold front should extend from the low over north- 
central Texas southwestward across parts of The Hill Country to the 
lower Pecos Valley. By 12z, the cold front will have crossed most 
of southeast and south-central TX, extending southwestward from the 
low near the la/Arkansas state line. 

Isolated to widely scattered strong/severe thunderstorms are 
expected to develop this afternoon over portions of northwest/ 
north-central TX, in a zone of maximized low-level convergence/lift, 
close to the surface low. This activity should move east- 
southeastward to eastward across parts of north-central or central 
Texas through the rest of the afternoon into the evening, offering the 
potential for hail and isolated severe gusts. The initial activity 
will be supported by a pocket of diurnal/warm-sector diabatic 
heating and related boundary-layer destabilization, behind an area 
of clouds, precip and non-severe thunderstorms now moving across 
parts of OK and North Texas. That increasing low-level instability, 
along with surface dew points generally upper 50s to mid 60s f, will 
be juxtaposed with cooling temperatures aloft as areas of DCVA pass 
through the southeastern quadrant of the mid/upper cyclone. 
Forecast soundings accordingly depict MLCAPE 1000-1500 j/kg over 
parts of north-central Texas in the 21z-00z time frame. Effective- 
shear magnitudes around 45-55 kt and curved low-level hodographs 
suggest potential for a supercell or two in the first few hours, 
followed by upscale development of a near-frontal convective band 
that would cross parts of north-central and east TX, reaching 
western la overnight. Isolated severe hail and gusts will be 
possible in that convective band as well. 

Coverage and boundary timing/placement uncertainties with respect to 
the convection are the main concerns precluding greater 
unconditional severe probabilities at this time, but a small area of 
greater risk may be added as the event gets closer and presumably 
better-focused on the mesoscale. 

.Edwards/Peters.. 04/21/2018 


Mesoscale Discussion

acus11 kwns 202047 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 202046 

Mesoscale discussion 0279 
National Weather Service Storm Prediction Center Norman OK 
0346 PM CDT Fri Apr 20 2018 

Areas affected...eastern New Mexico...West Texas 

Concerning...severe potential...watch unlikely 

Valid 202046z - 202315z 

Probability of watch issuance...20 percent 

Summary...a threat for large hail and wind damage may develop across 
parts of eastern New Mexico and West Texas late this afternoon into 
early evening. The threat is expected to remain isolated and weather 
watch issuance appears unlikely at this time. 

Discussion...the latest surface analysis shows a 1002 mb low eastern 
New Mexico with a dryline extending southward from the low into far 
West Texas. Surface dewpoints to the east of the dryline range from 
near 40 f at Clovis, nm to near 50 f at Midland, Texas. In response to 
surface heating, a narrow corridor of instability is analyzed by the 
rap from far West Texas extending northward into eastern New Mexico. 
Visible satellite imagery shows developing cumulus along the 
instability corridor with the most widespread cumulus located from 
near Roswell, nm northward to near Tucumcari, nm. A weak capping 
inversion is likely present but the cap should diminish over the 
next couple of hours as surface heating continues and large-scale 
ascent increases ahead of an approaching upper-level system in The 
Four Corners region. This should allow for the initiation of 
surfaced-based convection in east eastern New Mexico late this 

The hrrr solution seems reasonable which develops a line of 
thunderstorms oriented from north to south across far eastern New 
Mexico and moves this line eastward into West Texas by early 
evening. Regional WSR-88D vwps and objective analysis show strong 
deep-layer shear in place with 0-6 km shear estimated near 60 kt. 
This combined with steep mid-level lapse rates on forecast soundings 
may support strong updrafts capable of producing isolated large 
hail. A wind damage threat will also be possible. Instability should 
remain relatively weak and any severe threat should remain isolated. 

.Broyles/Hart.. 04/20/2018 

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


Latitude...Lon 33500236 34410243 34880269 35020325 34950402 34720436 
34070435 32830409 31750382 31240362 30960337 30930271 
31440243 33500236