SPC AC 031249
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0649 AM CST Sat Feb 03 2018
Valid 031300Z - 041200Z
...NO SEVERE THUNDERSTORM AREAS FORECAST...
...SUMMARY...
Thunderstorms are expected across the lower Mississippi Valley and
adjoining Gulf Coast region, mainly this evening and overnight.
...Synopsis...
In mid/upper levels, a longwave trough over eastern North America
will continue broadly cyclonic flow from the Rockies to the East
Coast. Two primary/influential shortwave perturbations currently
are embedded within the associated northwesterly flow field:
1. Leading trough now located from the Black Hills across
eastern/southern WY, northwestern CO and eastern UT. The northern
part will merge with a weaker/upstream vorticity lobe now over the
Canadian Rockies, then split eastward across the lower Missouri
Valley region tonight. By 12Z, the combined northern lobe will
extend from northern IL across the Ozarks, amidst height falls
related to the trailing/upstream trough #2 below. Meanwhile, the
southern lobe will dig southeastward to the southern High Plains by
00Z, then turn eastward, become much more positively tilted, and
weaken considerably tonight. By 12Z, this lobe should be located
from western MS to south-central TX.
2. Upstream trough and related upper speed max, now evident in
moisture-channel imagery over central SK, northern AB and northern
BC. The leading, strongest part of the related vorticity field
should move southeastward to southern MN and SD by 12Z.
At the surface, a large, continental/polar anticyclone covers the
southeastern 1/3 of the nation, with ridging extending to
northeastern MX. The associated high will move offshore VA today
while the ridge retreats eastward over the Gulf Coast and onshore
flow from the western Gulf gradually becomes less anticyclonic. A
wavy frontal zone -- currently warm in some segments and cold in
others, but slow-moving everywhere -- extends between weak lows over
eastern Upper MI and western SD, arching across southern MN in
between. The front continues northwestward from SD through another
weak low over central MT to the Canadian Rockies. As the northern
part of perturbation #1 advances, the SD low will move/consolidate
east-southeastward to IA by 00Z, with the front accelerating
southward across NE and northeastern WY behind it. By the end of
the period, the low should reach southern lower MI, with cold front
to western IL, the KS/OK border region, and northeastern CO. A
prefrontal trough and confluence zone should develop tonight over
the warm sector (such as it is, with a very immature return-flow
regime at the surface), reaching parts of mid TN, northern AL and MS
by 12Z.
...Lower Mississippi Valley and adjoining Gulf Coast region...
Isolated to scattered thunderstorms are possible, with episodes of
convection shifting generally eastward over the outlook area. The
greatest convective coverage should be from this evening through
overnight hours, shifting from LA/AR across parts of MS/AL.
The prefrontal surface trough will have little direct influence on
convective potential, given the lingering stable/low-thetae air from
the antecedent anticyclone. However, it will be a manifestation or
symptom of the intensifying low-level warm advection and convergence
underway tonight above the surface, ahead of the southern branch of
mid/upper trough #1. As the perturbation approaches, a 50-65-kt
southwesterly LLJ (locally exceeding 70 kt) will support that
regime, along with enough moisture transport to yield nearly
saturated parcels in the 800-900 mb layer near the coast, becoming
more elevated northward. Forecast soundings suggest MUCAPE 300-700
J/kg over the middle/southern parts of the outlook area, diminishing
northward. The stable boundary layer, weak lapse rates aloft, and
the lack of both greater buoyancy and effective shear, should
combine to keep severe potential too low for unconditional
probabilities.
..Edwards.. 02/03/2018
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