Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 101530
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1030 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/...
Cloud coverage has varied this morning across the CWA with a
swath of stratocumulus and mountain waves visible on satellite
imagery stretching from the panhandle of WV eastward across
northern VA and the D.C. metro area. Areas along and north of
Interstate 70 in MD have seen skies clear a bit this morning,
and across southern portions of our area in central VA. These
clouds are in response an upper level shortwave slowly exiting
the region and cold air advection. Snow showers have been
reported in the far western zones of WV thanks to an upslope
component with some minor accumulations.

Clouds will continue in those ares mentioned above for the
remainder of this morning before clearing out this afternoon,
allowing for the sun to make a return. Temperatures will be on
the cool side and winds are expected to pick up by early
afternoon and become gusty as clouds start to clear.
Temperatures today will struggle to breach the 40 degree mark,
remaining in the upper 30s for most.

A weak shortwave trough will approach the area tonight, bringing
an increase in cloud cover, but not much in the way of fanfare
in terms of precipitation. The exception would be a few
scattered snow showers along the western ridges of the Allegheny
Front. Winds will diminish this evening and become light out of
the west. Low temperatures will range in the mid-upper 20s,
likely toward the higher end of that range in areas where cloud
cover remains.

&&

.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 /14Z SUNDAY THROUGH THURSDAY/...
VFR conditions forecast today across all terminals, and will
persist through Monday. Winds will become gusty by this
afternoon out of the west, gusting to near 20 knots. Winds will
diminish this evening, becoming light and holding on to a
westerly component. Light southerly flow AOB 10 kts is expected
Monday into Monday night, then gusty NW flow returns Tuesday.

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.

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...
Potential exists for gusty winds over the waters today as a cold
airmass surges in to the region. Mixing over relatively warmer
waters could produce gusts of 20 to 25 knots today.

Gusts diminish over near shore waters, but likely persist well
into the night over open waters. Winds are then expected to be
light on Monday. Southerly channeling is possible ahead of a
frontal system Monday night which may require another Small
Craft Advisory. Strong northwest flow behind said frontal system
will likely necessitate a Gale Warning Tuesday into Wednesday.

No marine hazards are expected Thursday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 PM EST this evening for
     ANZ530>533-535-536-538>542.
     Small Craft Advisory until 5 AM EST Monday for ANZ534-537-543.

&&

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



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