Scientific Forecaster Discussion
Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
922 PM EST Wednesday Dec 17 2014
high pressure will build into the area through Thursday and
persist through Friday night..except for a weak disturbance that
will cross the area Thursday night. Low pressure will track south
of the area Saturday through Sunday. A secondary low is expected
to move up from the southeast coast early next week. A large area
of low pressure looks to develop in the Ohio Valley during the
middle of next week.
Near term /until 6 am Thursday morning/...
as of 02z...999mb surface low over southern Nova Scotia and 1031mb surface high
over far western Ontario. 4mb surface pressure gradient persists across
the County Warning Area...allowing occasional 20 miles per hour northwest gusts. Mixing persists
over night...so mins limited in spite of rather clear conditions
south of the first tier of counties from the Mason-Dixon line.
Will raise min temperatures a couple degrees for all but sheltered
Short term /6 am Thursday morning through 6 PM Thursday/...
sunny start to Thursday with middle-level trough entering the area
in the afternoon. Could squeeze out some snow flakes for the
alleghenies...but just higher clouds elsewhere. Downsloping westerly flow
with gusts to around 20 miles per hour...but colder air mass in place...only
middle 40s for maximum temperatures at lower elevations.
Long term /Thursday night through Wednesday/...
vertically stacked low pressure will drift toward Newfoundland
Thursday night while high pressure builds across the Great Lakes. A
dampening shortwave will approach in the fast southern stream.
Despite a few models trying to produce quantitative precipitation forecast in upslope areas...think
the moisture profiles only support an increase in middle level clouds.
Good guidance agreement with temperatures ranging from the middle 20s
in the mountains to lower 30s metros/east.
Dry and tranquil weather for Friday and Friday night as the surface
ridge builds in fully. However...low level winds take on a bit more
of a northwest direction behind the departing trough...leading to weak cold air advection.
Highs generally in the 40s. High/middle clouds increase Friday night
ahead of the approaching southern stream system. Speed/thickness of
clouds may determine how low temperatures drop since dew points remain low
with light winds. Trended toward slightly warmer GFS/European model (ecmwf) raw temperatures
with lows near or just below freezing.
Models continue to trend down the intensity of low pressure affecting the
middle-Atlantic over the weekend and take the highest quantitative precipitation forecast over the
Carolinas. In the last 3 days guidance has taken the system from a
potent closed low off Norfolk at 06z Sunday to an open wave with
the majority of its energy well to our south. Confidence is growing in
the current solution that the majority of precipitation with the
system...outside of the higher elevations...should be rain...and quantitative precipitation forecast
will be fairly low over the higher elevations where the temperatures are
lowest. Any accumulations should be focused along and west of the
Blue Ridge...though there is still the possibility for much of the
area to see at least a couple snow flakes. Precipitation should be out of
the area by midday Sunday. High pressure will build in Sunday
evening through midday Monday.
A large upper level trough developing over the central Continental U.S. Monday
will keep weather active through the middle of next workweek. Broad
surface low pressure progressing across the northern Great Plains
could help lift a stalled coastal front across the area as a warm
front sometime Monday night or Tuesday. Have included low end chance
probability of precipitation in this time frame to account for this. A wave of low pressure
may then form within the cold front of the larger parent low and
move through our area Tuesday night and Wednesday bringing high end
chance probability of precipitation into the forecast. Early impressions are that p-type
will be largely elevation dependent will lower elevations mostly
seeing rain. Overall confidence is low in the timing and details of
Temperatures should remain around normal through the long term with the
exception of Tuesday night and Wednesday/Post warm front...when
temperatures will be 5 to 10 degrees above normal.
Aviation /02z Thursday through Monday/...
VFR conds expeceted through Friday night. Northwest gusts to around 20 knots on
occasion tonight and more frequently Thursday. Flow weakens a bit
but remains northwesterly with pressure gradient Thursday night and Friday.
MVFR and IFR conditions are possible in showers as a wave of low
pressure passes well to our south Saturday night and Sunday morning.
northwesterly 18 to 20 knots surge currently crossing the waters late this
evening...with gusts returning to southern Maryland waters expected for the
first time in several hours. Small Craft Advisory for all the waters into Thursday
Winds will diminish Thursday evening. Northwest wind gusts Thursday
afternoon will be marginal for a Small Craft Advisory...but one may be needed for
portions of the waters depending on the efficiency of atmospheric
mixing. Light winds Friday night.
Confidence is low in the occurrence of any marine hazards in the
District of Columbia...none.
Marine...Small Craft Advisory until 6 PM EST Thursday for anz530>534-
Small Craft Advisory until 1 PM EST Thursday for anz535-536.