Valid 241300Z – 251200Z
…NO SEVERE THUNDERSTORM AREAS FORECAST…
…SUMMARY…
Isolated thunderstorms may occur across parts of the northeastern
U.S., southeastern Florida and the coastal Pacific Northwest.
…Synopsis…
A progressive mid/upper-level shortwave pattern will persist over
most of the CONUS through the period, while a cut-off cyclone drifts
southward over the Pacific, well west of Baja. Moisture-channel
imagery indicated a leading vorticity max over the eastern WV
Panhandle, with troughing southwestward across the western Carolinas
and GA. This perturbation is forecast to move offshore most of the
Atlantic coast by 00Z, except for a 500-mb low over southern New
England by that time. The trough then will deamplify considerably
and eject northeastward across the Canadian Maritimes overnight. By
12Z, shortwave troughs now over the Canadian Rockies and south of
the Gulf of Alaska will reach the Dakotas and southern portions of
ID/OR, respectively. In response to the latter, isolated thunder
may be noted this evening over parts of the coastal Pacific
Northwest, as cooling aloft occurs over the moist marine layer.
At the surface, the 11Z analysis showed a low over NJ, with occluded
front arching southeastward over the Atlantic to a triple point well
south of Long Island. The cold front extended from there across
central FL to the east-central/south-central Gulf. Isolated
thunderstorms may develop along/ahead of this front near parts of
the southeast FL coast, where low-level lift will be relatively
maximized compared to farther southwest in veered warm-sector flow,
and low-level theta-e advection with diurnal boundary-layer heating
may combine to overcome a strong morning inversion near 700 mb. The
front should proceed southeastward overnight, clearing all of FL by
the end of the period.
…Northeast…
Isolated to widely scattered thunderstorms are possible through
early/mid afternoon. As the deep-layer cyclone moves northeastward,
it will be preceded by a supportive conveyor of large-scale lift,
mainly warm advection with associated moistening, and related
steepening of midtropospheric lapse rates, rooted above a stable
boundary layer. Modified RAOBs and forecast soundings suggest
elevated MUCAPE 200-800 J/kg is possible. Locally/briefly higher
values are progged over southeastern New England, but confidence is
low in substantial elevated buoyancy, given the extensive precip
area that will act to prevent major midlevel destabilization. The
surface-based warm sector should remain offshore throughout the
episode, which will end from southwest to northeast during the
afternoon.
..Edwards/Kerr.. 11/24/2019
$$