-- Discussion containing changed information from previous version are highlighted. --
032
FXUS61 KLWX 181930 CCA
AFDLWX

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

.SYNOPSIS...
Low pressure over the southern Appalachians will continue to weaken
today while another area of low pressure forms off the Virginia and
North Carolina coast tonight into 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/...
-- Changed Discussion --
Vertically stacked low pressure continues to weaken while
drifting northeast across the western NC/SC mountains this
afternoon. Meanwhile, another area of low pressure is slowly
taking shape along the VA/NC coast and 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.

The highest coverage for rain will be in areas along and east of I-
81 through the afternoon where an easterly low-level jet combined
with a piece of shortwave energy rotating around the decaying upper-
level low remain most pronounced. This is evident on both satellite
and radar this afternoon (as of 2:30pm) with a few thunderstorms
traversing central and eastern VA. Additional isolated thunderstorm
activity is possible in this area this afternoon given the cooler
air aloft and leftover surface convergence from the occluded
boundary accompanying the stacked low. Confidence is medium given
the fact that the jet will continue to weaken this afternoon and the
deeper moisture will likely contract into the developing low of the
VA/NC coast. Abundant cloud cover will also stunt the potential for
widespread convective activity.

However, current 12z CAM guidance continues show a wide array of
solutions in regards to rain coverage, intensity, and amounts over
the next 12-24 hours. Rain rates will generally sit between 0.10-
0.25"/hr although heavier amounts are possible in any elevated
convective elements that can form. We`ll have to continue to monitor
areas southwest of Charlottesville and directly along the Blue Ridge
given some localized heavy/multi-inch rain totals. Current drought
conditions and transient/progressive nature of the precipitation
means any flood threat will be fairly isolated/localized at most.
High temperatures will struggle to get out of the mid to upper 60s
and low to mid 70s today given the increased cloud cover and
showers.

Rain chances 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 east of the Blue Ridge. 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
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.
-- End Changed Discussion --

&&

.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
-- Changed Discussion --
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.
-- End Changed Discussion --

&&

.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 /19Z WEDNESDAY THROUGH MONDAY/...
-- Changed Discussion --
Sub-VFR conditions are expected through the valid TAF period at all
terminals as scattered light to moderate showers pivot through. This
shower activity is in association with an upper-level low pressure
system that continues to weaken over the Carolinas and an area of
low pressure developing off the VA/NC coast. Cigs and vsbys this
afternoon will bounce between MVFR to low end VFR in most locations
with intermittent pockets of IFR mainly at terminals close to the
Blue Ridge/Alleghenies and down in the central VA
Piedmont/Shenandoah Valley. Overall shower coverage will decrease
this afternoon and evening as winds turn toward more of a northerly
direction allowing drier air to sneak in. The exception will be at
KCHO where IFR conditions will hold on for much of the afternoon and
into the overnight hours. North to northeast winds will gusts up to
15 kts at times especially at terminals close to the waters and
along the higher ridges. Sustained speeds will range between 5 to
13 kts given the stability aloft.

An isolated thunderstorm is possible mainly south of the corridor.
Did not include VCTS at KCHO although 15 percent probs are possible
between 19-23z. MVFR to IFR conditions may redevelop late tonight,
although there is some uncertainty in whether this would be in the
form of low clouds or fog and the location of the restrictions.
Highest probabilities for fog will be at terminals west of KIAD
where a few more breaks in the clouds are possible.

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.
-- End Changed Discussion --

&&

.MARINE...
-- Changed Discussion --
Small Craft Advisories continue for the lower tidal Potomac River
and the open waters of the Chesapeake Bay through Thursday. This is
in association with continued northeast flow as low pressure
develops off the VA/NC coast. Scattered showers with perhaps a
spotty thunderstorm are possible over the waters for the remainder
of the afternoon and into 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 night 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.
-- End Changed Discussion --

&&

.TIDES/COASTAL FLOODING...
-- Changed Discussion --
Anomalies remain highest this afternoon along the tidal Potomac
River. Several advisories have been issued through this evening`s
high tide cycle. 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. 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.
-- End Changed Discussion --

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until 11 PM EDT this evening for DCZ001.
MD...Small Craft Advisory until 9 PM EDT this evening for MDZ008.
     Coastal Flood Advisory until 6 PM EDT this evening for MDZ016.
VA...Coastal Flood Advisory until 11 PM EDT this evening for VAZ054.
     Coastal Flood Advisory until 6 PM EDT this evening for VAZ057.
WV...None.
MARINE...Small Craft Advisory until 9 PM EDT this evening for ANZ530-
     538-539-542.
     Small Craft Advisory until 6 AM EDT Thursday for ANZ531>534-
     537-540-541-543.
     Small Craft Advisory until 6 PM EDT this evening for ANZ536.

&&

$$

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