Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 151939
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
239 PM EST Fri Dec 15 2017

.SYNOPSIS...
Low pressure near off the Delmarva coast will move out to sea
tonight. High pressure will return for Saturday into Saturday
night before weak low pressure passes through later Sunday into
Sunday night. Weak high pressure will build overhead for Monday
before a cold front passes through Tuesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Low pressure continues to develop off the Delmarva coast this
afternoon while an upper-level trough swings through our area. A
potent jetmax has caused the low and mid-level flow to back just
enough for weak overrunning...and extra lift is occurring since
our region is in the left exit region of the jetmax.

This has caused light snow to develop overhead this afternoon.
There have been a few reports of freezing drizzle as well. Will
continue with the Winter Weather Advisory for the Washington and
Baltimore Metropolitan areas. However...with the precipitation
developing mostly in the 4-7kft layer there has been a lot of
riming and accretion of super cooled droplets causing snow
pellets and even the few reports of freezing drizzle.
Therefore...snow ratios will not be quite as high as previously
thought despite the cold temperatures. Have tweaked snow amounts
down just a bit for this reason with most locations in the
Washington Metropolitan area receiving a coating to an inch. 1
to 2 inches are still most likely near Baltimore. Having that
all been said...even the light snow amounts will cause
treacherous conditions on untreated surfaces due to the cold
temperatures. Temps will remain below freezing through the
evening commute...so any snowy and icy conditions will remain in
place.

Precipitation is most likely to end between 3 and 5 pm near
Washington and by 6 pm near Baltimore.

Across the rest of the area...a few flurries have occurred
today but little or no accumulation has taken place. A few
slippery spots cannot be ruled out though due to the cold
conditions.

High pressure will build to the south tonight while the low
moves out to sea and the jetmax moves off to our east. Dry and
seasonably chilly conditions are expected with temps in the
lower to middle 20s across most areas. Any snow covered or icy
surfaces will remain that way through tonight.

&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
High pressure will remain to our south for Saturday and Saturday
night. A southwest flow around the high will usher in somewhat
milder conditions Saturday afternoon after a chilly start. Max
temps will be in the 40s for most areas.

High pressure will move offshore Sunday and a southerly flow
will usher in more moisture and somewhat milder conditions. Max
temps will be in the 40s for most areas with lower 50s in
central Virginia. An upper-level disturbance will pass through
late Sunday and Sunday night. A few showers are possible...but
forcing from this system will be weak so any precipitation that
does fall will be light.

&&

.LONG TERM /MONDAY THROUGH FRIDAY/...
Mild conditions expected during the long term
period with dry conditions on Monday as we sit between two
boundaries and southerly flow settles over our region. Some
showers could develop over higher elevations later on Monday
and into Tuesday, but still expecting mainly dry conditions over
our CWA.

A cold front approaches Tuesday night increasing PoPs over our
area, but QPFs are not impressive. High pressure settles in
behind the front Wednesday into Friday with winds become
northwesterly... then becoming southwesterly Thursday into
Friday as high pressure moves offshore.

Temperatures will be above normal in general with highs reaching
the 50s and 60s in most locations, except on Wednesday. Low
temperatures will be in the 30s and 40s, but colder on Wednesday
night.

&&

.AVIATION /19Z FRIDAY THROUGH WEDNESDAY/...
Light snow will end by 23z for the eastern terminals. MVFR to
IFR conditions are expected during times of snowfall. VFR
conditions are expected tonight through Sunday morning. A few
showers are possible later Sunday and Sunday night along with
MVFR conditions...but confidence in precipitation is low at this
time.

VFR conditions expected Monday into Wednesday. Maybe
sub-VFR conditions on Tuesday night as showers could move
through the terminals. Gusty winds expected on Wednesday, up
to 20 kt.

&&

.MARINE...
Low pressure will move out to sea tonight. A Small Craft
Advisory is in effect for the Maryland Chesapeake Bay south of
Drum Point and the lower Tidal Potomac River...closer to the
gradient. High pressure will build to the south for tonight
through Saturday...but a tightening gradient will result in SCA
conditions. The gradient will subside Saturday night and
Sunday. A weak disturbance will pass through Sunday night.

Winds are expected to stay below small craft advisory
threshold Monday into Tuesday. Winds could increase Tuesday
night into Wednesday behind a front, requiring a small craft
advisory over the waters.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Winter Weather Advisory until 7 PM EST this evening for DCZ001.
MD...Winter Weather Advisory until 7 PM EST this evening for
     MDZ004>006-011-013-014-503>508.
VA...Winter Weather Advisory until 7 PM EST this evening for VAZ054.
WV...None.
MARINE...Small Craft Advisory from 9 PM this evening to 7 PM EST
     Saturday for ANZ530>533-539>541.
     Small Craft Advisory from noon to 7 PM EST Saturday for ANZ535-
     536-538-542.
     Small Craft Advisory until 7 PM EST Saturday for ANZ534-537-
     543.

&&

$$
SYNOPSIS...BJL
NEAR TERM...BJL
SHORT TERM...BJL
LONG TERM...IMR
AVIATION...BJL/IMR
MARINE...BJL/IMR


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