Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 290135

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
835 PM EST Sat Nov 28 2020

High pressure will build across the region for the rest of the
weekend. A strong storm system will impact our region late
Sunday night through early Tuesday. High pressure builds back
into our region from the south through the middle part of next


High pressure will settle overhead late tonight. With mainly
clear skies and light winds, this will provide a good setup for
radiational cooling. Leaned toward the chillier guidance for
lows tonight. Min temps will range from the upper 20s and lower
30s for most areas, to the the upper 30/lower 40s in downtown
Washington and Baltimore.

Sunday stays nice, but with clouds increasing during the
afternoon, high temperatures will be a couple degrees cooler
east of the Blue Ridge. The pattern favors warmer temperatures
out west, with Petersburg WV possibly exceeding 60 degrees.
Definitely Sunday will be less breezy than today.


A deepening surface low will track from the Central Gulf Coast
up the spine of the Appalachians Sunday night and Monday.
Widespread rain will move into the area, mainly after midnight
Sunday night. The heaviest rain is likely just ahead of and with
the passage of the surface low on Monday morning.

The most likely rainfall is expected to be 1-2 inches, most of
which falls in 6-9 hours. Can`t rule out some totals over two
inches and some isolated flooding, especially just east of the
Blue Ridge. Confidence is too low for a Flood Watch at this
time and stream guidance indicates no expected flooding
presently. The pressure gradient associated with the deepening
surface low will also lead to increased winds, with gusts up to
25 mph expected. Closer to the bay the winds may be even higher
for a time, sustained at near 20 mph with gusts up to 30-40 mph.

The other issue, as the low passes, is a surge of unseasonably
warm air (borderline hot for the end of November -- would not
be surprised to see places hit 70 degrees) with the passage of
a warm front. Very high shear and some modest CAPE, along with
the added instability from the higher low-level temps, will
promote some warm sector shower/storm development, especially
during the late morning and early afternoon hours. The strongest
of these could be capable of damaging winds, and an isolated
tornado cannot be ruled out either.

In the wake of this low pressure, an upper low will be centered
over the Mountain State (West Virginia) on Monday night, with
low-level flow promoting upslope snow west of the Allegheny
Front (continuing into Tuesday). Upped snow totals in the
upslope region slightly: 1-4 in Garrett, 1-2 in far western
Grant/Pendleton, and less than 1 elsewhere. Some guidance also
hints at precip further east with trailing upper energy Monday
night. Although I`m not yet sold on it, I`ve seen it happen
before in this setup, and will hold on to at least slight chance
PoPs all night.


An occluded surface low will be positioned near our region over the
northern PA/southern NY border on Tuesday and will move slowly
northeastward away from our region throughout the day. The
positioning of this low will lead to a general west to northwesterly
flow which will bring wrap around precip into our western upslope
region. The upslope flow will likely enhance the the precipitation
and combined with the near to below freezing temperatures to
generate show showers along the Allegheny Front. The threat for snow
showers should taper off Wednesday morning as the low to our north
shifts further away from our region. The pressure surge behind the
front low will lead to continued gusty winds with gusts of 20 to 25s
knots possible through the evening periods on Tuesday.

A general cooling trend is expected though the middle parts of this
upcoming week as a cooler air mass slowly moves into our region
behind the exiting low to our north. Daytime highs will run in the
40s both Tuesday and Wednesday with overnight lows in the 20s. The
surface low will be slow to exit the NE US and gusty winds will
continue to be possible through Thursday morning. High pressure will
build into our region from the south on Thursday and remain situated
over our area through early Friday. High temperatures will trend up
into the upper 40s to lower 50s on Thursday with overnight lows in
the upper 20s to 30s.

Another low pressure system is forecast to impact our region toward
the end of this upcoming week. Models are indicating another
occluded low pressure system that rides northward along the western
Appalachians on Friday and into our region on Saturday. Another
period of light to moderate rain will be possible once again for our
region but there remains a lot of uncertainty at this time.


The valid TAF period (through Sunday evening) should be quiet.

Rain spreads into CHO around midnight and the other terminals
after midnight on Sunday night, and continues through at least
midday Monday. Significant ceiling and visibility restrictions
are likely: MVFR for sure and IFR most likely. Wind gusts of
20-25 knots are possible. The cig/vsby restrictions will break
out in the afternoon with the passage of the low/warm front, but
that comes with potential at the major airports and MTN of
strong thunderstorms with further gusty winds and brief cig/vsby
restrictions. Could see some ceiling restrictions on Monday
night with showers and clouds given proximity to an upper level

With precipitation ending across area terminals, VFR conditions
are expected on Tuesday and Wednesday with prevailing westerly
winds each day. Gusts of 15 to 20 mph are possible at times on


High pressure overhead will allow for winds to remain below SCA
criteria through Sunday. The flow will turn southerly on Sunday.

Winds pick back up Sunday night ahead of an approaching
strengthening storm system. SCA-level winds are likely in the
open waters of the lower Maryland Bay and Tangier Sound by
daybreak Monday, with winds further increasing during the
morning as the strengthening low passes to the west. Gale force
gusts are possible over parts of the waters on Monday, but being
48 hours away, no headlines are yet raised. Strong thunderstorms
are possible mainly Monday afternoon, which could prompt Special
Marine Warnings for winds and perhaps for a waterspout.

Winds will subside Monday night as the low pulls away.

SCA conditions likely on Tuesday with Small Craft conditions
possible once again on Wednesday.


Onshore flow will increase Monday as a strong low pressure moves
up the Appalachians. Current anomaly in the tidal zone is
pretty consistently around 3/4 foot, and that will obviously
increase with the southerly winds, to at least 1.5 to 2.0 feet.
Although the ETSS and ESTOFS are in reasonable agreement, I
leaned toward the higher GMU tidal guidance given the flow
pattern. This yields minor tidal flooding at Annapolis,
Baltimore, Bowley`s Quarters, St. George Island, and Dahlgren.
Too early for issuance of any advisories at this point.

As has been noted by previous forecasters, some of the ensemble
guidance (Stevens System and experimental P-ETSS) have even more
significant flood potential -- maybe moderate at Baltimore and
Bowley`s. Given the quick nature of the surge, and the
potential that the highest surge could occur at low tide, the
GMU guidance solution seems most reasonable at this point.




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