Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 070134
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
834 PM EST Wed Dec 6 2017

.SYNOPSIS...
Surface ridge will remain in place through Friday.
Low pressure will pass to the east Friday night, then another
low pressure will cross the region Saturday night. High pressure
will then return Sunday into Monday.

&&

.NEAR TERM /THROUGH THURSDAY NIGHT/...
A longwave upper-level trough will remain positively tilted over
the Great Lakes into the southern Plains through Thursday night
while surface high pressure continues to build over the central
CONUS.

A southwest flow aloft will continue tonight through Thursday
night ahead of the longwave upper-level trough...and a light
westerly flow is expected during this time ahead of the building
high over the central CONUS. The light westerly wind will
continue to usher in a noticeably chillier and dry airmass
during this time.

A bkn/ovc deck is expected tonight across the central and
southern portion of the CWA due to the right entrance of an
upper-level jet. Farther northwest...there should be more breaks
in the cloud deck. Min temps will range from the mid and upper
20s/near 30 across the Potomac Highlands into the Shenandoah
Valley to the lower an middle 30s across most other locations
where cloud cover will persist.

There may be a break in the cirrus deck Thursday morning through
midday as the upper-level jet weakens and moves off to the north
and east. However...another upper-level jetmax will move in
Thursday afternoon and Thursday night...causing more high/mid-
level clouds. In fact...there could even be a few snow showers
along/west of the Allegheny Front late in the day into Thursday
evening due to the forcing from this disturbance along with weak
cold advection and an upslope component to the low-level flow.
Elsewhere...dry and chilly conditions will persist through
Thursday night despite the return of more clouds.

&&

.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
Low pressure will develop offshore along a stationary front
Friday night before moving well out to sea Saturday. Guidance
have remained generally inconsistent on how far northwest precip
shield gets, but latest guidance suggest that southern MD
should see some precip Fri afternoon and night. If precip were
to begin Fri, it would probably start as rain in southern MD
then mix and change to snow Fri afternoon and evening. There is
potential for some light snow accumulations across the southern
half of St. Marys county mainly Fri night into early Sat. There
is still some uncertainty on how far NW precip shield gets, so
some adjustments to PoP, QPF and snow totals may be required.
Dry low levels below 850 mb could also result in precip
evaporating before reaching the ground especially in northwest
St. Marys, northern Calvert and Charles counties and points
northwest. An inch or two of snow appears the most likely
scenario across the southern half of St. Marys county with a
trace or nothing northwest of there.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
We may have ongoing light precipitation across portions of the
region Saturday morning as the coastal wave moves northeastward.
Uncertainty remains high along the western flank of the precip
shield, but greatest chances of accumulating snow will be across
southern and eastern Maryland.

At the same time, a potent upper level trough and strong cold front
will be digging through the Midwest during the day, and will cross
the region Saturday night. This has the potential to bring a period
of snow showers and possibly some squalls late Saturday and Saturday
night, lingering into Sunday morning across the upslope regions
along the Allegheny Front. Behind the front, blustery and cold
conditions are expected for Sunday.

The next fast moving upper level feature and re-enforcing cold front
will move towards the east coast late Monday into Tuesday with
chances for some precipitation, followed by cold and blustery
conditions Wednesday.

Temperatures throughout the period will be below normal, possibly
significantly below, especially Sunday and Wednesday.

&&

.AVIATION /01Z THURSDAY THROUGH MONDAY/...
VFR conditions through Fri night. Any precip aloft Fri night is
not likely to cause any flight restrictions.

Potential for sub-VFR conditions in light precipitation Saturday
morning, with greatest risk at BWI/MTN/DCA. Additional periods of
sub-VFR conditions are possible in snow showers or possibly squalls
Saturday evening and Saturday night. VFR is expected Sunday into
Monday, with gusty northwest winds expected Sunday.

&&

.MARINE...
Winds have diminished for the time being but are
expected to pick up a bit late tonight across the southern
waters. Have modified SCA to just include the southern waters.
Winds may strengthen again some Fri night and reach SCA. For
now, will hold off on issuing any SCA given it appears a
borderline event.

Sub-SCA conditions expected Saturday, but winds will increase
Saturday night and Sunday associated with a strong frontal
system, and Small Craft Advisory conditions are likely.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from midnight tonight to 10 AM EST
     Thursday for ANZ533-534-537-541-543.

&&

$$
SYNOPSIS...BJL/LFR
NEAR TERM...BJL/LFR
SHORT TERM...LFR
LONG TERM...MM
AVIATION...BJL/MM/LFR
MARINE...BJL/MM/LFR



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