SPC 1300Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0650 AM CST Wed Dec 23 2020

Valid 231300Z - 241200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE CENTRAL
GULF COAST REGION...

...SUMMARY...
A threat for damaging gusts and a few tornadoes will shift eastward
across the central Gulf Coast region, this afternoon through
tonight.

...Synopsis...
The dominant upper-air feature over the CONUS will remain a high-
amplitude, progressive, synoptic-scale trough -- initially apparent
in moisture-channel imagery from MB south-southwestward across the
central High Plains to Chihuahua.  Reinforced by several shortwaves
and vorticity maxima, the associated cyclonic-flow field will shift
westward and expand through the period, covering most of the land
between the Rockies and Appalachians by 00Z.  By then, the major
trough should extend from MN-NM, where it will be reshaped by a
strong, basal shortwave perturbation now digging southeastward over
UT.  By 12Z, the synoptic trough should extend from the upper
Mississippi Valley across MO to central/southwest TX.

At the surface, a primary low was analyzed over southern MN.  A cold
front extended from there over western IA, eastern KS, western OK,
the TX Panhandle, and central NM.  The low should reach northern WI
or western Upper MI by 00Z and begin occluding, with cold front
extending across southern IL, northeastern AR, and east to south-
central TX.  By 12Z, with the occluded low over eastern Lake
Superior, the cold front should be sweeping eastward to
southeastward near an axis from DTW-JKL-MOB and across the
northwestern Gulf.  A secondary surface low may develop along the
cold front tonight, over parts of MS and AL, as the basal mid/upper-
level shortwave perturbation pivots eastward over central TX. 

...Central Gulf Coast and vicinity...
Widely scattered to scattered thunderstorms should form by mid/late
afternoon and expand in coverage this evening, in two primary early
episodes:
1.  A cold-frontal band across parts of AR and northeast/east TX,
shifting eastward across AR/LA. This activity initially will offer a
marginal severe threat as it impinges on a boundary layer somewhat
destabilized by both muted diurnal heating and synoptically forced
theta-e advection.  
2.  A nearly front-parallel but warm-sector low-level convergence
zone from southeast TX to northern LA, likewise shifting eastward
but not as quickly as the front.  As such, the first should overtake
the second tonight, probably across portions of MS to southern LA,
forming a single primary thunderstorm band.

The combined, primary thunderstorm band, in turn, should shift
eastward along or just ahead of the front over southeastern LA, MS,
and portions of AL and the western FL Panhandle by 12Z.  Late
tonight, a few thunderstorms also may move ashore in the western FL
Panhandle within a newer prefrontal convergence zone developing over
the adjoining Gulf.  

Damaging gusts will be possible throughout the outlook area, along
with at least a marginal supercellular and/or QLCS tornado threat
for most of the distance inland.  The greatest juxtaposition of a
favorably modified moist boundary layer with strengthening ambient
shear and low-level hodographs will be across the "slight" area this
evening and tonight.  Surface dew points in the low/mid 60s F --
apparent in the partly modified warm sector over the northwestern
Gulf now, should spread inland as far north as southern AR this
afternoon and southern portions of MS/AL tonight, offsetting
marginal lapse rates enough to yield around 300-800 J/kg MLCAPE. 
Meanwhile the approaching trough and its mass response will lengthen
the deep-shear vector and enlarge hodographs, with effective
shear/SRH values around 45-50 kt and 150-300 J/kg respectively,
where sustained surface-based inflow appears most probable.  

Already modest boundary-layer lapse rates and low-level CAPE will
diminish with northward/inland extent across the central Gulf Coast
region, but isolated severe gusts and/or a tornado cannot be ruled
out even near the north edges of the "marginal" area where MLCAPE
less than 200 J/kg is expected.  The blended severe regime should
reach the western FL Panhandle by 12Z, then continue into day-2 from
there eastward and southeastward across parts of FL/GA.

..Edwards/Smith.. 12/23/2020

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