Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 121439

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
939 AM EST Sun Nov 12 2017

High pressure will move east off the coast today. Low pressure
will then affect the region Sunday night into Monday, followed
by high pressure on Tuesday and Wednesday. Another front will
cross the region early Thursday with more high pressure for


Surface high pressure will continue shifting offshore through
today, with dry weather and light winds. Plenty of high clouds
are streaming overhead this morning, and while these will
generally persist through the day, they may tend to thin or
scatter out somewhat for a time later this afternoon and into
the evening before lower/mid clouds increase, leading to a few
breaks of sun. Otherwise, temperatures will be milder than
yesterday but still below normal with highs from the mid 40s to
around 50F.

Tonight is very tricky. We have a stronger shortwave moving into
the area, but it is still relatively weak and moisture-
starved. The surface low will traverse the Ohio Valley, then
dissipate as it crosses West Virginia and energy transfers to a
new low pressure developing near the VA/NC border. This sort of
energy transfer near us is always tricky, as it can often result
in both a persistent wedge of cold air, as well as most of the
precip bypassing the region. So, we have the questions of 1) How
much of the area will see precip overnight and 2) Will it be
cold enough where precip does occur for any of it to be icy or
snowy. Atmospheric profiles from various models are very
borderline, with many showing warm surface layers, but given the
relatively dry low levels and potential for a little radiational
cooling in the evening before thicker clouds move in overnight,
there remains the possibility of temps getting lower than
guidance suggests. However, the other fly is the precip itself
happening, and the latest guidance has trended a bit drier, with
more separation between the primary and the secondary low
pressures and their associated precip. Thus, do not feel any
more confident about any wintry precip than I did 24 hours ago,
and will maintain existing mention of potential wintry weather
as it was inherited. Certainly this is not a problem for the
I-95 corridor, with the main concern being north-central and
northwestern MD and eastern WV, along with the northern
Shenandoah Valley in VA. Most likely, any advisories will be
issued either very late today or this evening if their need
becomes more clear. Lows tonight will be chilly, but with clouds
moving in, feel most of area except region outlined above will
stay above freezing, despite some stat guidance to the contrary.


Most guidance has the secondary low pressure rapidly taking over
from the primary low early on Monday, with precip moving
out/falling apart over our western zones except for favored
upslope areas on the Allegany Front. Showers will likely linger
over central MD much of the morning, but should move out by
afternoon, with some sun breaking through. With a more Pacific
influence coming in behind this system, expect highs to actually
end up a smidge warmer than recent days, though this is
dependent on the sun breaking through. If for some reason the
clouds remain persistent, it could end up cooler than Sunday.

Any lingering upslope rain showers on the Allegany Front could
become snow showers on Monday night - otherwise, Monday night
through Tuesday night should be dry as high pressure moves in.
Highs Tuesday should have a better shot at getting into the 50s.


A vertically stacked ridge axis will pivot across the eastern
U.S. Wednesday resulting in seasonable temperatures and partly
to mostly clear conditions.

A cold front will cross the region from west to east Wednesday
night into Thursday morning. Though at the surface it will be
weakening, the mid-to-upper trough will be sharpening some,
which should still result in a few scattered showers.
Temperatures should be warm enough to support liquid as the
precipitation type. More clouds than sun and a few sprinkles or
a shower could linger Thursday as the upper trough axis pivots
across. 00z GFS/ECMWF are similar with timing/strength of the
shortwave, but GFS has a little more low- level moisture
resulting in upslope showers Thursday afternoon over the
Allegheny Front, with just cold enough air for a few wet
snowflakes by nightfall as precipitation ends.

A much stronger storm system will affect the area later Friday
into the first half of the weekend as a result of a deep trough
diving across the Great Lakes. We should be in the warm sector
for this with liquid rain showers and breezy conditions the most
likely outcome Friday night into Saturday, with upslope snow
showers plausible over the highlands as the system departs
Saturday night. Long range models/ensembles are in better than
average agreement, lending to a relatively high confidence


VFR through today. CIGS lower tonight as weak system moves in,
with MVFR looking most likely later tonight as a little rain
moves through. We could see some MVFR or even high end IFR vis
if there is enough moisture from the rain to generate mist late
tonight and early Monday. Should return to VFR on Monday
afternoon and remain VFR through Tuesday night, though a little
patchy fog is not impossible in the morning. Winds generally
less than 10 knots.

Patchy fog is possible Wednesday morning. Otherwise mainly VFR
Wednesday-Thursday with southerly flow of 5 to 10 knots.


High pressure today will keep winds light, while weak low
pressure Monday looks unlikely to bring winds up to SCA levels,
even as it pulls away Monday night. High pressure returns
Tuesday and Wednesday.

SCA conditions are possible Thursday behind a cold front.




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