Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 210739
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
339 AM EDT Fri Apr 21 2017

.SYNOPSIS...
A cold front will pass through the area late this afternoon
through this evening. The front will stall to the south Friday
night and low pressure will develop along the
boundary...impacting our area for the weekend through early next
week.

&&

.NEAR TERM /THROUGH TODAY/...
A cold front will approach and cross the region through today.
There are some showers out ahead of the front that have been
moving through portions of western Maryland/Virginia and eastern
West Virginia, and some of these will persist through he
morning hours, although coverage will be waning. As we head into
the afternoon, despite clouds, temperatures will rise into the
upper 70s and and mid 80s. This combined with dew points in the
upper 50s and lower 60s will lead to the development of 1000+
J/KG of MLCAPE across much of the region, especially east of
the Blue Ridge Mountains. As the front presses eastward into
this air mass, additional scattered showers and thunderstorms
are expected to develop in the afternoon ours out ahead of the
front. With the increased CAPE values and 0-6 KM shear near 40
knots, some of the thunderstorms may be strong to severe,
especially from I-95 south and east. Westerly surface flow may
tend to inhibit coverage west of there. Because of this, the
Storm Prediction Center has placed part of the area in a
Marginal Risk for severe thunderstorms. Most widespread
thunderstorm coverage expected from 18z-00z, with marginally
severe hail/wind the main threats.

&&

.SHORT TERM /TONIGHT THROUGH SUNDAY NIGHT/...
The frontal boundary will settle south of the area tonight and
remain relatively stationary through the weekend. Multiple waves of
moisture/energy will ride along this boundary through the period
ahead of a main low pressure system that will move along the
boundary from the southern plains tonight and to the east coast
early next week. There are still relatively significant
discrepancies as to northern extent and timing of these individual
waves, but several periods of rain and showers are likely from late
tonight through at least Sunday night, with the greatest coverage
and intensity across central and western Virginia, where model
consensus is growing for potentially several inches of rain during
this period. For areas to the north, have concentrated the highest
likelihood of rain from Saturday afternoon into Sunday morning,
although this is uncertain.

Temperatures will be quite cool during this timeframe given the
abundance of clouds and showers and northerly flow on the northern
side of the boundary. Temperatures tonight will drop into the 50s,
with not much variation expected once this happens, with highs
Saturday and Sunday in the 50s to near 60F, and lows Saturday and
Sunday nights in the 40s to around 50F.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
A cutoff upper-level low will swing through the Southeastern
CONUS Monday before moving off the Mid-Atlantic Coast Tuesday.
This system will cause coastal low pressure along the North
Carolina Coast Monday that will slowly move out to sea on
Tuesday. High pressure will remain over southern Canada into New
England during this time. An onshore flow will cause unusually
cool conditions for this time of year at the same time warm and
moist air is overrunning the cool air...providing a soaking rain
Monday. Rain will continue through Tuesday...but rain rates and
coverage may diminish some Tuesday afternoon as the low begins
to pull away.

The low will continue to move out to sea Tuesday night and high
pressure will develop over the central Atlantic. A south to
southwest flow around this high will likely bring a return to
warm and more humid conditions for the middle portion of next
week. Drier conditions are expected during this time...but
convection cannot be ruled out due to the warm and humid
air mass.

&&

.AVIATION /07Z FRIDAY THROUGH TUESDAY/...
Low clouds and areas of fog have developed overnight and early
this morning across portions of northeastern Maryland with
IFR/LIFR being observed at BWI/MTN. Elsewhere mainly VFR has
been maintained, although some patchy fog is around. As the
morning progresses, expect ceilings/visibilities to improve to
VFR at all sites. A cold front will then cross the region this
afternoon and evening, with scattered showers and thunderstorms
likely, some of which may contain strong gusty winds and hail.
Greatest chance for TSRA will be between 18z and 00z.

Otherwise, VFR expected for most sites much of tonight. Low
clouds and periods of rain will approach from the southwest
early Saturday morning through Sunday with the greatest chances
for significant rainfall and associated prolonged sub-VFR
conditions at CHO. However at least intermittent periods of
reductions expected just about every TAF site Saturday through
Sunday night.

Westerly winds today turn northerly tonight and Saturday then
northeasterly by Sunday.

Coastal low pressure will impact the terminals Monday through
Tuesday with Sub-VFR conditions and rain.

&&

.MARINE...
Sub-SCA west/southwesterly winds expected for much of today.
Scattered showers and thunderstorms this afternoon and evening may
produce gusty winds and small hail, and Special Marine Warnings may
be necessary.

Winds then turn northerly overnight following passage of the cold
front and increase on Saturday. Wind speeds may approach marginal
SCA-criteria, but confidence too low to issue a SCA at this time,
especially with potentially stable conditions due to rain. Similar
conditions expected Saturday night through Sunday night.

Coastal low pressure will impact the waters Monday and Tuesday
before moving out to sea during the middle portion of next week.
A Small Craft Advisory may be needed for Monday through Tuesday
night. High pressure over the central Atlantic will cause a
south to southwest flow to return for the middle portion of next
week.


&&

.TIDES/COASTAL FLOODING...
A persistent onshore flow will develop behind a cold front
Saturday through early next week. Elevated water levels are
expected during this time...and minor flooding cannot be ruled
out near times of high tide...especially Sunday into Tuesday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...None.

&&

$$
SYNOPSIS...BJL
NEAR TERM...MM
SHORT TERM...MM
LONG TERM...BJL
AVIATION...BJL/MM
MARINE...BJL/MM
TIDES/COASTAL FLOODING...BJL



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