U.S. Severe Weather Forecast (Convective Outlook)

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acus01 kwns 211300 
swody1 
Storm Prediction Center ac 211258 


Day 1 convective outlook 
National Weather Service Storm Prediction Center Norman OK 
0758 am CDT Mon Oct 21 2019 


Valid 211300z - 221200z 


..there is a slight risk of severe thunderstorms from the lower 
Mississippi Valley to parts of Alabama and the Florida Panhandle... 


... 
The severe-weather threat will shift eastward today through this 
evening from the lower Mississippi Valley to parts of Alabama and 
the Florida Panhandle. Damaging wind gusts and a few tornadoes are 
possible. 


... 
In mid/upper levels, a large area of cyclonic flow covers the 
western/central Continental U.S. To the Ohio/Tennessee valleys, anchored by a 
strong, stacked, occluded cyclone centered near Yukon. The associated 
500-mb low will pivot northeastward across the upper Midwest through 
the period. A speed Max and related vorticity Banner now behind the 
height trough -- over the central rockies -- will pivot through the 
trough and reach the mid-south region by 12z. This will lessen the 
synoptic trough's positive tilt and spread height falls across most 
of the eastern conus, aside from ridging crossing New England 
tonight. 


At the surface, a warm front was drawn across central Florida and the 
northeastern Gulf to near mob and Jan, northward to where it was 
being overtaken by a squall line in western Tennessee. The convection will 
continue to overtake the warm front across western Tennessee and northern 
MS through the afternoon. Meanwhile, a cold front -- initially over 
northern Arkansas to near txk and Sat -- will move southeastward across 
most of the rest of Texas and the lower Mississippi Valley by 00z. 
Weak frontal-wave cyclogenesis is possible late in the period over 
portions of eastern Alabama or GA, with a resultant low over eastern Georgia 
or SC by the end of the period. By then, the cold front should 
extend from that secondary low southwestward across southwestern GA, 
the western Florida Panhandle, and the north-central/west-central Gulf. 


... 
Through the remainder of the morning, a band of thunderstorms should 
continue to outpace the slow eastward shift of suitable warm-sector 
air, from northwest-southeast across the mid-south region, and into 
parts of the Tennessee Valley. Isolated severe gusts are possible 
in the meantime. For near-term details, refer to remaining portions 
of Storm Prediction Center watches 681 and 682, and related mesoscale discussions. 


Additional scattered thunderstorms should pose a risk of damaging 
gusts today and tonight, along with a few tornadoes closer to the 
Gulf Coast from portions of southern la to the Florida Panhandle. Some 
spatial and timing uncertainty exists on onset of greatest severe 
potential over the southern la area, because of an expansive shield 
of clouds/precip related to an mesoscale convective system offshore from the Texas/western la 
coast. Ultimately, the near-frontal convective band should overtake 
the remnants of the mesoscale convective system. The merged area of convection may 
intensify as it proceeds eastward near the coast, the inland portion 
encountering a destabilizing and favorably moist air mass. 
Additional storms, perhaps including a few supercells, may develop 
in a weakly capped, moisture-rich warm sector near the coast. 
Damaging winds and a few tornadoes are possible, given the very 
moist environment with strengthening vertical shear. 


Forecast soundings suggest that the preconvective environment will 
destabilize mainly from Theta-E advection, with some diurnal 
warming. This will offset modest mid/upper-level lapse rates enough 
to yield warm-sector/preconvective MLCAPE 1000-2000 j/kg near the 
coast, decreasing inland to around 300-800 j/kg over central 
sections of MS/al. Low-level mass response to the synoptic 
troughing will lead to enlarged hodographs with effective srh 
commonly in the 150-400 j/kg range within about 100 nm of the 
central Gulf Coast from late afternoon into tonight, where buoyancy 
should be greatest. Any relatively discrete cells sustained in this 
environment will pose a tornado threat, in addition to localized 
damaging wind, with better-organized wind threat from any Gulf 
mesoscale convective system/antecedent qlcs combination. 


.Edwards/Smith.. 10/21/2019 


$$ 

Mesoscale Discussion


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acus11 kwns 211400 
sels mesoscale discussion 
Storm Prediction Center mesoscale discussion 211359 
msz000-laz000-211530- 


Mesoscale discussion 2102 
National Weather Service Storm Prediction Center Norman OK 
0859 am CDT Mon Oct 21 2019 


Areas affected...northeast Louisiana through western and central 
Mississippi 


Concerning...Severe Thunderstorm Watch 682... 


Valid 211359z - 211530z 


The severe weather threat for Severe Thunderstorm Watch 682 
continues. 


Summary...threat for mainly isolated damaging wind is expected to 
persist another couple hours from northeast la through western and 
north central MS, followed by a diminishing trend. Storms are 
approaching the eastern bound of ww 682. While, the spatial and 
temporal extent of the threat appears too limited for a downstream 
watch, ww 682 can be extended in area as needed. 


Discussion...this morning an expansive squall line extends from 
northern MS southwest into northeast la moving east at 30 kt. Storms 
are moving through a narrow corridor of marginal instability with 
500-1000 j/kg MLCAPE along with 40-50 kt effective bulk shear. This 
environment should support maintenance of the squall line next 
couple hours. However, the downstream thermodynamic environment is 
very marginal due to a stable boundary layer and dry air between 
900-800 mb as evidenced on the 12z Jan radiosonde observation. Modest destabilization 
associated with Theta-E advection along the southwesterly low-level 
jet may occur east of ongoing storms. However, widespread 
multi-layer clouds will significantly limit surface-based 
instability, suggesting the squall line should begin a gradual 
weakening trend with eastern extent into MS. 


.Dial.. 10/21/2019 


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


Attention...WFO...Meg...Jan...lix...lch...shv... 


Latitude...Lon 31519195 32529080 33888944 33618894 32478949 31449057 
30529221 30589290 31519195