741
FXUS61 KLWX 190802
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
402 AM EDT Thu Sep 19 2024

.SYNOPSIS...
Low pressure will drift off the Mid Atlantic coast through
Saturday while high pressure builds north of the area. A weak
backdoor cold front may push into the area by Sunday. Strong
high pressure builds northeast of the area while a low pressure
system approaches the Great Lakes early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Upper level low has drifted toward central North Carolina this
morning while low pressure spins off the Mid Atlantic coast.
Plenty of moisture remains in place, so isolated light showers,
drizzle, and fog will remain possible through the morning. It
appears stratus is too solid to result in much dense fog except
perhaps where the ridges intersect the cloud base.

The southern upper low will open today while another upper low
develops near New England. This ultimately shifts the trough
axis to the east, allowing deeper layer northerly flow to advect
some drier air into the area. Therefore, it seems like there
should be some breaks in the clouds as the afternoon progresses,
and highs should rise into the mid 70s to lower 80s. A
disturbance rotating through the back side of the trough,
combined with diurnal instability and surface convergence, will
allow some scattered showers and thunderstorms to develop across
southwest portions of the forecast area this afternoon before
dissipating this evening.

There should be some additional clearing tonight. Patchy fog
will be possible in some of the rural valleys. Lows will range
from the mid 50s to mid 60s.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
Guidance has converged on a solution keeping the low far enough
away that Friday should be a dry day with partly cloudy skies.
One exception is the southwestern corner of the forecast area
where weak convergence and instability may again result in a few
showers or a thunderstorm. Highs will reach the lower to mid 80s
for most areas. Friday night`s lows will fall into the mid 50s
to mid 60s again, with patchy valley fog possible.

The forecast for Saturday appears to be in flux. With the low a
bit farther offshore and a slightly flatter upper level pattern
overhead, the strong Canadian high may be blocked to the north
and the previously advertised backdoor cold front associated
with it may not arrive until Sunday. However, there is potential
for a shortwave to move into the area in northwest flow which
could bring some showers and thunderstorms. The 00Z ECMWF, GEM,
and GFS all show this in some fashion. Capped PoPs at 20-30
percent (highest along the Appalachians) for now until there is
better agreement from the ensemble data. Highs Saturday should
now remain in the lower 80s while lows remain close to a
persistence forecast.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Cooler and drier air will move into the region on the northeast
flank of the backdoor cold front on Sunday. High temperatures will
be a couple of degrees cooler than average with highs in the middle
70s for most.

High pressure will build in from the northeast on Monday, while a
ridge of high pressure in the middle and upper levels sneak
into the region from the west during the same time. This
combination will keep most of our region dry and cool. A
lingering or perhaps dissipating backdoor cold front against the
Appalachians will be the focal zone for any developing showers
in the mountains and perhaps a few sneaking into the Shenandoah
Valley later Monday. High temperatures will be in the lower to
middle 70s.

The ridge of high pressure slowly flattens Tuesday into Wednesday as
waves of energy are expected to move into our region. A chance of
showers will accompany this energy and could venture as far as the I-
95 corridor on Tuesday. Temperatures will stay cool.

As for Wednesday, rain chances spread to the Chesapeake Bay with an
approaching cold front arriving sometime late Wednesday or Wednesday
night. The western half of our region stand the better chance of
showers and/or thunderstorms midweek into early Thursday.
Temperatures may be a few degrees warmer with a return flow ahead of
the front.

&&

.AVIATION /08Z THURSDAY THROUGH MONDAY/...
Drier air moving in from the north has maintained VFR conditions
across northern areas. However, it does seem ceilings are
trending toward MVFR even at CHO -- where it may take until this
afternoon to reach persistent VFR conditions, with some
uncertainty in timing noted. Spotty light rain or drizzle could
occur through the morning and a brief period of fog could occur
around dawn for CHO, IAD, and MRB...although it seems like
chances for IFR or lower conditions are lowering.

For the remainder of the day VFR ceilings will persist but with
dry conditions and light north winds. A shower or thunderstorm could
approach CHO late this afternoon but confidence is low. For
tonight, fog is possible at MRB and CHO.

High pressure will maintain VFR conditions Friday, with fog
possible Friday night at MRB and CHO. A disturbance may bring
some showers and thunderstorms Saturday afternoon or evening,
but confidence as low as the forecast for this period has been
changing.

VFR conditions Sunday through Monday night. Winds northeast to east
5 to 10 knots Sunday and Sunday night. Winds becoming more
southeasterly 5 to 10 knots Monday and Monday night.

&&

.MARINE...
Winds have been lessening this morning. Low pressure will remain
off the coast through the weekend while high pressure builds
north of the area. So north winds will continue, perhaps
becoming northeast or east at times. Wind forecasts have trended
down a bit, especially for Saturday, so Small Craft Advisories
are looking less likely. However, some marginal gusts up to 20
kt can`t be totally ruled out along the bay.

Small craft advisories possible late Sunday afternoon and Sunday
evening. Otherwise, no marine hazards expected Sunday morning
nor late Sunday night through Monday night. Winds northeast 10 to 15
knots gusts up to 20 kts late Sunday into Sunday evening, mainly
over the main stem Chesapeake and Tidal Potomac. Winds becoming east
then southeast 10 knots late Sunday night into Monday and Monday
night.

&&

.TIDES/COASTAL FLOODING...
Anomalies remain highest along the tidal Potomac River. Several
advisories remain in effect through this evening`s high tide.
Have not extended the advisories yet since the following high
tide cycle looks to be marginal minor flooding at most. After
remaining stable through tonight, water levels are forecast to
rise into the weekend as offshore low pressure traps water in
the Chesapeake. Combined with high astronomical tide levels,
additional, and more widespread, coastal flooding can be
expected. Some guidance suggests Annapolis and DC SW Waterfront
could even approach moderate flood levels. Coastal flood issues
look to persist into next week.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Coastal Flood Advisory until midnight EDT tonight for DCZ001.
MD...Coastal Flood Advisory until 10 PM EDT this evening for MDZ014.
     Coastal Flood Advisory until 7 PM EDT this evening for MDZ016.
VA...Coastal Flood Advisory until midnight EDT tonight for VAZ054.
     Coastal Flood Advisory until 7 PM EDT this evening for VAZ057.
WV...None.
MARINE...Small Craft Advisory until 4 AM EDT early this morning for
     ANZ534-537-543.

&&

$$

SYNOPSIS...ADS
NEAR TERM...ADS
SHORT TERM...ADS
LONG TERM...KLW
AVIATION...ADS/KLW
MARINE...ADS/KLW
TIDES/COASTAL FLOODING...ADS