Day 1 Severe Weather Outlook

Valid 301300Z – 311200Z


A few thunderstorms may impact portions of the Mid-Atlantic region
Monday afternoon, with a localized, low-end severe-weather risk.

In mid/upper levels, the synoptic pattern will be characterized by
two main features:
1. Split flow around a compact cyclone digging south-southeastward
offshore from southern CA and northern Baja, turning eastward over
northern Baja around the end of the period.
2. A broad, slowly filling synoptic cyclone crossing the upper
Great Lakes, from its current position over the upper Mississippi

At the surface, 11Z analysis showed an occluded low over central
Lower MI, with occluded front to a temporary triple point near CLE.
A cold front extended from there across WV, to near ROA. The cold
front extended southwestward from there across the western
Carolinas, southern GA, and the eastern FL Panhandle. A wavy warm
front was drawn across northeastern WV, northern VA, north-central
MD, northern DE, and southern NJ, with slow northward motion
possible before the cold front overtakes it. A broad area of low
pressure was evident from near the warm front over northern VA
southwestward to the southwestern VA cold-frontal segment.

The MI low will move northwestward to eastern Lake Superior today
and become vertically stacked with the mid/upper low, before
filling. Another low will consolidate by midday near the eastward-
propagating frontal triple point over MD, becoming better-defined
and shifting eastward across and offshore from NJ through 00Z. By
that time the cold front should extend from the low to near the NC
Outer Banks, northern FL and the eastern Gulf. By 12Z, the low
should move to near Cape Cod/Nantucket, with the cold front having
cleared all the Atlantic Coast except perhaps the FL Keys.

Isolated thunderstorms may form in a marginally unstable environment
from late morning into afternoon, offering the potential for
damaging/briefly severe gusts east of the cold front and south of
the low. If convective residence time in the warm sector is long
enough for a mature cell, a brief tornado cannot be ruled out, but
this threat is very marginal/conditional.

Theta-e advection will destabilize a narrow corridor of warm sector
near the coast, and south of the low, prior to cold frontal passage.
This will enable surface-based inflow parcels from midday into
mid-afternoon, prior to cold frontal passage, with forecast
soundings suggesting MLCAPE in the 300-800 J/kg range. Strong speed
shear will be present, with deep-layer flow largely parallel to the
cold front. This will enable effective-shear magnitudes of 45-55
kt, and some backing of flow with height in about the 850-700-mb
layer. Backed near-surface winds are possible along the warm front,
but instability should be very weak. Low-level convergence and
large-scale lift will decrease with southward extent from the
vicinity of the triple point, so the potential for sustained/
organized convection is in doubt south of about Chesapeake Bay. The
bulk of convection may be elevated, north of the triple point, in
the conveyor of warm advection and frontal forcing located above the
boundary layer.

..Edwards/Bunting.. 12/30/2019


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