Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 201524

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1024 AM EST Fri Jan 20 2017

A warm front will move into the area this afternoon, then will
stall out overhead through Saturday. Low pressure will impact the
area Sunday through Monday before moving away 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 moving across the region at
present, with temperatures in the upper 30s to mid 40s. 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. Steady rain will then taper off from west
to east during the afternoon hours, but low clouds and drizzle
will linger.

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.


Conditions deteriorating through the day as rain and low stratus
overspread the region. 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.

Sub-VFR 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 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 thresholds.

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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