Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 211324
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
924 AM EDT Thu Mar 21 2024

.SYNOPSIS...
Weakening winds, low relative humidity values, and cooler
temperatures can be expected today as high pressure builds into the
region. Widespread rain arrives from the south late Friday into
Saturday as an area of low pressure lifts north from the Gulf Coast
and cold front approaches from the Ohio River Valley. Drier
conditions are expected behind the front Sunday with high pressure
briefly building over the area Monday. Another front looks to impact
the area by the middle of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
A cold start this morning with temperatures in the upper 20s to
30s for most of the area (teens in the mountains). Temperatures
will be much cooler overall today with highs only reaching into
the upper 40s to mid 50s, and struggling to get out of the 30s
in the mountains.

Smoke from ongoing wildfires in the area is evident on visible
satellite, but smokiness should improve later this morning as
transport winds start to pick up. RHs will be low again this
afternoon, around 15-20% for much of the area. While lower
winds today will be less favorable for wildfire spread than
yesterday, elevated risk for spread does continue today. A
Special Weather Statement for enhanced fire weather conditions
is in effect. We`ll continue to monitor throughout the day.

Previous discussion follows...

Winds continue to decrease today although very dry conditions remain
as high pressure builds into the region. Winds remain elevated this
morning out of the northwest at 5 to 15 mph with gusts of 15 to 25
mph mainly along and east of the Blue Ridge. Piedmont/Shenandaoh
Valley locations where the bulk of Wednesday`s fire activity
occurred are seeing slightly less wind with intermittent gusts of 10
to 15 mph.

With very dry high pressure building in expect low minimum RH values
this afternoon hovering between 15 to 20 percent. The good news is
that the winds will continue to come down. There is enough of a
gradient though for gusts of 15 to 25 mph across northern portions
of the area along and north of I-66/US-50 as well as west of I-81.
Piedmont locations south of I-66/US-50 and down toward I-64 will
likely see a little less on gusts hovering between 10 to 20 mph.
With that said, an elevated risk for the spread of wildfires looks
to continue given the current ongoing fires and further drying of
fuels (leaf litter, dead grasses, brush) from the recent stretch of
dry/windy conditions.

High pressure will gradually slide north and east into the northern
Mid-Atlantic/southern New England late tonight into Friday. As the
surface high lifts north and east it will gradually wedge itself
southward along the eastern side of the Appalachians pulling in
cool/moist marine air off the Atlantic. This will generate a few mid
and high level clouds overnight especially in areas along and south
of I-64 where the better moisture uptick (isentropic lift/upslope)
will reside. Residual dry air will linger further north leading to
mostly clear skies and lows in the mid to upper 20s and low 30s.

&&

.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
Friday starts out dry as mid and high level clouds increase from the
south Friday morning into Friday afternoon. High pressure over
western New England will remain wedged south along the eastern
Appalachians as an area of low pressure lifts north from the central
Gulf Coast region. Surface dewpoints will climb from the teens
Friday morning into the 30s and 40s Friday afternoon into Friday
evening as low pressure pushes north toward southern Appalachians.
Meanwhile, a cold front will approach from the Ohio River Valley
late Friday night into Saturday before exiting the region Saturday
night into Sunday morning. The combination of low pressure lifting
north and the incumbent cold front will result in widespread
beneficial rain across the entire forecast region. The
steadiest/heaviest rainfall looks to be along and east of the Blue
Ridge where 1 to 2 inches of rain can be expected. Locally higher
amounts are possible along and east of the I-95 corridor 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 0.5 to 1 inch. Lower
totals 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.

The steadiest rain (i.e strongest isentropic lift/upslope flow) will
occur between 05-12z(1am-8am) Saturday 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 an issues at this time. Some rises on streams and creeks will
likely occur given the 6-8 hour window of moderate to perhaps heavy
rainfall. If any flooding were to occur it would be fairly localized
and isolated to poor drainage/urban areas.

With a cool air wedge in place and increasing southeasterly flow
Friday expect highs in the upper 40s and low to mid 50s. Slightly
warmer conditions can be expected along and west of the Allegheny
Front due to downsloping. Lows Friday night will fall back into the
low to mid 40s.

Coastal low pressure will continue off the SC/NC coast Saturday
afternoon and evening with the upper trough/cold front set to push
offshore Saturday night into Sunday morning. This will result in
decreasing rain chances from west to east across the area along with
cooling temperatures and increasing winds. Highs Saturday will push
into the low to mid 50s with south to southeasterly winds gusting 15
to 25 mph. Lows Saturday night will fall into upper 20s and low to
mid 30s.

&&

.LONG TERM /SUNDAY THROUGH WEDNESDAY/...

Ridging will build along the East Coast during the first half of
the week supporting fair weather conditions. A weakening front
will stall along the East Coast mid week. Meanwhile, southern
stream energy digging across the northeast Gulf of Mexico will
trigger sfc cyclogenesis across the Southeast with the low
pressure becoming cutoff as it moves slowly offshore. Global
models show rain developing across the region Wed night and Thu
associated with upper level diffluence on the northeast side of
the cutoff low and an upper level jet streak.

&&

.AVIATION /13Z THURSDAY THROUGH MONDAY/...
VFR conditions are expected through early Friday as high pressure
builds over the region. Winds will continue to decrease as a result
with northwest gusts hovering between 20 to 25 kts later this
morning and into the afternoon for terminals along and north of I-
66/US-50 and west of I-81. Areas further south toward KRIC will see
gusts between 10 to 20 kts. Some mid and high level clouds may pass
through during the late morning and afternoon hours with otherwise
mostly sunny skies expected. Similar conditions can be expected
overnight as winds continue to decrease.

Sub-VFR conditions return late Friday afternoon and evening as an
area of low pressure approaches from the Gulf Coast states. Light
rain will approach the southern terminals (i.e KCHO/KSHD) Friday
afternoon before spreading northward toward the corridor Friday
night. Light southeasterly winds will start the day Friday before
increasing late in the afternoon and evening hours. Speeds will
remain between 5 to 15 kts with gusts up to 25 kts especially in the
evening and overnight period as low pressure lifts north from the
southern Appalachians toward the NC/SC coast.

Sub-VFR conditions are likely at all terminals Saturday morning and
Saturday as rain and gusty winds impact the area. North/northwest
winds will likely gust 20-30kts knots Saturday afternoon into
Saturday night especially across terminals east of the Blue Ridge
Precipitation exits the area, from west to east, Saturday night into
Sunday with gusty northerly winds lingering.

Winds will be diminishing Sunday as high pressure builds in.

&&

.MARINE...
SCA level winds look to continue Thursday afternoon as brief high
pressure builds into the region. Northwest winds will gusts 15 to 25
kts before falling back below SCA levels Thursday night into Friday.
Rain chances return to the waters Friday evening into Saturday
afternoon as an area of low pressure approaches from the Gulf Coast
states. Additional SCA level winds out of the south/southeast can be
expected from this passing coastal low pressure system Friday into
Saturday. The confidence for gale conditions continues to increase
for middle and southern portions of the bay/tidal Potomac Saturday
evening into Sunday due in part to a very tight gradient overhead.
Winds diminish slightly on Sunday, but are expected to stay above
SCA criteria.

SCA conditions expected Sunday, but winds will be on the
downward trend.

&&

.FIRE WEATHER...
Lower wind speeds are expected today although a very dry air mass
remains in place as high pressure builds in from the Ohio River
Valley. Expect another day of good mixing with northwesterly winds
gusting between 15 to 25 mph later this morning and into the mid-
afternoon hour. Highest gusts will be along the ridges and in areas
along and north of I-66/US-50 where the gradient is a bit tighter.

Minimum RH Values fall into the 15 to 20 percent range for much of
the forecast region. There were be a tad bit more moisture along and
north of I-70 and west of the Allgheny Front. Even with that said,
an elevated threat for the spread of wildfires remains given the
dry/gusty conditions of the past few days, ongoing fires, and dry
fuel moistures in the 9-15 percent range. The window for most
favorable fire weather conditions will be this afternoon and
evening.

The next chance for widespread wetting rain arrives Friday evening
into Saturday. Rainfall amounts of 1 to 2 inches can be expected
east of the Blue Ridge with 0.5-1 inch in areas further west.

Sunday and Monday look like mild days with relatively low wind
speeds.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 8 PM EDT this evening for
     ANZ530>543.

&&

$$

SYNOPSIS...EST
NEAR TERM...CAS/EST
SHORT TERM...EST
LONG TERM...LFR
AVIATION...LFR/EST
MARINE...LFR/EST
FIRE WEATHER...LFR/CAS


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