U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 190546 
Storm Prediction Center ac 190545 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
1145 PM CST sun Feb 18 2018 

Valid 191200z - 201200z 

..there is a marginal risk of severe thunderstorms over a portion 
of the Southern Plains... 

A few strong to severe storms are possible over a portion of the 
Southern Plains, mainly later this afternoon into tonight. 


Synoptic upper trough will continue to amplify over the western U.S. 
With a building upper ridge over the southeast states. Between these 
two features, several weak vorticity maxima embedded within a 
southwesterly flow regime will move through the Southern Plains 
during the period. At the surface a cold front from the upper MS 
valley into northwest Kansas will advance slowly south, possibly 
becoming stalled near the Kansas/OK border. South of this boundary, a 
dryline will mix ewd through western OK and western Texas during the 
afternoon before retreating overnight as a Lee cyclone becomes 
established over the Central High plains. 

..Southern Plains region... 

Within a broad southerly flow regime, partially modified Gulf air 
with low to mid 60s f dewpoints will advect through the Texas and OK 
warm sector today. Meanwhile, a modest elevated-mixed layer will 
spread eastward and overlap the western fringe of the moist warm 
sector contributing to a corridor of 500-1000 j/kg MLCAPE primarily 
from western and central OK through southwest Texas. However, warmer 
temperatures aloft will contribute to a capping inversion over the 
more unstable portion of the warm sector during the day. Model 
consensus is that showers and a few thunderstorms may be ongoing 
early in of the period along warm conveyor belt, primarily from 
north central Texas through eastern OK into the middle MS valley. 

Primary uncertainty this forecast is extent of surface-based 
thunderstorm development given presence of widespread clouds, 
likelihood of at least a modest capping inversion, and tendency for 
the low-level jet to veer and shift eastward during the day. 
Potential for heating and mixing in vicinity of dryline may 
contribute to at least isolated storms by late afternoon or evening. 
Other storms may initiate over the higher terrain of northern Mexico 
and spread northeast into southwest Texas. Another area of potential 
surface-based development will be near the dryline/cold front 
intersection over northern OK or southern KS, though any storms 
initiating in this area may have a tendency to become elevated as 
they move northeast and cross the frontal zone. 

Very strong (55+ kt) 0-6 km shear will support potential for 
supercells with any storms developing along the dryline or higher 
terrain from southwest Texas into western OK. Conditional upon storm 
initiation, a small window will exist for a tornado during the early 
evening, mainly from western OK into northwest Texas as low-level 
hodographs become more favorable due to the strengthening low-level 
jet, and before the boundary layer stabilizes. Otherwise, primary 
severe threat is expected to be large hail. Additional storms may 
develop later Monday night along evolving warm conveyor belt but 
within a strongly sheared environment. Extent of severe threat 
overnight is somewhat uncertain given a stable to marginally 
unstable near-surface layer. Nevertheless, some of the updrafts 
could become near the surface. 

Have introduced a marginal risk this update, given the conditional 
nature and uncertainties regarding extent of thunderstorm initiation 
during the more favorable time of day and evening. An upgrade to 
higher probabilities could be needed in later updates over a portion 
of this area if these uncertainties can be mitigated. 

.Dial/jirak.. 02/19/2018 


Mesoscale Discussion

acus11 kwns 182148 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 182147 

Mesoscale discussion 0071 
National Weather Service Storm Prediction Center Norman OK 
0347 PM CST sun Feb 18 2018 

Areas affected...parts of West Texas 

Concerning...severe potential...watch unlikely 

Valid 182147z - 182245z 

Probability of watch issuance...5 percent 

Summary...at least isolated strong to severe storm development 
appears possible during the 4-6 PM CST time frame, with severe hail 
and locally strong wind gusts the primary potential hazards. 

Discussion...weak to moderate destabilization is ongoing near the 
dry line across West Texas, with continued boundary layer heating 
and mixing. Latest rap output suggests cape is now on the order of 
500-1000 j/kg. Low-level convergence appears weak, and mid/upper 
forcing for ascent unclear, but inhibition appears to be in the 
process of becoming increasingly negligible. At least isolated 
thunderstorm development does not appear out of the question within 
the next hour or two, probably near or west/southwest of Midland, 
based on latest visible imagery. 

Given favorable deep layer shear beneath southwesterly mid-level 
flow on the order of 40-50 kt, any sustained convection could take 
on a supercell structures. This probably would be accompanied by at 
least the risk for severe hail and localized strong surface gusts, 
before boundary layer instability wanes after nightfall. 

.Kerr/Thompson.. 02/18/2018 

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


Latitude...Lon 31250339 32360272 33110158 33120058 32100184 31550236 
30670330 30880347 31250339