Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 161927

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
227 PM EST Tue Jan 16 2018

A cold front will cross the area tonight. High pressure will build
into the area Wednesday, then shift south and east of the region
later this week into the weekend.


Snow continues to gradually increase in coverage and slowly
push south and east ahead of an approach cold front/upper
trough. Forward progress is slow but steady. Expecting
appreciable snow to begin in the I-81 corridor between noon and
5 PM, and the I-95 corridor between 8 PM and midnight. Hi-res
guidance has been remarkably consistent on a "snow hole" over
much of the Shenandoah Valley in the lee of the Allegheny
highlands, with around an inch expected in the valley and around
inches on the ridges (the Allegheny Front itself is expected to
see 2 to 4 inches).

This relative minima in QPF extends east into northern Virginia and
central Maryland including the Baltimore/DC metros due to a weakness
between better jet forcing to the north and strong PVA with the 500
mb low to the south. Given that the one inch will be falling into
the morning commute Wednesday, an advisory has been issued for the
Interstate 95 corridor/metros. The advisory has also been expanded
to include portions of central Virginia closer to the upper
low/better moisture where 1 to 2 inches is expected. Areas adjacent
to better jet forcing in northern Maryland likely see 1 to 3 inches.

Despite favorable upper level dynamics, low level dry air intrusion
on NW flow begins by daybreak across much of the area, and this is
expected to eat away at the snow from north to south. Snow should
end around midnight across northwestern Maryland, around daybreak
from the northern Baltimore suburbs southwest into the Shenandoah
Valley and by mid-morning elsewhere, though some light snow may
linger across the central VA piedmont until noon as the upper low
moves across. If this drier air moves in quicker than currently
forecast, snow will end sooner and totals will be lower. On the flip
side, if upper level dynamics overcome low level dry air, snow
totals could be a little higher, especially considering the strength
of the upper jet and colder temperatures resulting in higher ratios
toward the end of the event. Overall, a relatively dry and powdery
snow is expected.

Temperatures will fall as the sun sets and precipitation starts,
quickly tumbling below freezing shortly after snow begins in any one
location. Temperatures are expected to then remain below freezing
through the day Wednesday as another Arctic airmass spills into the
region on gusty northwest winds.


Clearing is expected on brisk northwest winds through the night
Wednesday night. Low temperatures will fall into the teens for most
areas, single digits at the higher elevations. With a tight pressure
gradient and cold air advection in place, wind gusts could approach
Wind Advisory criteria over the higher elevations Wednesday into
Thursday. The combination of these strong winds and cold
temperatures could also result in dangerously low wind chills
approaching -10 F over the ridgetops. Winds should be somewhat
lighter at the lower elevations precluding both Wind Advisory and
significant wind chill threats in the lowlands, though it will be
far from mild (wind chills likely in the single digits).

As the building high pushes to the south of the area on Thursday,
winds become more westerly and temperatures quickly begin to
moderate both as warmer air advects in aloft and westerly
downsloping component helps boost surface temperatures east of the
higher elevations. Highs a few degrees on either side of 40 are
expected. If winds go light Thursday night under clear skies, the
ambient airmass in place could allow temperatures to drop below the
20s currently forecast.


Surface high pressure will cover the southeastern states on Friday
as an upper level trough axis slowly moves offshore. Heights build
aloft as a broad upper level ridge becomes more established across
the central and eastern U.S. Temperatures on Friday continue to
moderate with dry weather expected.

The weekend will feature high pressure settled over the southeastern
and mid-atlantic states with a continued moderation in temperatures,
10 to 15 degrees above normal and continued dry weather.

By Monday, the area of low pressure that developed in the central
plains on Sunday will move into the western Great Lakes region early
Monday. The low level flow will bring an increase in moisture and
continued relatively mild temperatures on Monday. Best rain chance
(not snow) will be Monday into Monday night. The cold front
extending from the Great Lakes surface low will sweep across the
region on Tuesday. 85 hPa temperatures behind the cold front on
Tuesday finally go back below freezing, with some upslope snow
possible in the western mountains.


Conditions at most terminals have returned to VFR as of early this
afternoon. MTN still has MVFR CIGS and this may continue immediately
adjacent to the Chesapeake Bay in light onshore flow. Lower CIGS are
still on the doorstep of BWI as well and likely re-develop around
sunset. Meanwhile, -SN is expected to move into MRB momentarily,
which will start causing VSBY to drop there.

-SN expected at all terminals tonight with a period of IFR likely.
LIFR can`t be ruled out in any briefly moderately heavy snow bands.
Winds will be light and variable tonight as the cold front causing
the light snow moves overhead.

NW flow behind the front should bring drying and a return to VFR
shortly after daybreak Wednesday. Winds could gust 25-30 kts into
Wednesday evening before gradually diminishing.

Expect VFR conditions through the extended. Due to the cold water
temperatures on the Potomac, there could be IFR or LIFR issues at
both DCA and MTN due to fog during the morning hours Fri-Sun.


Light/variable winds are expected over the waters as a cold front
moves overhead through tonight bringing light snow and lower
visiblities. Patchy fog is possible as well. NW winds increase
behind the front Wednesday morning into SCA territory, and likely
remain there well into Wednesday night. Winds should be lighter
Thursday, but SCA gusts are still possible.

With surface high pressure established, expect wind conditions
on the waters to be below SCA levels Friday-Sunday. With the
moderation in temperatures expected this weekend, some breakup
in ice will occur by late this weekend.


DC...Winter Weather Advisory from 9 PM this evening to 9 AM EST
     Wednesday for DCZ001.
MD...Winter Weather Advisory until 10 PM EST this evening for
     Winter Weather Advisory from 9 PM this evening to 9 AM EST
     Wednesday for MDZ011-013-014-016-503>506-508.
     Winter Weather Advisory until 7 AM EST Wednesday for
VA...Winter Weather Advisory until 10 PM EST this evening for
     Winter Weather Advisory from 9 PM this evening to 9 AM EST
     Wednesday for VAZ036>040-050>057-501-502-505>508.
WV...Winter Weather Advisory until 10 PM EST this evening for
MARINE...Small Craft Advisory from 3 AM Wednesday to midnight EST
     Wednesday night for ANZ530>533-535-536-538>542.
     Small Craft Advisory from 3 AM Wednesday to 6 AM EST Thursday
     for ANZ534-537-543.


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