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Area Forecast Discussion (AFD)

Issued by Baltimore/Washington (LWX)

FXUS61 KLWX 140236 AAA

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
936 PM EST Wed Dec 13 2017

An Alberta Clipper will pass nearby tonight through early 
Thursday. Developing low pressure will pass off the east coast 
late Friday and high pressure will return for Saturday. Low 
pressure may impact the area early next week.



Band of very light snow went through our local area here in
northern VA about an hour or two ago producing only a trace.
Snow has shifted north and east into Baltimore and northeast MD
where a few snow reports up to half inch have been received. After
06Z, all snow is expected to shift well north into Pennsylvania
with snow ending except in the upslope areas of far western MD
and eastern WV. Becoming breezy again late tonight into early
Thu and windy over the mtns where gusts could reach wind
advisory criteria. 

Breezy again tomorrow with highs only in the 30s and wind gusts
up to 35 mph in the morning diminishing in the afternoon.


Weak high pressure crosses the region Thursday night into early
Friday, with partial clearing and dry conditions with lighter
winds. Trough passing aloft however will generate a surface low
off the East Coast. While the chance of this low affecting our
region remains small, with the trough crossing the region have
kept slight chance to low chance pops for snow showers over the
region. Highs will cool a bit with the passing trough, with 30s
generally expected. System heads east of the region at night,
with drying likely, except for the upslope areas, where
accumulations are likely. Lows will be below freezing once


By Saturday, the upper level pattern will begin to change, with
upper level troughing pulling away to the northeast due to 
developing ridging downstream of a west coast trough. Surface 
high pressure will be building into the area, although 
temperatures may remain slightly below normal in continued 
northwest flow. The high will begin to move to the east on 
Sunday, which will allow a more southerly flow to develop with 
warmer temperatures.

For the remainder of the long term, while there is moderate 
model agreement on the large scale pattern, there are 
considerable smaller scale differences. One shortwave trough 
will be crossing late Sunday, with deeper troughing digging 
toward the eastern United States in the Tuesday to Wednesday 
time frame. These two time frames represent the best opportunity
for some precipitation, although with low confidence due to the
spread in timing, locations and moisture sources. With a milder
westerly flow aloft overall, temperatures should remain above 
normal (even above freezing at night), so any precipitation 
should be in the form of rain, except minimal chances for 
upslope snow.


VFR conditions at present have an increasing chance of
deteriorating this evening as a brief period of snow is looking
more and more likely at the terminals near and north of DC. Cigs
and vis are likely to drop to at least MVFR at BWI/MTN/MRB, and
quite possibly doing likewise at DCA/IAD. CHO is most likely to
stay VFR. Conditions should improve after midnight and
generallye expect VFR to dominate Thursday through Friday night,
though the risk of snow showers will be elevated again on Friday
as another weak system crosses the area. Winds will relax
tonight but become reinvigorated from the northwest on 

VFR with high pressure for Saturday and most of Sunday. A weak 
weather system could bring some showers late Sunday into Monday,
but it's uncertain at this time if sub-VFR conditions would 



Gale warning has been issued for the lower waters through 17Z
Thu. Marginal gusts up to 35 kt possible. SCA elsewhere through
Thu afternoon.

We then drop back below SCA over the course of Thursday night, 
but another system crossing the area could bring SCA back on 

Small Craft Advisory conditions will remain possible through 
Saturday in west/northwest flow. Winds will become lighter 
Saturday night into Sunday with high pressure. Southerly flow on
Sunday will become westerly on Monday as a weak system passes. 
At this time, it appears winds will remain below criteria 
through this period.


MD...Winter Weather Advisory until 7 AM EST Thursday for MDZ501.
WV...Winter Weather Advisory until 7 AM EST Thursday for WVZ501-503.
MARINE...Small Craft Advisory until midnight EST Thursday night for 
     Small Craft Advisory until 6 PM EST Thursday for ANZ530-535-
     Small Craft Advisory from 6 PM Thursday to midnight EST 
     Thursday night for ANZ534-543.
     Gale Warning until noon EST Thursday for ANZ534-543.