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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
526 PM EST Thursday Nov 27 2014

Synopsis...
an upper-level disturbance will pass through the area tonight and
high pressure will build overhead Friday. The high will move off
the coast during the weekend. A cold front is expected to move
through Monday.

&&

Near term /through Friday/...
surface high pressure will build toward the region tonight behind
the cold front...but another upper-level disturbance will swing
through the area during this time. A pressure surge will cause
blustery conditions and it will be cold due to a northwest flow.
An upslope northwesterly flow will bring accumulating snow to
locations along and west of the Allegheny Front. Moisture will be
partially lined up with the maximum dendritic snow growth zone
underneath the subsidence inversion. The subsidence inversion will
also remain relatively high and there is plenty of moisture around
as well. Therefore...a Winter Weather Advisory has been issued for
the ridge tops in West Virginia and Virginia along and west of the
Allegheny Front. Two to four inches of snow is expected across
these areas. A few snow showers may spill east of the mountains
across northern Maryland...extreme northern Virginia and eastern
West Virginia...but little or no snow accumulation is expected.
Forecast min temperatures will range from the teens along the Allegheny Front
to the low 30s near Washington and Baltimore.

The upper-level disturbance will move off to the east Friday while
high pressure builds overhead. Any leftover snow showers along the
Allegheny Front will diminish and sunshine is expected.
However...it will be cold due to the northwest flow ahead of the
building high. Maximum temperatures will be below climatology...ranging from the
20s in the mountains to the lower 40s in the Virginia Piedmont and
southern Maryland.

&&

Short term /Friday night through Saturday night/...
pattern shift as western Continental U.S. Ridge breaks down as an upper trough shifts
south into the Pacific northwest. An associated warm front will move east from
the Midwest states and into the northern mid-Atlantic. 00z European model (ecmwf)/12z GFS
suggest lift from this front will cause precipitation into the central
Appalachians Friday night. In spite of warm air advection...thermal
profiles cool enough for all snow. Limited probability of precipitation for now to the northwestern
three zones (west of the Allegheny front). Middle-level clouds
elsewhere...though plenty of cold air and little mixing will allow
for min temperatures in the low to middle 20s inland...upper 20s near shore.

Warm air advection continues through Saturday night in southwesterly flow
with plenty of cloud cover. Temperatures moderate through the
weekend...reaching normal by Sunday evening. Temperatures a blend of
mav/met and sref overnight/mosguide during the day. By the time
precipitation returns to western zones Saturday evening...it should be warm
enough for rain. Not expecting freezing rain at this time due to the
warm nose descending...will have to lookout for that though. Dry
east of the Allegheny Front with southwesterly/southerly flow 10 miles per hour or so
maintained through Saturday night in decent gradient.

&&

Long term /Sunday through Thursday/...
high pressure sets up offshore for the second half of the weekend
into early next week. This will set up a southerly flow and bring in
warmer temperatures...with highs a few degrees above normal expected
Sunday and Monday.

A cold front will approach from the west late Monday...and cross the
area through Tuesday. This will bring the region its next chance of
precipitation...but temperatures should be warm enough for all rain.
Some wet snowflakes may mix in once the cold front passes with any
leftover showers Monday evening along the Mason-Dixon line and west
of the Blue Ridge mountains Monday evening. Northwest flow behind
the front will likely result in upslope snow showers overnight
Monday night along the western ridges of the Allegheny Front once
temperatures drop...but airmass will be marginal so any accumulation
potential is questionable.

A strong area of high pressure /I.E. 1040 mb/ will build to the
north Tuesday into Tuesday night. Meanwhile...the aforementioned
front stalls to the south...then slowly returns north as a warm
front Tuesday night. There is a strong signal for cold air damming
in the models...with even lower resolution global models showing
temperatures below freezing as precipitation begins over much of the
Shenandoah Valley and the Potomac Highlands. European model (ecmwf) is warmer at 850
mb and deeper with low pressure to the west resulting in more
precipitation and suggesting a higher ice potential especially west
of the metropolitan. GFS is colder at 850 mb and weaker with low pressure
to the west suggesting lighter quantitative precipitation forecast and a more generic wintry mix of
snow and rain. For now...sticking with a chance of rain/snow...but
will definitely need to monitor and fine-tune over the coming days
as potential is there for overrunning precipitation. Over cold air to
result in wintry precipitation over at least part of the area Tuesday night
into Wednesday. Left out of severe weather potential statement for now because 30 percent
confidence is not there that we will be issuing hazards given large
spread in timing/temperatures.

Models/ensembles begin to diverge after this time in response to a
pattern change aloft with more zonal flow of Pacific origin taking
hold.

&&

Aviation /22z Thursday through Tuesday/...
VFR conditions are expected through Friday. Brief period of ceilings
around 3000 feet possible with light precipitation showers through 00z as a trough
moves through. A pressure surge behind this boundary will cause
northwest to gust around 20 knots tonight through much of Friday.
Winds will diminish toward Friday evening.

VFR Friday night through Saturday night. Calm Friday night then
southwesterly/southerly flow increases to 10-15 knots Saturday...10 knots through
Saturday night.

VFR expected Sunday west/ S flow 10-15 kts. Sub-VFR possible in scattered
showers Monday evening west/ frontal passage. Winds become west and diminish to less than 10
kts Monday. Flow becomes north then east behind frontal passage Tuesday west/ sub-VFR possible
in precipitation late Tuesday.

&&

Marine...
with wind gusts to 20 kts already being reported on multiple
mesonet...have gone ahead and begun the Small Craft Advisory on all the
waters as the reinforcing cold front moves through the waters. The
Small Craft Advisory remains in effect through Friday.

High pressure overhead Friday evening...with return flow increasing
Saturday. Southerly channeling expected with Small Craft Advisory likely by late morning
Saturday continuing well into Saturday night.

Southerly flow with gusts exceeding Small Craft Advisory criteria is
likely Sunday into Sunday evening ahead of a cold front that will
slowly cross the waters late Monday. Winds diminish as the front
passes...then stalls near the waters through the middle of next week.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...none.
Virginia...Winter Weather Advisory until 7 am EST Friday for vaz503.
WV...Winter Weather Advisory until 7 am EST Friday for wvz501-505.
Marine...Small Craft Advisory until 6 PM EST Friday for anz530>543.

&&

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