Day 1 Severe Weather Outlook

Valid 161300Z – 171200Z


Organized severe-thunderstorm potential appears minimal over the
U.S. today and tonight.

In mid/upper levels, a progressive synoptic pattern will exist this
period, featuring eastern troughing, ridging over the rockies, and
height falls across the Pacific Coast. The latter will be related
to a strong trough — now apparent in moisture-channel imagery over
the northeastern Pacific between 135W-140W. Once the trough moves
inland this evening, associated cooling in midlevels will steepen
lapse rates above a marine air mass, leading to buoyancy of marginal
magnitude and depth for isolated lightning production from
associated convection moving ashore.

Downstream, a well-defined mid/upper cyclone is evident over the
upper Great Lakes, with trough south-southwestward across the
Mid-South region. The cyclone should shift eastward to New England
and deepen by 12Z, while the trough pivots across the southern
Appalachians, then offshore from the Mid-Atlantic region. A
southern-stream perturbation — initially located between AMA-LBB —
should move slowly southward and become a small, closed cyclone,
increasingly cut off from the prevailing westerlies by the
eastward-moving synoptic ridge.

At the surface, 11Z analysis showed a deeply occluded low,
corresponding to the mid/upper cyclone, over northern Lake Huron.
The occluded front arched eastward then southward across the central
Appalachians, becoming a cold front over the southern Appalachians
to central MS, central LA and south-central TX. A diffuse warm
front extended east-northeast/west-southwest across eastern NC and
central SC, amidst a large area of precip. A weak low — initially
drawn along the warm front near CAE — will deepen gradually through
the day and move northeastward across the Delmarva Peninsula region,
while being overtaken by the cold front. By 00Z, the low should be
near coastal NJ, with cold front just offshore from the Carolinas
and across parts of southern GA and the FL Panhandle.

…Coastal NC…
A low-CAPE/strong-shear parameter space, marginally favorable for
supercells, may brush the immediate coastal areas from ILM
northeastward across the southern Outer Banks and past HSE through
early/mid afternoon. Northward/inland extent of favorable
destabilization is uncertain, given extensive precip evident over
the region and even offshore. As the surface low deepens to the
north, a few hours of relatively backed surface winds and higher
theta-e are possible over the sounds/banks amidst suitable
isallobaric forcing. However, the most favorably buoyant air, and
the great majority of surface-based deep convection, will remain
offshore. Weak deep-layer lapse rates will prevail, supporting only
around 500-700 J/kg MLCAPE over the beaches, increasing with
southward/offshore extent across the Gulf Stream. For now, the
severe potential appears too conditional, isolated, and limited in
coverage to warrant an unconditional outlook area.

..Edwards/Goss.. 10/16/2019


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