U.S. Severe Weather Forecast (Convective Outlook)

Day Three

acus01 kwns 221251 
Storm Prediction Center ac 221250 

Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0750 am CDT sun Oct 22 2017 

Valid 221300z - 231200z 

..there is a marginal risk of severe thunderstorms from 
southeastern Louisiana to parts of the Florida Panhandle... 

..there is a marginal risk of severe thunderstorms over parts of 
the northern High Plains... 

A few severe storms will be possible this afternoon through tonight 
near the central Gulf Coast. 

The main upper-air feature for this forecast will be a 
full-latitude, synoptic-scale trough -- currently arching from 
southern mb across western Minnesota, eastern KS, central OK, and southwest 
Texas. Approximately the northern half of this trough will break 
northeastward across the upper Great Lakes and on through the 
period, with a cyclone forming on its northern extremity over 
northern Hudson Bay. Meanwhile, the southern portion will shift 
eastward from the Southern Plains across the lower/mid Mississippi 
Valley. An embedded/500-mb low should develop this evening over the 
west-Central Ozarks and move generally eastward near the MO/Arkansas 
border toward the Mississippi River. By the end of the period, the 
trailing shortwave trough should become slightly negatively tilted, 
extending south-southeastward from the low across MS to near bix and 
over the adjoining northern Gulf. 

Northern-stream shortwave perturbations -- now evident in 
moisture-channel imagery near the coastal Pacific northwest and 
coastal British Columbia -- will cross the northern rockies today. These features 
phase into a strengthening, high-amplitude shortwave trough over mb, 
the Dakotas, and NE by 12z, contributing to some acceleration of the 
southern trough/low near the end of the period and into day 2. 

At the surface, the 11z analysis showed a cold front from western 
Upper Michigan across western IL, then southwestward in more diffuse form, 
temporarily hung up in one convectively related wake low over 
southwestern/south-central MO and another near Dal. The front then 
became better-defined again southwestward across the Southern Hill 
country to near drt. A warm/marine frontal zone was evident from 
southeastern Arkansas across southwestern and coastal MS, 
east-southeastward over shelf waters to near aaf, then in diffuse 
form over the north-central Florida Peninsula. The warm front should 
move slowly east-northeastward across the north-central Gulf coastal 
plain through the period. By 00z, the cold front should reach 
western portions of lower mi, in, KY, and MS, southwestward over the 
northwestern Gulf. By 12z, the cold front should become more 
meridional from eastern lower Michigan through an area of low pressure 
over eastern portions of Kentucky/TN, southward across the western Florida 

..central Gulf Coast... 
An extensive area of scattered convection is ongoing across portions 
of southeastern la and southern MS, over and southwest of the 
effective warm/marine front. While deep shear is not strong over 
this region, expect backed flow and well-curved hodographs near the 
warm front, along with rich low-level moisture and related low local. 
This will Foster a conditional risk for a brief/small tornado or two 
and isolated damaging gusts from the most heavily water-loaded 
downdrafts, until the main/pre-cold-frontal convective band 
overtakes it from northwest-southeast late morning through early 

Areas of diurnal destabilization are likely ahead of that convection 
today, augmented by low-level Theta-E advection; however, the amount 
and pace of that destabilization is in question given the cloud 
cover associated with the ongoing convection. Regardless, surface- 
based effective-inflow parcels should be available across most or 
all of the risk area as the marine air shifts slowly inland, 
displacing a recycled Continental/polar air mass from the frontal 
passage a few days ago. Meanwhile, with the approach of the basal 
trough aloft, overlying mid/upper winds should contribute to an 
increase in deep shear over the warm sector and the potential for 
sporadic organized convection, with damaging gusts the main concern; 
a tornado cannot be ruled out. 

Overnight, as the mid/upper trough and associated vorticity Banner 
become more negatively tilted, the resulting adjustment of kinematic 
geometry will result in an eastward acceleration of the southern 
part of the corridor of most favorable lift near the front. That, 
in turn, should reorient the main convective plume more north-south 
with some acceleration possible the last 3-6 hours of the period. 
Spreading of large-scale ascent (from both DCVA and warm advection) 
over more of the warm sector also is possible, supporting 
development of surface-based or nearly surface-based convection over 
the Gulf, moving inland with some merger into the main line 
possible. All of these factors support an eastward expansion of the 
existing outlook area at least to near the Apalachicola/ 
Chattahoochee River system for tonight. Some subset of the "mrgl" 
area may need an upgrade in succeeding outlooks, once uncertainties 
are better-resolved regarding the impact of the large area of 
ongoing convection on favorable destabilization to its east. 

..northern High Plains... 
As the northwestern shortwave trough consolidates/intensifies and 
approaches, both large-scale lift and deep-layer flow will intensify 
markedly. Accordingly, high-wind warnings are in effect for much of 
this region. Isolated thunderstorms may supplement the already 
strong-severe gradient flow at the surface late this afternoon, 
resulting in damaging/severe gusts in convective downdrafts. 
Forecast soundings, modified for high temps in the lower 60s f, 
yield steep low/middle-level lapse rates and well-mixed boundary 
layers conducive to convective downward Transfer of momentum through 
the short vertical distance from a field of 50-60-kt flow in the 
850-700-mb layer. Essentially uninhibited MLCAPE of 100-200 j/kg is 
possible with Low Cape density, but also, buoyancy extending into 
the icing layer suitable for at least isolated lightning production. 
The threat should diminish markedly after dark with surface 
diabatic stabilization and resultant loss of both cape and dcape. 

.Edwards/Dean.. 10/22/2017 


Mesoscale Discussion

acus11 kwns 221041 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 221041 

Mesoscale discussion 1734 
National Weather Service Storm Prediction Center Norman OK 
0541 am CDT sun Oct 22 2017 

Areas affected...southeast la...southern MS 

Concerning...severe potential...watch unlikely 

Valid 221041z - 221245z 

Probability of watch issuance...5 percent 

Summary...persistent thunderstorm clusters will pose a threat for 
locally damaging wind and/or a brief tornado this morning. 

Discussion...at 1030z, a very persistent thunderstorm cluster 
continues to impact portions of southeast la and adjacent southwest 
MS. Broad rotation has occasionally been noted with individual cells 
within this cluster, and some eastward propagation has recently been 
noted as cells interact and continually feed into the cluster from 
the south. This activity is being sustained within a region of rich 
moisture and broadly confluent low-level flow, possibly being aided 
by divergent upper-level flow downstream of an amplified upper 
trough over the central Continental U.S.. 

While midlevel flow is modest at best across the region, low-level 
easterly/southeasterly flow veering to southwesterly aloft is 
resulting in sufficient effective shear/helicity for organized storm 
structures. Locally damaging winds and a brief tornado will be 
possible with the aforementioned long-lived thunderstorm cluster, 
and also potentially with ongoing storms further south across 
southeast la, where weak rotation has occasionally been noted with 
the strongest cells. 

Watch issuance is not currently anticipated, given the marginal and 
relatively localized nature of the threat. 

.Dean/Edwards.. 10/22/2017 

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


Latitude...Lon 29609211 30289185 31109132 31399069 31428929 30828865 
30428852 30038926 29468932 29168903 29019016 29289151