U.S. Severe Weather Forecast (Convective Outlook)

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Day Three

000 
acus01 kwns 211946 
swody1 
Storm Prediction Center ac 211944 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0144 PM CST sun Jan 21 2018 


Valid 212000z - 221200z 


..there is a slight risk of severe thunderstorms from East Texas to 
Arkansas... 


..there is a marginal risk of severe thunderstorms surrounding the 
slight risk from southeast Texas to MO... 


... 
Severe storms are expected from parts of East Texas/Oklahoma into 
Arkansas and northwest Louisiana into tonight. A few tornadoes, 
damaging winds, and isolated large hail will be possible. 


... 


Changes are not warranted to 1630z outlook. 


Large-scale forcing for ascent is quickly spreading across the High 
Plains and shallow convection is now developing from north-central 
Texas into eastern OK. Earlier thoughts regarding the evolution of 
thunderstorms into the evening hours remain. 


Refer mesoscale discussion #32 for more information regarding the short-term threat 
across this region. 


.Darrow.. 01/21/2018 


Previous discussion... /issued 1028 am CST sun Jan 21 2018/ 


..East Texas/OK to AR/la... 
Overall minor changes to the slight and marginal risks in this region. 
Considered an upgrade to enh centered on Texarkana but continued 
concerns over the degree of buoyancy and potential weakness in 
forecast hodographs around 700 mb suggest the tornado threat may not 
yet warrant greater probabilities. 


12z soundings sampled mean mixing ratios of 10-12 g/kg within the 
modifying western Gulf air mass that is characterized by surface dew 
points of 60-65 f. These dew points should spread as far north as 
eastern OK and western/southern Arkansas during the afternoon/evening. 
Abundant low cloud cover will limit diabatic surface heating within 
the moist sector. However, low-level convergence will intensify 
along the dryline during the late afternoon as mid-level height 
falls associated with the southern rockies shortwave trough 
overspread the boundary. This should yield storms gradually 
increasing along/ahead of the dryline across eastern OK into 
east-central Texas. 


Forecast soundings show modest low/middle-level lapse rates across 
much of the moist sector. Nearly saturated low-level thermodynamic 
profiles should result in predominant MLCAPE values of 300-800 j/kg. 
Low-level and deep-layer shear will strengthen tonight and become 
increasingly favorable for updraft rotation, however some forecast 
soundings indicate weaknesses in the hodograph around 700 mb. Given 
the positive tilt of the mid/upper wave, the related geometry of 
winds aloft should lead to a substantial component of mean flow 
parallel to the developing convective corridor. These factors should 
contribute to a mixed Mode of semi-discrete cells transitioning to a 
largely quasi-linear Mode. Damaging winds should be the primary 
hazard, but a few tornadoes are possible, particularly centered from 
northeast Texas into southeast OK and southwest Arkansas between 00-06z where 
the combination of buoyancy/shear should be relatively maximized. 


A broken band of storms will likely expand/develop slowly eastward 
towards southern MO through the Sabine valley tonight. The severe 
threat should diminish towards the lower MS valley during the early 
morning as the warm sector Narrows from north to south ahead of the 
convective band, though isolated damaging wind potential may persist 
through 09-12z. 


... 
Relative to areas farther south, colder midlevel temperatures will 
offset weaker boundary-layer Theta-E to maintain weak buoyancy for 
storms, amid an intense warm-advection conveyor. Forecast soundings 
suggest MLCAPE will be meager, below 500 j/kg along and south of the 
warm front in the 00-06z time frame. A shallow, near-surface stable 
layer reinforced by sporadic precip should temper downdraft strength 
somewhat. Still, convective downward momentum Transfer and related 
isolated damaging winds may penetrate that layer. Low wind 
probabilities have been maintained in this regime of favorably 
strong deep-layer lift and weak buoyancy immediately preceding the 
ejecting low-level cyclone. 


$$ 

Mesoscale Discussion


000 
acus11 kwns 211914 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 211913 
arz000-laz000-txz000-moz000-okz000-212115- 


Mesoscale discussion 0032 
National Weather Service Storm Prediction Center Norman OK 
0113 PM CST sun Jan 21 2018 


Areas affected...northeast Texas...eastern Oklahoma through western 
Arkansas 


Concerning...severe potential...Tornado Watch likely 


Valid 211913z - 212115z 


Probability of watch issuance...80 percent 


Summary...thunderstorms are expected to develop by 21z across 
northeast Texas into eastern Oklahoma and subsequently increase in 
coverage and intensity. Supercells and bowing line segments with 
damaging wind and a few tornadoes should be the primary threats. A 
Tornado Watch will probably be needed before 21z. 


Discussion...this afternoon a dryline extends from northeast Kansas 
through west-central OK to north-central and southwest Texas. Pacific 
front will merge with the dryline later this afternoon, and this 
boundary will continue through the Southern Plains this evening as a 
surface low consolidates over eastern Kansas in association with a 
progressive shortwave trough. Modifying Continental-polar air with 
low 60s f dewpoints has advected through the warm sector beneath 
modest lapse rates where objective analysis shows a corridor of 
500-800 j/kg MLCAPE from central and northeast Texas into southeast OK. 
WV imagery indicates mid-upper level moistening and clouds resulting 
from a band of deeper forcing for ascent moving through eastern OK 
and northeast TX, and scattered showers are already developing over 
northeast Texas in association with this process. Rap soundings 
indicate a capping inversion still in place around 750 mb, but 
ascent in the 850-500 mb layer layer will eventually erode the 
inversion, contributing to thunderstorm development by 20-21z. 
Vertical wind shear will increase as the low-level jet strengthens 
within exit region of the approaching mid-upper jet with large 0-1 
km hodographs and effective-bulk shear exceeding 40 kt. This 
environment should support organized storms including a few discrete 
supercells and bowing line segments as storms intensify later this 
afternoon and evening. 


.Dial/grams.. 01/21/2018 


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


Attention...WFO...lzk...sgf...shv...tsa...hgx...fwd...oun... 


Latitude...Lon 30929680 34139629 36149588 36619432 35819334 33839335 
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