Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 212000

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
300 PM EST Tue Nov 21 2017

High pressure will move offshore today. A cold front will pass
through tonight into early Wednesday, while weak low pressure
moves up the coast. High pressure will build back overhead
later Wednesday through Thanksgiving Day. High pressure will be
overhead Friday. A front approaches from the west on Saturday,
High pressure builds later on Sunday and will remain in control
through Tuesday.



High pressure ebbs away tonight as we get caught in between a
cold front moving in from the west, and weak low pressure moving
up the coast. While the front is not expected to bring much if
any precip to our forecast area, the coastal low will bring rain
up the coast and into lower southern MD and the southern MD Ches
Bay. Precip PoPs fall back to chance levels over the major metro
areas, and then to slight chance and less for the Shenandoah
Valley. Looking at the latest convective allowing meso models
and LAMP, the timing looks to be a few hours slower...keeping
most of the precip in our CWA after midnight and possibly
lingering into Wed morning closer to the Bay. Amounts for those
who get rain are expected to be a quarter inch or less.

A period of upslope snow showers are expected late tonight into
Wed morning behind the cold front for locations along/west of
the Allegheny Highlands. Accumulations should be an inch or
less since moisture will be limited...but this may cause
slippery conditions by the early morning hours Wednesday.


The cold front will move off to the south and east early
Wednesday morning. Canadian high pressure will approach from the
north and west and a northwest flow ahead of the high will usher
in drier and chillier conditions. Max temps will hold in the 40s
for most locations north and west of Interstate 95 and in the
lower to middle 50s farther south and east.

High pressure will build overhead Wednesday night...bringing dry
and cold conditions. Min temps will drop into the 20s for most
areas, except lower 30s downtown DC/Balt. High pressure will
remain overhead Thanksgiving Day through Thanksgiving
night...bringing more dry and chilly conditions.


High pressure will be overhead Friday, keeping dry conditions
over our region, before it moves offshore late on Friday. A
front approaches from the west on Saturday, as upper trough
deepens over the eastern CONUS. PoPs increases with this
boundary on Saturday, but deterministic guidance and GEFS are
not too aggressive with precipitation over our CWA. At the same
time a low pressure system will develop off of the southeast
Atlantic coast but it is expected to stay offshore with little
to no impacts to our area.

Upslope showers expected on Sunday with the northwest flow and upper
trough axis moving through our area. Surface high pressure builds
later on Sunday and will remain in control through Tuesday.


Southwest winds will gust around 20 knots this afternoon over
the metro airports. A cold front will cross late tonight. A few
showers cannot be ruled out (especially for DCA/BWI/MTN)...but
any precipitation amounts will be light. MVFR conditions are
expected late tonight into early Wed, most prolonged for
DCA/BWI/MTN. Northwest winds will develop behind the cold front
early Wednesday morning and gusts around 20 to 25 knots are
expected for most of Wednesday behind the cold front.

VFR conditions expected Wednesday into early Saturday.
Precipitation possible later on Saturday as a cold front moves
trough our area. Dry/VFR conditions return on Sunday.


With the southwest flow ahead of a cold front today, a Small
Craft Advisory is in effect for the waters. The SCA continues
through this evening for most of the Bay and lower Tidal
Potomac. Winds should decrease overnight as the gradient
relaxes. A cold front will pass through early Wednesday morning
and northwest winds will increase behind the boundary with gusts
20 to 25 knots most likely. An SCA is in effect for the waters.

High pressure will build over the waters for Wednesday night
through Thursday night. An SCA was extended through Wed night
for most of the Bay and lower Potomac.

Winds are expected to stay less than 20 kts Friday into
Saturday night. Winds will increase on Sunday as low pressure
moves NE offshore the Delmarva Peninsula, therefore, a small
craft advisory is anticipated on Sunday.


MARINE...Small Craft Advisory until midnight EST tonight for ANZ530>534-
     Small Craft Advisory from 6 AM to 6 PM EST Wednesday for
     Small Craft Advisory until 6 PM EST this evening for ANZ535-


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