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Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1002 AM EST Tue Dec 6 2016

.SYNOPSIS...
Coastal low pressure impacts the area through tonight before high
pressure briefly returns Wednesday. A potent cold front will pass
through Thursday. Arctic high pressure builds overhead Friday into
the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
Rain will continue to spread northward across the Mid-Atlantic
region this morning. The precipitation has been fighting low level
dry air as evident on the 12Z IAD raob and has been light. Sleet
has been reported on the leading edge as evaporative cooling
occurs. Sub- freezing temperatures are present at higher
elevations above 1.5- 2k feet along and west of the Blue Ridge
Mtns. Freezing rain is expected at these locations through the
afternoon.

Any breaks of precipitation will fill in by afternoon and a steady
rain is expected through the afternoon and into the evening.
Moderate rain is also possible across central VA and southern MD
this afternoon. 

Previous discussion...

Low pressure over the Tennessee Valley this morning will transfer
its energy to a coastal low along the North Carolina coast this
afternoon. The coastal low will slowly move off to the east
tonight. Warm and moist air from the Gulf and Atlantic Ocean will
overrun the surface cooler air in place...resulting in a soaking
rain for most areas today into this evening. Rainfall amounts
around one inch are expected. Evaporative cooling at the onset may
result in sleet at the onset...but surface temps should be above
freezing for most locations. Precipitation will overspread central
Virginia around daybreak...eastern West Virginia into northern
Virginia and the the Washington Metropolitan area between 8 am
and 10 am this morning...and across northeastern Maryland by noon.  

A subfreezing layer of air aloft...generally above 1500 feet is
expected to develop due to the evaporative cooling that will take
place at the onset. This means that the ridges in the
Allegheny and Potomac Highlands as well as the Blue Ridge
Mountains will experience a period of freezing rain this morning
into this afternoon. Warmer air should work its way in
aloft...allowing for all locations to change to rain later this
afternoon into this evening. Winter Weather Advisories are in
effect for these areas. The main threat will be for slick spots on
elevated surfaces due to freezing rain with temperatures near 32
degrees. 

Precipitation will end from southwest to northeast this evening
into the overnight hours as the low slowly moves off to the east.

&&

.SHORT TERM /WEDNESDAY THROUGH WEDNESDAY NIGHT/...
High pressure will return for Wednesday into Wednesday evening.
Dry and seasonable conditions are expected during this time. A
cold front will approach from the west overnight Wednesday. A
strong jetmax ahead of the front may bring a few showers...but
details are uncertain at this time.

&&

.LONG TERM /THURSDAY THROUGH MONDAY/...
Strong cold front crosses the area Thursday into Friday. Potent
upper level dynamics...with 140 kt jet streak over the area by
late afternoon. Model spread with regards to QPF
continues...with GFS/NAM far more bullish than ECMWF. Best chance
for precipitation will be along/east of I-95...with snow showers
across the upslope areas. Elsewhere...p-type appears to initially
be rain as low-levels remain just warm enough. Though...could be a
changeover to rain/snow mix or all snow on the back edge of precip
shield. Any remaining precip will exit the area Thursday
night...with CAA commencing.

Coldest weather so far this season Friday and Saturday...as high
pressure builds into the area. Highs generally 10-15F below
normal...with most areas struggling to get out of the 30s...all
areas AOB 30F during the overnights. 

High pressure moves offshore Sunday...with southerly flow allowing
for slight airmass modification. Highs back into the 40s. Next
chance for precipitation arrives late Sunday into Monday as
another cold front approaches.

&&

.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
Cigs/Vsbys will deteriorate this morning as rain overspreads the
terminals from southwest to northeast. IFR/SubIFR conditions are
expected this afternoon through this evening. Conditions may
gradually improve late tonight as drier air works its way in
behind a departing coastal low. However...confidence with the
timing of improving conditions is low. Any leftover subvfr
conditions early Wednesday morning will give way to VFR conditions
as high pressure builds overhead. VFR conditions will continue
Wednesday night. 

Brief period of sub-VFR conditions possible in rain/snow showers
as cold front passes through the area Thursday. Return to VFR
behind the front into the weekend...though, gusty NW winds in
excess of 25 mph are possible Friday afternoon in the strong cold
air advection.

&&

.MARINE...
Coastal low pressure will develop along the North Carolina coast
today. The gradient will continue to strengthen and a Small Craft
Advisory is in effect for middle portions of the Bay and the lower
Tidal Potomac River. The Small Craft Advisory continues through
this evening...but there may be a break in the gradient overnight
as the low moves away. 

High pressure will approach for Wednesday and a northwest flow is
expected. Wind gusts around 20 knots are possible...but confidence
is too low at this time for a headline. Winds will become light
for Wednesday night.

Light winds across the waters through midday Thursday...before winds
increase above SCA criteria Thursday afternoon as cold front crosses
the waters. Solid SCA gusts continue Thursday night into Friday
morning. Increased mixing during the day Friday could lead to
marginal gale force winds...regardless...solid SCA will continue.
Winds drop back below SCA Saturday morning as large area of high
pressure builds overhead.

&&

.TIDES/COASTAL FLOODING...
Persistent and strengthening easterly flow will lead to an
increase in tidal anomalies today. Action stage currently
forecast at the sensitive sites for this evenings high tide cycle
(e.g., Annapolis, Straits Point)...though marginal minor tidal
flooding is possible if anamolies are slightly higher than
currently forecast.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...Winter Weather Advisory until 3 PM EST this afternoon for 
     MDZ502.
     Winter Weather Advisory until 7 PM EST this evening for MDZ501.
VA...Winter Weather Advisory until 3 PM EST this afternoon for 
     VAZ507-508.
     Winter Weather Advisory until 7 PM EST this evening for VAZ503-
     504.
WV...Winter Weather Advisory until 3 PM EST this afternoon for 
     WVZ050-055-502-504.
     Winter Weather Advisory until 7 PM EST this evening for WVZ501-
     503-505-506.
MARINE...Small Craft Advisory until midnight EST tonight for ANZ532>534-
     537-540>543.
     Small Craft Advisory from 1 PM this afternoon to midnight EST 
     tonight for ANZ531-538-539.

&&

$$
SYNOPSIS...MSE
NEAR TERM...BJL/HSK
SHORT TERM...BJL
LONG TERM...MSE
AVIATION...BJL/MSE
MARINE...BJL/MSE
TIDES/COASTAL FLOODING...LWX

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