Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 170258

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
958 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.


Snowfall this evening has been relegated mainly along our
northern and western zones, slowly moving eastward across
western MD and along the Allegheny Front. Reported observations
so far this evening have been ranging 1 to 3 inches across
northwestern MD and the WV panhandle, with lesser amounts to the
adjacent areas to the south and east. Model guidance continues
to depict lower QPF, thus reduced snow amounts to the east of
these areas overnight and early Wednesday morning. As such, have
adjusted snow totals down, with a dusting to up to an inch
looking more likely in the metro areas.

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 a half inch to up to an inch is
possible into the morning commute Wednesday, an advisory
continues for the Interstate 95 corridor/metros. The advisory
also includes portions of central Virginia closer to the upper
low/better moisture where an inch to an inch and a half is

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
will end by 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

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.


Light snow is expected across the terminals tonight, resulting
in MVFR/IFR conditions. Expect light and variable winds for much
of the overnight period before starting to increase out of the
northwest towards daybreak as a cold front crosses the

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 and variable winds expected today as a cold front
approaches from the west. 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 until 9 AM EST Wednesday for DCZ001.
MD...Winter Weather Advisory until 10 PM EST this evening for
     Winter Weather Advisory until 9 AM EST Wednesday for MDZ011-
     Winter Weather Advisory until 7 AM EST Wednesday for
VA...Winter Weather Advisory until 10 PM EST this evening for
     Winter Weather Advisory until 9 AM EST Wednesday for
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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