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Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
310 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.


Near term /through Thursday/...
weak ridging of surface high pressure is over the area at
present, but is quickly moving east. Wind is slowly but surely
diminishing as it starts to shift from northwest to south. The
system which affected the area yesterday is now over the
Canadian martimes, while an Alberta clipper is over northern
Illinois heading southeastward.

Tonight, this clipper will move by near or just north of the
cwa, crossing eastward along or just north of the Maryland/PA state
line. However, a warm front will advance northeast ahead of it,
resulting in significant isentropic lift over our region. While
surface air remains dry, the lift looks strong enough to
generate some light snow across northern portions of the cwa,
potentially reaching District of Columbia itself. Model guidance has shifted
southward the southern extent of precip, so that while the
operational NAM and GFS are still dry in DC, the operational
global and regional Canadian, the hrrr, and ec all now bring
measurable to District of Columbia itself. While the amounts will be light,
generally less than 1 inch, and probably less than a half inch
in District of Columbia if it occurs, with cold ground temps leading into it, the
potential for icy/snow covered roads is elevated.
Fortunately, nearly all guidance holds the precip back until
after 7pm, so the bulk of the rush hour should stay dry, but as
we head beyond 7pm the odds of encountering light snow increase

The period of accumulation across the Metro looks quite brief,
accompanying the warm advection period this evening, and after
midnight we should dry out by comparison. This can't be said
further northwest along the Allegheny Front, where upslope snow
showers will continue through the night. Thus, the Winter
Weather Advisory for 2-4 inches remains in effect in this

With the warm front crossing the region, temps will likely drop
down to the high 20s across the Metro early before staying
steady or even slowly rising late tonight. Colder temps will
dominate along the Allegheny Front.

Cold front crosses the region early Thursday, and while a snow
shower can't be ruled out with it, the impact at this moment
looks lower than potential impact this evening. The bulk of the
day will be behind the front as the low pressure heads east into
the Atlantic, with a gusty northwest wind but temps actually
rebounding as a slightly warmer air mass dominates. Upslope snow
will also end early. Highs will rise towards 40 in many areas.


Short term /Thursday night through Friday night/...
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


Long term /Saturday through Wednesday/...
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.


Aviation /20z Wednesday through Monday/...
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 District of Columbia. 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


Small Craft Advisory remains in effect for all waters through rest of today, then
drops off the more sheltered waters tonight as winds shift
southerly and we get some channeling ahead of an approaching
cold front. This front crosses the area early Thursday and Small Craft Advisory
redevelops across all waters. We then drop back below Small Craft Advisory over
the course of Thursday night, but another system crossing the
area could bring Small Craft Advisory back on Friday.

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.


Tides/coastal flooding...
strong northwest winds have caused tidal anomalies to drop
sharply. Anomalies are however already starting to rebound this
afternoon through tonight as winds turn to the southwest. They
should drop again on Thursday behind a cold front, but likely
not as much as they dropped last night.


Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...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 this evening for anz535-
Small Craft Advisory from 8 am to 6 PM EST Thursday for anz535-
Small Craft Advisory until 6 PM EST Thursday for anz530.


near term...rcm

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