Area Forecast Discussion
Issued by NWS Sterling, VA

Current Version | Previous Version | Text Only | Print | Product List | Glossary On
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50
350
FXUS61 KLWX 101058
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
558 AM EST Sun Dec 10 2017

.SYNOPSIS...
A gyre of upper level low pressure over Hudson Bay Canada will
send multiple clipper-like systems through the area over the
next several days.

&&

.NEAR TERM /THROUGH TONIGHT/...
Small update early this morning as of 6 AM to boost sky cover a
bit this morning a mention a few flurries as energetic upper
level shortwave has been slow to cross the region. Any flurries
will be of minimal consequence with no accumulation expected.
Mainly sunny and dry conditions are still anticipated this
afternoon.

Previous discussion...
Weak and transient surface ridging will slide across the Mid-
Atlantic today. Upper troughing aloft remains, but we will be in
the subsidence region behind departing low pressure which gave
us the first accumulating snowfall of the 2017-18 season. This
will result in a partly to mostly sunny day, but breezy as a
reinforcing shot of cold air advects into the region.

A small but sharp 500 mb shortwave trough crosses the region
tonight. Believe this will bring an increase in sky cover and a
few scattered snow showers along the western ridges of the
Allegheny Front, but of little consequence. Winds should
diminish in the evening but it will be cold overnight with lows
generally in the 20s. Light winds and cloud cover would argue
against radiational cooling, but I could see a few favored
spots in the central VA piedmont briefly dropping a few degrees
below what is currently forecast.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
Another transient surface ridge will traverse the area Monday,
the axis of which should be east of the area by midday along for
light southerly flow and slightly milder temperatures (a few
degrees milder than Sunday). A mostly sunny day seems likely in
the wake of the departing Sunday night shortwave.

The big wheel of low pressure keeps turning into Tuesday,
sending another spoke (shortwave/clipper) towards the area. I
think it`s quite possible there is a little more light
precipitation than model guidance is explicitly printing out at
the moment due to a subtle coupled upper jet, strong mid-level
vorticity advection and low-level isentropic upglide.
Temperature profiles will still be quite cool, so precipitation
type should be wintry, though at the moment cold air appears
deep enough to result in snow or sleet as opposed to freezing
rain. The highest chances are along and west of the Allegheny
Front and near the Mason-Dixon line Monday night into Tuesday.

As the trough pivots through, another one fast on its heels
catches up and phases with it, resulting in cyclogenesis to our
northeast. The tightening pressure gradient behind this low as
well as strong cold air advection in the wake of the upper
trough will result in strong and gusty northwest winds.
Trajectories are favorable for upslope snow showers and squalls
and substantial accumulations are possible. The strong winds and
very cold air (-12 C or colder at 850 mb) may also result in
wind chills well below zero over the ridges of the Allegheny
Front as well. Both Winter Weather and Wind Chill Advisories
could be required for these areas.

Despite the very strong cold air advection, model wind fields
suggest winds should stay mostly below Wind Advisory criteria,
but it is quite possible gusts over 45 mph may be observed for a
time Tuesday into Tuesday night, even at lower elevations.

&&

.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
Windy and cold Wednesday as an area of high pressure builds in
behind a departing storm system. Winds should diminish Wednesday
night but temperatures will not modify. They will stay cold.

As winds back around from the northwest to the southwest Thursday,
temperatures will not be as chilly. There is a chance for snow
showers across much of the region Thursday, particularly in the
western zones and along the Mason-Dixon region, as the next storm
system pivots across the lower Great Lakes region. This chance
will linger into Thursday night and Friday as well due to the
upper level cold air support behind the surface storm.

A modifying area of high pressure is expected to build eastward
behind the low pressure system Friday night through Saturday night,
bringing milder temperatures and temperate southwesterly breeze.

By Sunday, the next threat for precipitation will come with a
cold front sagging southward across the region. High
temperatures, next weekend, could reach the 50s.

&&

.AVIATION /12Z SUNDAY THROUGH THURSDAY/...
Conditions have improved to VFR for most terminals as of early
Sunday morning, and VFR should persist through Monday. Flight
restrictions are possible Monday night into Tuesday as a frontal
system approaches bringing the potential for lower ceilings or
brief visibility restrictions. A wintry mix is possible, most
likely north of a line from MRB to MTN Monday night.

Winds will become a bit gusty shortly after daybreak out of the
northwest (10-20 G 25+ kts). Expect winds to abate with the loss
of daytime mixing and a relaxing pressure gradient by nightfall.
Light southerly flow AOB 10 kts is expected Monday into Monday
night, then gusty NW flow returns Tuesday.

VFR conditions Wednesday through Thursday night. Any brief snow
showers could reduce conditions briefly. Winds northwest 15
knots gusting 20 to 25 knots Wednesday, diminishing Wednesday
night, before becoming southwest at 5 to 10 knots Thursday and
Thursday night.

&&

.MARINE...
18-22+ kt gusts were being observed over the waters as of early
Sunday morning. As a cold airmass surges toward the region and
mixing increases over relatively warmer waters after daybreak,
anticipate solid 20-30 knot gusts over the waters. The gradient
relaxes late this afternoon so gusts should diminish in concert
with the setting sun. Winds are then expected to remain light
through Monday. Southerly channeling is possible ahead of a
frontal system Monday night which may require a Small Craft
Advisory. Strong northwest flow behind said frontal system will
likely warrant the issuance of a Gale Warning Tuesday into
Tuesday night.

Small craft advisories likely Wednesday and gales possible.
Otherwise, no marine hazards expected Wednesday night through
Thursday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...Winter Weather Advisory until 9 AM EST this morning for VAZ503.
WV...Winter Weather Advisory until 9 AM EST this morning for WVZ501-
     505.
MARINE...Small Craft Advisory until 6 PM EST this evening for
     ANZ530>543.

&&

$$
SYNOPSIS...DHOF
NEAR TERM...DHOF
SHORT TERM...DHOF
LONG TERM...KLW
AVIATION...KLW/DHOF
MARINE...KLW/DHOF



USA.gov is the U.S. government's official web portal to all federal, state and local government web resources and services.