Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 211432

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
932 AM EST Sun Jan 21 2018

High pressure will shift east off the coast of the Southeast
today. A cold front will approach the region Monday before
passing through Monday night into Tuesday. High pressure will
then return for Wednesday through Saturday before another system
approaches next Sunday.


Frontal zone is nearby this morning as moisture continues to
increase across the region this morning, noted by low level
theta-e advection. This has led to a strato-cumulus deck of
clouds across the area, with coverage varying from near
overcast across much of Maryland and across the terrain to
basically clear across central Virginia. Varying degrees of
cloud cover are expected to persist through the day as moisture
advection continues. With mean westerly low level flow, general
pattern of more clouds north, less south should be maintained.
Some light upslope rain showers remain possible along and west
of the Allegheny Front as well. Temperatures, while a bit cooler
than yesterday, should still be able to rise into the 50s, with
low 60s likely south where sunshine is more prevalent.


Warm advection/isentropic lift will continue tonight and Monday
as the high continue slipping eastward and the storm in the
Plains strengthens and moves towards the Great Lakes. However,
moisture and forcing will remain limited, so not expecting much
more than some clouds. A few showers or a little drizzle can`t
be ruled out along the Allegheny Front tonight, shifting towards
the Mason-Dixon line Monday morning, but otherwise expect dry
weather. Temps will be held up tonight with the clouds, and
should warm considerably on Monday thanks to the strong warm
advection and some limited sun reaching the surface. Lows
tonight should stay in the 40s overall, with highs Monday
reaching the 50s and 60s.

Strong cold front approaches from the west Monday night and
crosses the region Tuesday morning. Still some timing
differences among the models, but overall expect showers to move
across the region starting late evening Monday and exiting the
area Tuesday afternoon. Southerly low level jet combined with
strong forcing aloft and weak instability could be enough for a
low-topped squall line, which could mix down some stronger
gusts, and given the weak instability and strong forcing, have
included slight chance thunder late Monday night and Tuesday
morning. Rainfall totals look relatively light, mainly less than
an inch, since the system should be moving along fairly well,
but isolated totals could exceed an inch. Temperatures Monday
night will stay very mild, with 40s and 50s common, and may
spike back into the 60s on Tuesday across most of the area
either just ahead of, or just behind, the cold front.

Behind the front later Tuesday and Tuesday night, cold advection
on a gusty NW wind will get underway. This isn`t an arctic air
mass, but we do expect snow showers to get going across the
higher terrain. However, given the marginally cold air mass,
partly frozen lakes, and blocked flow, have kept pops in the
chance range for now. Further east, Tuesday night will be dry
and colder, but actually staying above normal for late January,
with lows mainly in the 20s to low 30s. Will need to watch
winds, especially in western Virginia, with a very strong 850
max coming through - we may need an elevation Wind Advisory here
later Tuesday into Tuesday night.


A secondary vort max looks to pivot around upper troughing that
will be situated over the northeastern CONUS on Wednesday. East
of the mountains, moisture is not that impressive or deep,
upper jet positioning is relatively unfavorable and the
shortwave is opening up (as opposed to deepening) as it crosses.
But there could still be a (non-accumulating) flurry or two as
the shortwave moves overhead Wednesday afternoon (GFS) or
evening (EC). The Great Lakes are largely frozen, so the
moisture source along the upslope areas likely won`t be
tremendous either, but it would likely still be enough for some
snow showers Wednesday afternoon/evening, gradually tapering off
through Thursday morning as drier air moves in and the upper
trough departs.

High pressure will then dominate the weather pattern later
Thursday into Saturday bringing dry weather. The high looks like
it will move off to the east as opposed to retreating to the
north, which would result in a warming southwesterly return flow
Friday into Saturday.

Another rather large/deep upper trough moves toward the area
Saturday night into Sunday which would likely result in
inclement (wet) weather for the second half of next weekend, if
timing holds. For what it`s worth, even when it was beyond day 7
the guidance has been in decent agreement already showing this
rather robust system passing to our west, bringing a soaking
rain to the region.


VFR expected through today despite low level strato-cumulus
deck as ceilings hold steady generally in the FL040-FL060
range. Tonight, increasing moisture even at low levels will
likely lead to cigs lowering to MVFR. There is a chance we reach
IFR late tonight with continued warm/moist advection from the
south, and some drizzle is not impossible, but the atmosphere
may not moisten enough to allow it, so confidence on this is
very low.

Any sub VFR cigs will likely go back to VFR for the balance of
Monday, with lowest confidence in this being at MRB/MTN where
the warm air may have more difficulty scouring out the low level
cool air, but even here, moisture looks limited in low levels.
Winds will increase from the south as a cold front approaches.

Sub-VFR, quite possibly IFR at times, later Monday night and
Tuesday morning as cold front crosses the region. May even get a
few thunderstorms to develop, with low level wind shear also
likely to be an issue. Outside chance that one of those storms
taps into the shear and brings it to the surface, which could
cause an isolate 40-50 knot gust, but odds on this are low.
Going back VFR Tuesday afternoon with gusty NW wind and staying
VFR Tuesday night.

Mainly VFR expected Wed-Thu. NW flow 10-20 kts and gusty Wed,
becoming light/variable late Wed night into Thu as high pressure
moves overhead.


Sub-SCA winds expected through tonight. Southerly flow increases
just above the surface on Monday, with winds likely to reach SCA
criteria over land. However, with very mild temps expected
(60s), expecting mixing over the water to be much poorer, so
have kept gusts below SCA criteria for now. Its possible there
is a little more mixing than currently expected, especially
right along southerly shores, so we may eventually need an SCA
for Monday for some waters, but there is little confidence in

Winds continue to increase just above the surface Monday night
as cold front approaches, so expect enough of that to mix down
to result in SCA gusts Monday night. Cold front pushes through
Tuesday with SCA likely. Could be close to gale at some points
Tuesday and Tuesday night, mainly in potential squalls Tuesday
ahead of the cold front, then with cold advection behind the
front Tuesday night, but confidence in this is also low at this

SCA gusts/possible gales Wednesday in the wake of a departing
cold front. Winds become light Thursday as high pressure builds
over the waters.


Increasing southerly flow ahead of an approaching cold front
will cause water levels to increase Monday into Tuesday. Minor
coastal flooding is possible, especially during the midday high
tide cycles Tuesday if the cold front isn`t through by then.




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