Day 1 Severe Weather Outlook

Valid 091300Z – 101200Z


Severe thunderstorms are unlikely today and tonight.

In mid/upper levels, the large-scale pattern over North America will
continue to be dominated by a high-amplitude mean ridge from
northwestern Mexico to northwestern Canada and the adjoining
Beaufort Sea. Meanwhile, a positively tilted mean trough will
persist from a Hudson Bay vortex across the upper Great Lakes,
south-central Plains, and north-central/northwestern MX. A series
of shortwave troughs will traverse the associated cyclonic-flow
field through the period, the most notable of which is evident in
moisture-channel imagery over southern CA. This perturbation will
deamplify somewhat as it moves east-southeastward across AZ, Sonora,
NM, and northern Chihuahua through the period. By 12Z, this
perturbation should reach west TX and the TX South Plains region,
anchoring an extensive fetch of southwest flow that will cover much
of the Gulf Coast and eastern CONUS.

Farther north, another prominent shortwave trough is apparent over
the Dakotas and northwestern NE. This feature should pivot across
the upper Mississippi Valley today, then weaken as it ejects
northeastward over the upper Great Lakes overnight. The associated
surface cyclone — analyzed at 11z over eastern IA — will eject
northeastward over northern parts of Lower MI and Lake Huron through
00Z, then accelerate across adjoining parts of ON and QC south
through east of James Bay. The trailing cold front was drawn across
northwestern MO, eastern KS, west-central OK, the northern TX
Panhandle, and northeastern NM. By 00Z this front should reach
southern ON, OH, middle TN, northern MS, and south TX. By 12Z the
front should extend across northern New England, central/eastern PA,
the southern Appalachians, and southern MS, then across the
northwestern Gulf to near or just south of BRO.

Isolated to widely scattered thunderstorms will continue to develop
and shift generally eastward across this region into this evening,
supported by cooling/destabilization aloft overlapping marginal
low-level moisture/theta-e. Favorable buoyancy should continue with
MUCAPE 300-800 J/kg over portions of AZ, and MLCAPE near 500 J/kg
developing today as pockets of sustained surface heating underlie
cooling aloft. Weakness of both low/middle-level winds and shear
contraindicates assigning a severe-risk area.

A band of low-level warm advection and moisture transport, with
increasing moisture content in low/middle levels, is forecast
tonight across parts of west TX. While isolated thunder cannot be
ruled out in this band, weak lapse rates/buoyancy cast considerable
uncertainty on coverage, which still appears most probable at less
than 10% over most of the regime.

…Gulf Coast to lower Great Lakes…
Isolated, perhaps widely scattered thunderstorms are possible within
a plume of low-level warm advection and moisture transport covering
the prefrontal (warm) sector, as well as in the zone of frontal
forcing located along and just behind the surface boundary. This
afternoon into tonight, somewhat steeper lapse rates aloft in the
northern area will support MUCAPE in the 200-800 J/kg range, some of
which will extend into icing layers suitable for lightning

Meanwhile, in the southern area, increasing moisture/instability in
low levels will offset weak mid/upper lapse rates enough to yield
areas of 500-1000 J/kg MUCAPE. Forecast soundings also suggest some
MLCAPE near 500 J/kg with surface-based effective-inflow parcels
this evening and into the overnight hours over parts of the
Mid-South and Delta regions, with enough deep shear (effective-shear
magnitudes potentially exceeding 50 kt) to raise conditional concern
for severe potential. However, lack of stronger prefrontal lift, in
a regime of some stable layers and capping above the boundary layer,
suggest convective coverage/strength will be in doubt.

A relative minimum in thunder potential still is evident across the
gap between the two general/10%-coverage outlook areas. This
doesn’t mean no thunder will occur — indeed, isolated lightning
cannot be ruled out anywhere in the plume. Instead, the relative
displacement of the area from stronger deep-layer forcing and
mid/upper cooling to the north, and greater low-level moisture/
theta-e to the south, suggest more uncertainty and lower coverage/
duration for any thunderstorm(s) that do occur.

..Edwards/Broyles.. 12/09/2019


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