Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 200814

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
314 AM EST Fri Jan 20 2017

A warm front will move into the area today and it will stall out
overhead through Saturday. Low pressure will impact the area
Sunday through Monday before finally moving away from the area
Tuesday. A cold front will pass through the area Wednesday into
Wednesday night and an upper-level trough may build overhead late
next week.


Wave of warm air advection-driven rain will overspread the region
from southwest to northeast over the course of the morning.
Temperatures out ahead of the rain have fallen to near freezing in
some locations, but they are expected to rise into the mid 30s to
low 40s by onset time, so no ice is anticipated. Forcing
associated with the rain bands will be weakening through the day
as wave of low pressure moves northward into the lower Great Lakes
and weakens, with warm air advection weakening as a result as
well. Therefore rain amounts will be light, generally from one to
two tenths of an inch. Rain will then taper off from west to east
during the afternoon hours, with low clouds and drizzle possibly
lingering after widespread rain ends.

Temperatures will be above normal for late January, more mild
south and west across the Shenandoah Valley and portions of
eastern WV where rain ends earliest, coolest north and east across
Maryland. Highs from the mid 40s to mid 50s.


With little flow and lingering low level moisture, widespread low
clouds and areas of fog and drizzle are expected overnight and
into Saturday morning. As moisture thins, expecting improvement
for areas west of the Blue Ridge by Saturday afternoon, with
breaks of sun even possible. For locations east, solid overcast is
more likely, although fog/drizzle should come to an end.

Lows tonight around 40F under clouds/drizzle/fog, rebounding to
the low to mid 50s for areas east of the Blue Ridge on Saturday,
and low to mid 60s west.

As large upper level low digs into the southern US Saturday into
Sunday, deep south/southwest flow aloft develops along with low
level easterly flow, establishing a moisture feed with the
Gulf/Atlantic. This in combination with multiple waves of
energy/moisture will lead to several rounds of rain moving across
the region for the latter half of the weekend and into next week.


Cutoff surface low pressure in the southern branch of the jet
will track northeast off the North Carolina coast before emerging
off the Delmarva Coast Monday night. The low may stall out or
remain nearly stationary off the Delmarva coast as it runs into
blocking high pressure downstream...but the blocking high should
dissipate Tuesday and the low will track off to our north and east
during this time.

A soaking rain is expected Monday through Monday night. Moisture
from the Gulf of Mexico and Atlantic will overrun the surface
cooler air in place...and strong forcing is expected with the low
being close by. Latest GEFS indicates around 1-2 inches of rain
possible during this time. The cutoff upper-level low associated
with the surface low is expected to track overhead Monday through
Monday night. This may cause enough dynamical cooling for rain to
mix with or even change to a period of snow across the ridge tops
of the Catoctin...Blue Ridge and Potomac Highlands. Details remain
highly uncertain at this time but it does appear that the main
ptype in the valleys will be rain.

Drier conditions are expected Tuesday as the low moves away from
the area...but a couple showers cannot be ruled out. A cold front
will pass through the area late Wednesday into Wednesday night.
Unusually mild conditions are expected ahead of the boundary. More
seasonably chilly conditions are expected behind the boundary late
next week.


VFR conditions to start this morning will markedly deteriorate
through the day as rain and low stratus overspread the region.
Initially VFR/MVFR expected as rain overspreads the area, but will
see widespread IFR/LIFR develop during the afternoon hours and
persist into the evening.

Areas of drizzle, fog, and low stratus will continue tonight and
into Saturday with IFR/LIFR conditions expected. Some improvement
expected Saturday afternoon, especially MRB/CHO.

Additional Sub-VFR conditions expected Saturday night through at
least Sunday night with additional periods of rain likely.

IFR/Sub-IFR conditions are likely Monday through Monday night
along with rain. Drier conditions are expected Tuesday as the low
moves away from the area. Gusty northeast winds Monday will turn
to the northwest later Monday night and Tuesday.


Sub-SCA conditions through Saturday night with southerly winds
today, gradually turning southeasterly on Saturday. Winds increase
markedly for Sunday and Sunday night out of the east/northeast
with Small Craft Advisory conditions likely Sunday, with Gales
possible Sunday night.

Low pressure will move northeast toward the Delmarva Coast Monday
night before slowly moving away from the area Tuesday. Gusty winds
are likely with the low close by. A Small Craft Advisory will
likely be needed Monday and Tuesday. A Gale Warning may be needed
for portions of the waters Monday through Monday night.

A cold front will pass through the waters later Wednesday into
Wednesday night.


Tidal Anomalies have decreased a bit due to the light winds. A
southeast wind will develop today...but speeds will remain light.
A light onshore flow will persist through early Sunday. Anomalies
may not drop too much from where they are now...but they should
remain low enough for water levels to remain below minor flooding

The onshore flow will strengthen later Sunday through Monday.
Tidal flooding is possible during this time. The flow will turn
offshore later Monday night and Tuesday.




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