164
FXUS61 KLWX 190030
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
830 PM EDT Wed Sep 18 2024

.SYNOPSIS...
An area of low pressure will continue to form off the Virginia
and North Carolina coast through Thursday. Shower chances and
extra cloud cover will continue as a result especially in areas
along and east of the Blue Ridge. Low pressure will drift along
the coast Friday before a backdoor cold front sweeps it eastward
into the start of the weekend. Strong high pressure builds
northeast of the area Sunday into Monday while a low pressure
system approaches the Great Lakes by the middle of next week.

&&

.NEAR TERM /THROUGH THURSDAY/...
Vertically stacked low pressure has devolved into an open wave
while drifting northeast across the western NC/SC mountains this
evening. Meanwhile, another area of low pressure continues to
slowly take shape along the VA/NC coast. This latter low will
become the dominant low pressure feature tonight. As a result,
expect a continuation of scattered to widespread light to
moderate showers pivoting through at times during the remainder
of the afternoon into the front half of the overnight hours.

Rain chances will gradually decrease tonight as northerly winds
bring dry air back into the region. This is due largely in part
to low pressure taking over along the VA/NC coast and the
upper-level low overhead. Any shower activity will be isolated
to scattered in areas mainly near/east of I-81. Areas further
west including the mountains may see a few breaks in the clouds
which will warrant fog development. Lows will be in the mid 50s
to mid 60s.

By Thursday, the upper-level trough and coastal low will continue to
drift east, although differences in track remain amongst the latest
guidance. Shower chances will remain, but they will remain isolated
to scattered in nature given lingering moisture over the region.
The highest confidence for showers and perhaps a thunderstorm
will be closest to the Chesapeake Bay compared to the Appalachians
where slightly drier air will filter in as upper-level ridging
builds from the west. High temperatures Thursday will push into
the low to mid 70s west of the Blue Ridge with values closer to
80 further east.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
Both the upper-level low and coastal low will continue to drift
eastward Friday as upper-level ridging builds in from the Ohio River
Valley and central Appalachians region. Current 12z model guidance
continues to suggest drier conditions with gradual clearing mainly
in areas along and west of I-95. Areas east of I-95 will likely hold
onto extra clouds and perhaps a few leftover showers/sprinkles as
low pressure sits off the Delmarva and NJ coast. For now, went with
a split amongst the guidance which has trended drier in the last few
runs.

Outside of the low end shower chances, some patchy fog is possible
late Thursday night into Friday morning, especially in areas west of
Blue Ridge. Lows Thursday night will fall back into the mid to upper
50s in most locations outside of the immediate bay. Highs Friday
will push into the upper 70s and lower 80s with added sunshine.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
A backdoor cold front will be exiting the area Saturday morning with
cooler air expected to filter in behind it. While high temperatures
on Saturday will be similar to Friday with highs in the upper 70s to
low 80s, temperatures are expected to gradually cool each day behind
the front. High temperatures will be in the mid 60s to mid 70s
Sunday through Tuesday. Overnight low temperatures will be in the
50s to low 60s each night.

Surface high pressure will keeps conditions primarily dry Saturday
and Sunday, though energy offshore will bring slight chance PoPs to
the easternmost portions of the forecast area. Precipitation chances
increase from west to east Monday into Tuesday as a low pressure
system and associated warm front approach from the west. Highest
rain chances will be in our western zones.

&&

.AVIATION /00Z THURSDAY THROUGH MONDAY/...
Sky conditions remain FEW/SCT at FL015-025 as of 00Z for IAD,
BWI, and DCA. Light NE flow leads to a bit of a challenge in the
CIG forecast for the metro terminals overnight. If the wind has
a slightly more northerly component, drier air will continue to
filter in favoring VFR. But, if the wind has a slightly more
easterly component, a bit more moisture could linger resulting
in these FEW/SCT clouds becoming BKN/OVC at times. The general
theme through the Thursday AM push should be gradual improvement,
though some patchy fog may form where clouds break. MTN is a bit
further north closer to the source of dry air, but is also
closer to the Chesapeake Bay which is a moisture source, so
similar challenges are in the forecast here overnight. Lingering
moisture in lighter low-level flow could result in the re-
development of lower CIGs or patchy fog near MRB, with CHO having
conditions as low as IFR most of the night given deeper and
more persistent moisture further south in that area.

A few showers may continue Thursday, but overall there should be a
drying trend into Friday. There is some potential of low clouds
and/or fog developing both Thursday night and Friday night, however.

VFR conditions are expected at all terminals on Saturday and Sunday
with no precipitation in the forecast. Northeasterly winds on
Saturday shift to easterly Sunday afternoon. Winds increase slightly
on Sunday in the eastern portions of the forecast area with KIAD,
KDCA, KBWI, and KMTN gusting to 15 knots in the afternoon.

&&

.MARINE...
Small Craft Advisories continue for the lower tidal Potomac River
and the open waters of the Chesapeake Bay through tonight. This
is in association with continued northeast flow as low pressure
develops off the VA/NC coast. Scattered showers are possible
over the waters for the overnight hours. The position of the
low and intensity will determine how long SCA conditions last
over the waters Thursday and perhaps into Friday. It does appear
winds should be relatively lighter Thursday into Friday, but
may strengthen again Friday night amongst a tightening pressure
gradient.

Northeasterly winds on Saturday shift to easterly on Sunday with
Small Craft Advisories possible both days. Winds gust 15 to 20 knots
each afternoon.

&&

.TIDES/COASTAL FLOODING...
Anomalies remain highest this evening along the tidal Potomac
River. Several advisories remain in effect. As low pressure
turns winds more northerly tonight, there may be a brief
decrease in tidal levels. However, as the low strengthens
offshore, surge guidance shows increasing water levels by
Thursday evening into the weekend as water becomes trapped near
the mouth of the Chesapeake Bay. Combined with high
astronomical tide levels, additional coastal flooding can be
expected. Some guidance suggests Annapolis and DC SW Waterfront
could even approach moderate flood levels.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until midnight EDT Thursday night for
     DCZ001.
MD...Coastal Flood Advisory until 10 PM EDT Thursday for MDZ014.
     Coastal Flood Advisory until 7 PM EDT Thursday for MDZ016.
VA...Coastal Flood Advisory until midnight EDT Thursday night for
     VAZ054.
     Coastal Flood Advisory until 7 PM EDT Thursday for VAZ057.
WV...None.
MARINE...Small Craft Advisory until 8 AM EDT Thursday for ANZ531>534-
     537-540-541-543.

&&

$$

SYNOPSIS...DHOF/EST
NEAR TERM...DHOF/EST
SHORT TERM...DHOF/EST
LONG TERM...AVS
AVIATION...AVS/DHOF/EST
MARINE...ADS/AVS/DHOF/EST
TIDES/COASTAL FLOODING...LWX