434
FXUS61 KLWX 210705
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
305 AM EDT Tue May 21 2024

.SYNOPSIS...
High pressure will move offshore through Wednesday. A cold front
will approach from the Midwest late Wednesday, then cross the
northern Mid-Atlantic Thursday. The front will stall to the
north across Pennsylvania keeping the local area in a warm and
humid air mass through early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Early morning satellite and surface observations indicate high
cirrus tumbling in from the northwest, with an area of low
clouds and stratus developing near the Chesapeake Bay. Patches
of fog have developed where there are breaks in the high clouds.

Any fog or low clouds should lift by mid morning resulting in a
mainly sunny day. Slightly lower heights associated with a weak
lead shortwave, coupled with heating and orographic lift could
result in a few showers and thunderstorms generally west of the
Blue Ridge and south of US-50 mid to late afternoon into this
evening. Flow aloft is relatively weak (W 15-20 kts 700-300 hPa,
SE 5-10 kts 850 hPa). Although light, the veering nature of the
flow could help prop or lock cells to the terrain, with
subsequent outflow collisions resulting in only a slow east-
southeastward drift. There is a stable layer around 600 hPa in
forecast soundings, and this plus a lack of a large scale
trigger may inhibit more widespread deeper convection for much
of the day. Although storm motion may be slow, any excessive
rainfall threat should be mitigated by lower PWs of around 1
inch. Still, isolated totals of 1-2 inches are possible under
any slow-moving/repeat storms. CAPE around 1500 J/kg and
steepening low-level lapse rates could result in some spotty
small hail or gusty winds and frequent lightning.

Any shower and thunderstorm activity should dwindle this evening
with additional mid and high level clouds beginning to move in
from the west. These clouds may help reduce the threat of more
widespread and/or dense fog tonight, but some patchy fog or
perhaps marine stratus are possible.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
High pressure at the surface will shift offshore Wednesday into
Wednesday night. Meanwhile, a broad area of troughing and
associated surface low pressure will trek gradually eastward
from the Midwest. This will lead to a period of unsettled
weather starting as early as Wednesday afternoon, and lingering
through at least Thursday evening.

Deep layer shear is expected to remain weak Wednesday (AOB 20
kts sfc-500 hPa). However, flow will increase subtly later in
the day, and heights will begin to come down slightly as well.
This, coupled with above normal temperatures and increasing
humidity and instability will lend to a threat of at least
widely scattered showers and thunderstorms come afternoon. The
threat appears greatest west of I-95 further away from the
departing ridge and any associated subsidence. A couple storms
could produce a little hail or gusty winds given the increased
instability, but the lack of shear will probably keep a more
organized or widespread severe weather threat marginal, and
relegated to north of US-33/west of US-15 for the most part.

Heights fall more appreciably on Thursday as a surface cold
front impinges on the area. Latest guidance times the front
favorably with peak heating. The background airmass will be very
warm and humid, but cloud debris from previous day`s convection
over the middle of the country could inhibit more significant
CAPE. If higher instability is realized, a more organized threat
for strong to severe thunderstorm clusters could develop given
the strong surface front and increasing flow/shear aloft in the
right entrance region of a seasonably strong upper jet. The
threat for strong to severe thunderstorms may be greater south
of US-50/I-66 and east of US-15 where the potential for more
heating is highest.

Activity should gradually dissipate heading into Thursday night
as daytime instability wanes and the front sags south. But, the
front`s progress will be slow, so the potential of showers may
linger well into the night south of US-50/I-66.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...

A front will remain generally north of the Pennsylvania Turnpike
through early next week keeping the local area in a warm and humid
air mass with the risk of almost daily afternoon and early evening
showers and thunderstorms. A well defined 700 mb shortwave-trough is
fcst to approach the area Fri night with its axis crossing the area
around 18Z Saturday. With shear vectors on the weaker side and slow
storm motions, storms Fri afternoon and evening may pose a risk of
heavy rainfall and flash flooding. Saturday`s threat really depends
on the timing of that shortwave as it appears the trough axis will
be too far east to enhance t-storm activity.

On Sunday, the models show significant 925-850 mb drying under low-
mid level ridging, which is likely to keep t-storm activity isolated
at best.

On Monday, a potent shortwave-trough and attendant sfc cyclone will
move across Ohio and northern Mid-Atlantic. This system could pose a
more substantial severe wx threat given strong winds aloft
especially if the timing of the frontal passage coincides with
peak heating. The latest timing suggests a severe threat
during the morning hours, but things could definitely still
change between now and next Monday.

&&

.AVIATION /06Z TUESDAY THROUGH SATURDAY/...
As of early this morning (06Z), IFR stratus was developing near
the Chesapeake Bay. This layer will attempt a slow NW drift
through daybreak, most likely affecting KMTN/KBWI, with a bit
more uncertainty towards KDCA and especially KIAD. High clouds
were moving overhead, but there were enough breaks to allow some
cooling to near the dew points. This could result in patches of
fog through daybreak, as well. VFR should return to all TAF
sites by 13-15Z. Any shower or thunderstorm activity this
afternoon into this evening should be sparse in coverage and
west of the TAF sites. Light south flow is expected.

Some patches of fog or low clouds are possible again overnight
tonight into early Wednesday with continued light south flow.
Widely scattered showers and thunderstorms may develop Wednesday
afternoon and early evening, especially west of the metros.
Otherwise, VFR is expected through Thursday morning. Sub-VFR
conditions are possible with potentially more widespread shower
and thunderstorm activity Thursday into Thursday night as a cold
front crosses. Winds will shift to northwest.

Isold-sct afternoon t-storms Fri and Sat.

&&

.MARINE...
Generally light south to southeast flow is expected through
Thursday, becoming northwest by Thursday night as a cold front
crosses. A brief period of sporadic and marginal SCA-level gusts
is possible late this afternoon into this evening, with a bit
better chance Wednesday afternoon and evening as high pressure
moves offshore. There is a low potential of a shower or
thunderstorm approaching the waters Wednesday evening, with a
much higher chance Thursday.

SCA conditions possible Fri night across the southern waters in
southerly channeling.

&&

.TIDES/COASTAL FLOODING...

Minor coastal flooding is occurring at Annapolis and expected to
occur shortly at DC Waterfront. After this morning`s high tide,
water levels are expected to recede, but may increase again with
tonight`s high tide especially at Annapolis.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 9 AM EDT this morning for DCZ001.
MD...Coastal Flood Advisory until 9 AM EDT this morning for MDZ014.
     Coastal Flood Advisory until 3 AM EDT early this morning for
     MDZ017.
VA...None.
WV...None.
MARINE...None.

&&

$$

SYNOPSIS...DHOF
NEAR TERM...DHOF
SHORT TERM...DHOF
LONG TERM...LFR
AVIATION...LFR/DHOF
MARINE...LFR/DHOF
TIDES/COASTAL FLOODING...LFR