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970
FXUS61 KLWX 211409
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1009 AM EDT Sat Sep 21 2024

.SYNOPSIS...
A frontal system will move into the area this afternoon before
stalling through Sunday. A strong front and area of low pressure
over the Great Lakes and Upper Midwest will approach the area during
the early or middle part of next week. High pressure will follow.

&&

.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --
Satellite imagery this morning shows a mix of mid-level clouds
near and west of the Blue Ridge, with continued low-level
clouds near and east of I- 95. Clouds should generally lift and
scatter this morning ahead of a shortwave approaching in NW flow
from the Great Lakes.

As this shortwave moves overhead, its attendant surface cold front
will pivot across the Mid-Atlantic this afternoon and evening. Lift
associated with this system should result in scattered to numerous
showers and thunderstorms mid to late afternoon generally west of US-
15 which will then propagate southeastward into this evening. Modest
lapse rates and shear aloft, and CAPE of 800-1600 J/kg should be
enough to organize storms at times into small clusters, lines, or
transient supercells with a risk for localized gusty/damaging winds
and hail. Although cold temperatures aloft could result in some
added updraft vigor, a lack of stronger shear and low-level flow as
may keep the severe risk more isolated/marginal. SPC has
maintained a Level 1 out of 5 (MRGL Risk) for much of the area
west of I-95. Mesoscale trends will be monitored closely for the
potential for more focused corridors. There is a low end
flood/flash flood risk given the approaching disturbances in the
NW flow interacting with the backdoor front and easterly flow.
Not exactly tall skinny CAPE and high PWs, so not a classic FF
setup, but model guidance has been hinting at local areas of
2-3" of rain. Will continue to monitor.

Stronger storms should tend to wane this evening, though a continued
risk for showers and a few rumbles of thunder look to linger through
the night as added lift from a passing shortwave moves atop a
backdoor cold front. This overnight precip may focus closer to the
metro areas.

After high temperatures in the low-mid 80s today (70s mountains),
lows will fall into the upper 50s to mid 60s tonight.
-- End Changed Discussion --

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
A backdoor front will stall over the area Sunday. To its southwest
(over the central Shenandoah Valley toward the Allegheny and Potomac
Highlands), it will be mainly sunny and warm. To the northeast of
the boundary across much of the rest of the area, cool, cloudy, and
perhaps drizzly conditions are expected in onshore flow.

The pattern doesn`t change much heading into Monday. Thus, expect
lingering cloud cover for much of the area along with muted diurnal
temperature spread.

Another trough and area of low pressure/frontal system begin to
approach from the OH Valley/Great Lakes by later Monday resulting in
increasing rain chances from west to east.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
The overall synoptic pattern looks to be active, and becomes very
uncertain as we get to the end of next week. At the surface, an area
of high pressure over far southeast Canada remains in control across
much of the Northeast to Mid-Atlantic. A prolonged CAD wedge remains
over the area through at least Thursday, bringing cooler conditions
and cloudy skies, especially east of the Blue Ridge. Highs only
reach the low to mid 70s, with Tuesday being the coolest as some
locals struggle to reach 70F north of I-66. Overnight lows mostly in
the upper 50s to low 60s. Expect lots of low clouds and fog each
morning. The Mid-Atlantic looks to be in a favorable spot where
numerous shortwave troughs move over the area prolonged period of
southerly , bringing high chances for showers and a few
thunderstorms Tuesday through Thursday.

The forecast becomes highly uncertain Thursday into Friday due to
major discrepancies in the global models. The GFS is the most robust
indicating a very amplified mid/upper-level pattern across the
eastern two thirds of the country. The mid-level trough to our west
cuts off into a large low over the Mid-South, with deep mid-level
ridging centered just offshore of the Carolinas. The ECMWF is much
less amplified, and does not have a strong mid-level ridge over the
Southeast. Similar patterns are seen in each of the ensemble suites.
Need to continue monitoring in the days ahead.

&&

.AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
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Scattered to numerous showers and thunderstorms are expected to
accompany a frontal system late this afternoon into this
evening. At this time, confidence is highest in TS near and west
of IAD. Winds will generally be S/SE 5-10 kts with a few gusts
to 15 kts possible.

Lower CIGs and/or patchy fog can be expected later tonight into
Sunday morning in the wake of a backdoor cold front. Additional
shower activity is possible near the metros through the night. Winds
will become easterly but remain light AOB 10 kts. Restrictions are
expected to persist much of Sunday into Monday as onshore flow
continues, with MVFR most likely during the day and IFR
overnight.

A prolonged period of sub-VFR conditions is likely to continue
Tuesday through the end of next week. Persistent low clouds and fog
each morning are likely at all terminals. Cloud ceilings rise a bit
during the afternoon, then drop back down each evening and
overnight. Passing showers Tuesday through Thursday also contribute
to aviation restrictions.
-- End Changed Discussion --

&&

.MARINE...
S/SE flow is expected through this evening ahead of an approaching
cold front. Winds will switch around to the E/NE behind the
(backdoor) front tonight through early next week. Showers and a few
thunderstorms are possible tonight as the front crosses. Overall,
winds look light, but there may be a period of SCA conditions behind
the backdoor front through Sunday.

Surface high pressure well north of the area brings steady southeast
winds around 10 knots, gusting 15 knots Tuesday, that become
southerly Wednesday. Winds could approach SCA conditions (gusting
close to 18-20 knots) at times over the Lower Tidal Potomac and
middle Chesapeake Bay due to local channeling of winds over those
open waters. Additionally, marine fog is possible each night into
the morning each day.

&&

.TIDES/COASTAL FLOODING...
Southerly flow today gives way to a very prolonged period of onshore
flow (east to southeast) that begins tonight and continues for much
of next week. Minor coastal flooding is likely at high tide at all
sensitive locations for the next few days. Coastal Flood Advisories
are in effect for the entire shoreline of the Chesapeake Bay and
Lower Tidal Potomac, including Washington DC. A steady rise in
anomalies is expected to produce Moderate flooding at Annapolis
tonight, and likely during the two high tides Sunday. As a result
Coastal Flood Warning has been issued at Annapolis through Sunday
evening. Also, Washington DC SW Waterfront could approach Moderate
flooding later this weekend, something to continue to monitor.
Expect Advisories to be extended through the weekend, and likely for
much of the upcoming week.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 4 AM EDT Monday for DCZ001.
MD...Coastal Flood Advisory until 4 AM EDT Monday for MDZ011.
     Coastal Flood Advisory until 1 PM EDT this afternoon for
     MDZ014.
     Coastal Flood Warning from 5 PM this afternoon to 4 AM EDT
     Monday for MDZ014.
     Coastal Flood Advisory until 11 PM EDT Sunday for MDZ016-018.
     Coastal Flood Advisory until 8 PM EDT Sunday for MDZ017-508.
VA...Coastal Flood Advisory until 4 AM EDT Monday for VAZ054.
     Coastal Flood Advisory until 11 PM EDT Sunday for VAZ057.
WV...None.
MARINE...None.

&&

$$

SYNOPSIS...DHOF
NEAR TERM...DHOF/CPB
SHORT TERM...DHOF
LONG TERM...KRR
AVIATION...DHOF/CPB/KRR
MARINE...DHOF/CPB/KRR
TIDES/COASTAL FLOODING...LWX