Day 1 Severe Weather Outlook

Valid 291300Z – 301200Z


The most concentrated severe threat (thunderstorm wind and a couple
tornadoes possible) appears to be this afternoon and evening from
northeastern Louisiana and central Mississippi to northwestern
Alabama and middle Tennessee.

Moisture-channel imagery depicts a large cyclone covering most of
the Great Plains and central/southern Rockies regions, with a
primary circulation center in the eastern part of the broader
vortex, over eastern NE. The associated 500-mb low should deepen
and move to southwestern MN by 00Z, then oscillate roughly eastward
near the MN/IA border through tomorrow morning. Meanwhile, a strong
shortwave — initially apparent over southeastern NM and far west TX
— will intensify and pivot through the southern semicircle of the
larger gyre. By 00Z, the shortwave trough should reach western AR
and the Arklatex region, accelerating to southern Lower MI by 12Z.

Meanwhile, a compact cyclone — now offshore from Vancouver Island
— will weaken while the southern part of the associated
perturbation forms another low. This second, also compact cyclone
will dig south-southeastward just offshore from OR and northern/
central CA through the period. Its large-scale lift and
destabilization will contribute to general thunderstorm potential
near the CA coast this evening and tonight.

At the surface, 11Z analysis showed the main synoptic low over
northwestern IA, with an occluded front southeastward to
northeastern IL, and a cold front across western KY, southeastern
AR, north-central LA, and the middle/upper TX coastal plain. The
initial/occluded synoptic low will weaken over the upper Mississippi
Valley through most of the period, while the low-level isallobaric/
mass responses become driven more by the southern/basal shortwave
trough. The front may become quasistationary across northern LA and
the Mid-South for several hours as that perturbation approaches.
Meanwhile, a secondary surface low should develop along the cold
front by early afternoon over the northern LA/extreme west-central
MS area.

The low should strengthen and ripple northeastward along the
boundary, and across western/middle TN between MEM-BNA by 00Z. The
cold front then should extend southwestward across north-central
through southwestern MS to near BTR, then across the northwestern/
west-central Gulf. Overnight the low will deepen greatly, becoming
the center of a well-organized synoptic surface cyclone over lower
MI during the last few hours of the period. By 12Z, the cold front
should arc from there across Lake Erie, eastern OH/western PA, WV,
western NC, central GA, and the western/central FL Panhandle.

…Southeastern States…
Scattered thunderstorms are possible along and ahead of the cold
front, initially this afternoon from the lower Mississippi Valley
into the eastern Mid-South, spreading/developing northeastward
across the severe-outlook areas through tonight. The potential for
damaging to severe gusts and a couple tornadoes now appears most
concentrated from central MS to middle TN later this afternoon
through this evening.

The main severe threat should arise from a spatial composite of two
regimes, which will encounter the most favorable parameter space
over portions of the “slight risk” area, separated by a few hours.
First, a well-organized band of convection is expected to develop
near the cold front over parts of northern LA and northern/central
MS and move/expand northeastward, evolving into a QLCS. This
convection will impinge on a narrow warm sector characterized by
strong deep-layer winds/shear and modest yet surface-based buoyancy
typically associated with cool-season events in the interior South.
Surface dew points generally in the 60s F, and boundary-layer
destabilization mainly from theta-e advection, will contribute to
300-800 J/kg prefrontal MLCAPE from the lower Delta region into
middle TN. 50-70-kt effective-shear magnitudes are possible over
the same area beneath very strong mid/upper flow. Hodograph
curvature/size may vary greatly, but effective SRH 200-400 J/kg is
possible, especially over eastern parts of the “slight risk” area.

To its east, a separate area of convection should develop in the
zone of low-level warm advection, moisture transport, and weak
MLCINH characterizing the eastern rim of the surface-based warm
sector, initially near the MS/AL border. This activity should move
roughly north-northeastward, offering some severe threat until it
outruns already marginal boundary-layer instability in the northern
AL/middle TN area. Then the QLCS is expected to move over the same
region. The southern extent of the severe threat will be somewhat
diffuse, but marked by a combination of weakening frontal/prefrontal
lift and stronger MLCINH with southward extent, as per the stout
inversion near 700 mb sampled by the 12Z LIX sounding.

..Edwards/Bunting.. 12/29/2019


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