Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 230736
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
336 AM EDT Sat Mar 23 2024

.SYNOPSIS...

Widespread soaking rainfall will end from west to east later today
as a cold front and associated area of low pressure push offshore.
Gusty winds and drier conditions return in the wake of the system
Saturday night into Sunday as high pressure builds into the region.
Winds decrease Monday into Tuesday with wedging high pressure
overhead. Another front and area of low pressure look to approach by
the middle and latter half of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...

A cold front will approach from the Ohio River Valley with an
incumbent low pressure system ejecting northward from the southern
Appalachians toward the VA/NC coast early this morning into mid-
afternoon. The resultant front will push low pressure off the lower
Delmarva coast later this afternoon and evening putting an end to
the rain from west to east across the area. A widespread beneficial
soaking rainfall will occur until then due largely in part to
increased isentropic lift between the low/front as well as
enhancement from east to southeasterly onshore flow. This is
especially true in areas east of the Blue Ridge where high PWATS of
1.0" + look to pool.

The steadiest/heaviest rainfall looks to be along and east of the
Blue Ridge where 1.25 to 2.25 inches of rain can be expected.
Locally higher amounts are possible along and east of the I-95 (in
particular northeast MD where ensemble probs have amounts up around
3") and along the eastern facing slopes of the Blue Ridge where the
upslope component will come into play. Areas further west of the
Blue Ridge will see rainfall amounts on the order of a 0.75 to 1.25
inches. Lower totals of 0.50-1" may be found west of I-81 given
where the potential deformation axis sets up on the northern fringe
of the coastal low passing to our south and timing of the squeeze
play with the approaching front.

The steadiest/heaviest rain (i.e strongest isentropic lift/upslope
flow) will occur between now and 11am this morning per the latest
00z deterministic/ensemble guidance. Given the recent dry stretch of
weather and a lack of a convective element flooding does not appear
to be a widespread issue at this time. Rain rates up to 0.5"/hr
could lead to some rises on streams and creeks given the 6-8 hour
window of moderate to heavy rainfall especially within the urban I-
95 corridor. If any flooding were to occur it would be fairly
localized and isolated to poor drainage/urban areas. Flood Watches
remain in effect for both the Baltimore/Washington DC metro areas as
well as areas east of the Blue Ridge.

Coastal low pressure will continue off the lower Delmarva coast this
afternoon and evening with the upper trough/cold front set to push
offshore tonight. This will result in decreasing rain chances from
west to east across the area along with cooling temperatures and
increasing winds. Highs today will push into the low to mid 50s with
north to northeasterly winds gusting 25 to 35 mph. Lows tonight will
fall into upper 20s and low to mid 30s.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...

Dry high pressure returns Sunday with gusty northerly flow
continuing. Gusts of 20 to 30 mph can be expected with high
temperatures in the upper 40s and low to mid 50s under mostly sunny
skies. Lows Sunday night will fall into the upper 20s and low to mid
30s as winds slowly diminish.

1040 mb high pressure over eastern Canada will wedge its way
south along the Appalachian Mountains before shifting offshore
through the start of the upcoming workweek. This will lead to
decreasing winds, dry conditions, and near normal temperatures
Monday. Highs will push into the mid to upper 50s and low 60s
with overnight lows in the 30s and 40s.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...

Wedging high pressure will gradually slide offshore Tuesday allowing
for slightly warmer temperatures and increasing cloud cover as a
weak cold front pushes in from the Ohio River Valley. Highs Tuesday
will push into mid to upper 50s and low 60s under increasing
southeast flow.

Rain chances will return late Tuesday into Wednesday as the weak
cold front crosses the region. The bulk of the dynamics will remain
well north and west of the region leading to scattered to perhaps
widespread showers across the region. Precipitation amounts appear
to be light due to a lack of deep moisture with increasing winds
Tuesday afternoon into Wednesday morning as the front crosses.

Model uncertainty continues to remain beyond Wednesday in regards to
an area of low pressure developing somewhere along the Gulf
Coast/southeast U.S coast. 00z deterministic/ensemble guidance
continues to diverge on the placement, strength, and track of a
coastal low which will be highly dependent on where/if a sharp mid-
level trough cuts off over the southeast CONUS. With the residual
front just along the coast, it could act as catalyst to push
additional rain chances northward Thursday into Friday. Currently
the 00z GFS/GEM/GEFS remain the wettest with low pressure working
north over eastern NC into southeast VA while the 00z ECMWF holds
the low further south shunting the moisture mainly east of the Blue
Ridge. For now, kept a blend of the 3 model solutions with increased
precipitation chances between 30-50 percent late Wednesday through
Thursday afternoon. Trended the forecast drier Thursday night
through Saturday as high pressure builds back into the region.
Additional shower chances are expected Sunday into early next week
as a series of fronts and area of low pressure approach the region.
Temperatures will remain at or slightly above average for the
remainder of the extended period.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...

IFR/LIFR conditions through 18Z, then clearing rapidly in the
afternoon. NNW to N winds will strengthen this afternoon and
peak around 06Z Sun with gusts 30-35 kt. Winds begin to dimnish
Sun but still gusty around 20 kt.

VFR conditions are expected Sunday through Tuesday as strong high
pressure builds into the region. The departing front and low
pressure system offshore combined with the building high pressure
will lead to the continuation of gusty northerly winds through
Sunday evening. Gusts of 15 to 25 kts can be expected with the
highest gusts likely at terminals east of the Blue Ridge and in
particularly along the corridor Sunday afternoon. Winds will subside
(5-10 kts) and turn toward more of a east/southeast direction as
high pressure strengthens over the area Monday into Tuesday.

Next chance of sub-VFR conditions arrives with with a weak cold
front late Tuesday into Wednesday. The front will bring a few
scattered showers to the terminals along with increasing
southeasterly winds. Additional sub-VFR conditions are possible late
Wednesday into Thursday as an area of low pressure works up the
coast.

&&

.MARINE...

Small Craft Advisories remain in effect this morning before
transitioning to Gale Warnings this afternoon and tonight as deep
low pressure passes south and east of the region. Gusts of 35 to 40
kts are expected especially later evening and into the first half of
Sunday as the gradient tightens.

SCA level winds are expected late Sunday into early Monday morning
as high pressure builds back over the region. Sub-SCA level winds
out of the east and southeast return Monday into Tuesday with
wedging high pressure overhead. SCA level winds return with a weak
front and coastal low pressure by the middle and end of next week.

&&

.FIRE WEATHER...

Beneficial soaking rain is expected today which should help
containment efforts with ongoing wildfires over the Shenandoah
Valley and eastern WV. With the added rain, smoke left over at the
surface should scatter out leading to improved air quality within
the aforementioned region. The heaviest rain looks to occur this
morning into midday before quickly tapering from west to east later
this afternoon and into the evening hours. Rainfall amounts will
range between 0.50-0.75 over the fire locations.

The rain combined with low/mid level cloud cover will yield highs in
the low to mid 50s. Mountain locations will be a touch cooler in the
upper 40s with coastal locations in the upper 50s. Light winds will
start the day before increasing out of the south and southeast at 10
to 20 sustained this afternoon and evening. Gusts of 25 to 35 mph
are expected during this time as the gradient tightens between low
pressure passing to the south/east and incoming high pressure from
the eastern Canada/Great Lakes region. Maximum RH values of 85 to
100 percent are expected this morning before falling into the 60 to
70 percent range this afternoon.

Dry and breezy conditions look to continue Sunday although winds
will slowly diminish. North/northeasterly winds look to gusts 20 to
30 mph with minimum RH values falling back into the 25 to 35
percent range. Portions of the central and southern Shenandoah
Valley and eastern slopes of the Alleghenies could see minimum RH
Values as low as 20 to 25 percent Sunday afternoon. This is due to
the gradient wind and added sunshine as high pressure builds
overhead.

Winds continue to decrease Monday before returning Tuesday as a weak
front approaches the region. Minimum RH values Monday will remain
between 25 to 30 percent before increasing into the 40 to 45 percent
Tuesday.

&&

.HYDROLOGY...

Moderate to heavy rain will overspread the area this morning and
exit by 18Z if not sooner as a strong area of low pressure
passes south and east of the region. Heaviest rainfall looks to
fall this within a 6 to 8 hour window (i.e through 11am). Rain
will quickly diminish in intensity and coverage as the cold
front pushes low pressure further offshore late this afternoon
into early evening.

PWATS will be on the order of a 1-1.25" per the 00z
NAEFS/GEFS/HREF/EPS with a focus east of I-95. Modeled QPF
values per the 00Z NAM/GFS/ECMWF/HREF sits between 1-2 inches
across areas east of US-15. 2"+ inch totals are likely along and
east of I-95 where a combination of enhanced isentropic lift
and onshore flow will be maximized. Areas further west toward
the I-81 corridor will likely see 0.5-1" of rain with this quick
event.

6 hour FFG values sit between 1.75-3 inches across most of the
region outside the metros where runoff poses more of a concern. Any
flooding issues as a result, appear to be localized/isolated
given the progressive nature of the system. Even with that said,
the amount of heavy rainfall with the shortened window could
lead to quick rises along small streams and creek especially
within the I-95 urban footprint which is noted per the latest
forecast per the NASA Sport and HEFS guidance (i.e Rock Creek in
northwest DC, Western Branch at Marlboro, Great Mills, etc.)

A Flood Watch remains in effect for both the Baltimore/Washington DC
metro areas along with northeast areas east of the Blue Ridge
through midday. For more info visit weather.gov/lwx.

&&

.TIDES/COASTAL FLOODING...

Increasing onshore flow ahead of approaching low pressure will cause
an increase in tide levels through tonight. Minor tidal flooding is
possible during this time, especially for typically more vulnerable
shoreline locales, though the magnitude of flooding is uncertain and
will depend on the strength and duration of onshore flow. Some
guidance has stronger onshore flow for a few hours longer, resulting
in notably higher water levels.

In the wake of low pressure, strengthening northerly flow will cause
a decline in water levels Saturday night through Sunday. Low water
level issues may arise during the second half of the weekend,
especially over northern portions of the Chesapeake Bay.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flood Watch until 2 PM EDT this afternoon for DCZ001.
MD...Flood Watch until 2 PM EDT this afternoon for MDZ004>006-008-
     011-013-014-016>018-503>508.
VA...Flood Watch until 2 PM EDT this afternoon for VAZ038>040-050-
     051-053>056-501-502-505-506-526-527.
WV...None.
MARINE...Small Craft Advisory until 6 PM EDT this evening for
     ANZ530>532-536-538>540.
     Gale Warning from 6 PM this evening to noon EDT Sunday for
     ANZ530>532-539-540.
     Small Craft Advisory until 10 AM EDT this morning for ANZ533-
     534-537-541>543.
     Gale Warning from 10 AM this morning to noon EDT Sunday for
     ANZ533-534-537-541>543.
     Small Craft Advisory from 10 AM this morning to 6 PM EDT this
     evening for ANZ535.
     Gale Warning from 6 PM this evening to 6 AM EDT Sunday for
     ANZ535-536-538.

&&

$$

SYNOPSIS...EST
NEAR TERM...EST
SHORT TERM...EST
LONG TERM...EST
AVIATION...LFR/EST
MARINE...LFR/EST
FIRE WEATHER...EST
HYDROLOGY...EST
TIDES/COASTAL FLOODING...EST


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