Finally posted
DISCUSSION...
KEY MESSAGE 1: Severe t-storm risk confined to southeast
quadrant of CPA through early evening
Focus for severe t-storms appears confined to the southeast
quadrant of the CWA over the Lower Susquehanna Valley. This
area is on the northern end of linear convective line evolving
eastward from DCA/BWI. Within a deeply meridional wind profile,
damaging winds and embedded tornadoes will be the main hazards.
Latest WOFS is more bullish on severe risk (both wind and tor
probs) to the south of the MD line toward Chesapeake Bay. We
plan to trim the Tornado Watch #68 based on the latest SPC watch
status report.
KEY MESSAGE 2: Quick burst of moderate to heavy snow and
sharply falling temperatures to bring slippery travel across
the Alleghenies this evening into early tonight
Well-defined surface/arctic cold front is on the western
doorstep at 18-19Z. Observed 1hr temp drops are on the order of
10-15 degrees along with wind shift to the west. In wake of
this FROPA, temperatures will continue to plummet due to strong
CAA, while mid and upper level flow remains southerly in
response to the strongly tilted and digging upper trough.
Combined with upslope enhancement, this will cause precipitation
to rapidly transition to snow from the Laurel Highlands across
the western and central Alleghenies through the evening commute.
While some snow accumulation may be initially limited to non-
paved surfaces particularly at lower elevations, the higher
elevations will see a two-part winter wx event consisting of
front-end "thump" followed by lake-enhanced upslope flow. A
brief period of snowfall rates surpassing 1"/hr (>60% chance) is
expected btwn roughly 00-04UTC where a quick 1-4" of snow is
fcst. Typically favored locations +2,000ft elevation should
continue to see additional minor accums into Tuesday with local
amounts 4+ possible.
Pressure rises and steeper lapse rates/colder air aloft will
maintain gusty winds with max gusts approaching advisory levels.
KEY MESSAGE 3: Much colder with a couple of locally heavy snow
showers possible Tuesday
As the trailing vorticity maxima and secondary shortwave
embedded within the trough over the OH Valley moves east Tuesday
afternoon, convective cellular snow showers and isolated snow
squalls could develop across the area. Additional snowfall
accumulations will be light with any of these convective
elements, but briefly heavy snow rates and gusty winds could
cause local travel impacts due to rapid reductions in
visibility. A SPS may be needed due to the broad coverage of
locally heavy snow showers.