Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 101521
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1021 AM EST Sat Dec 10 2016

.SYNOPSIS...
High pressure over the center of the continent builds east today
before moving off the Mid-Atlantic coast Sunday morning. A warm
front develops over the area and lifts north Sunday before a cold
front crosses Monday. A reinforcing cold front crosses Wednesday
before cold high pressure builds back into the area.

&&

.NEAR TERM /THROUGH TONIGHT/...
Northwest flow continues across the Mid-Atlantic as high pressure
builds eastward from the Ohio and Tennessee Valleys. There will
still be enough of a gradient for gusty winds of 15-25 mph at
times, particularly the northern part of the area. While
accumulating snow is likely ending across the Allegheny Front,
traffic cams show flurries continuing to fall, so have added that
mention into the early afternoon. Max temps will range from the
20s at higher elevations to the 30s elsewhere...and perhaps a few
40s south/east.

Even though the high will be moving overhead tonight, mid-high
level clouds will increase from the west late tonight as low
pressure moves into the Central Plains. Warm air advection will
begin across the Ohio Valley and light snow showers are possible
across the northern Allegheny Front and Potomac Highlands late
tonight. Little to no accumulation is expected through Sunday
morning. Calm conditions will result in another cool night with
temperatures dropping into the teens/20s tonight...with the
thickness of cloud cover potentially impacting the outcome.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
Moisture advection will spread across the Mid-Atlantic region
Sunday. Snow showers are expected to spread eastward but hug the
MD/PA border as the area of best mid-level convergence shifts
northward into PA. High pressure will be off the Mid-Atlantic
coast by Sunday afternoon and southerly flow will increase across
the region. Temperatures will still be on the cool side due to
cloud cover across much of the region especially across the north.
An area of lower pressure will move northward along the SE U.S.
coast Sunday afternoon associated with Atlantic moisture. Rain
showers are possible across the Tidewater region and Southern MD
into Sunday night and amts will be light.

Low pressure will move into the Great Lakes region Sunday night
and warm air advection aloft will continue... 850mb temperatures
will cross 0 deg and keep rising into Monday morning. Sfc
temperatures are expected to drop into the low to mid 30s across
the region with warmer temperatures along and east of I-95. This
will make for multiple p-types across the region Sunday night into
Monday morning. Guidance shows sfc temperatures rising overnight
into Mon morning and after analysis of boundary layer winds it may
actually occur as SW winds will be 10-20kts. This can be a flaw in
model guidance as they can overestimate sfc warm air advection. A
wintry mix is expected across the region as precipitation moves
into the west Sunday night. Snow will likely change to sleet and
then freezing rain for places below freezing. As precipitation
moves eastward by Monday morning...SN/IP will change to rain.
Temperatures will climb into the 50s Monday and rain is definitely
expected. Overall...a brief period of ice accumulation is possible
mainly across the Allegheny Front and Potomac Highlands Sunday
night into Monday. Confidence is low at this time to issue any
advisories.

Decent gulf moisture associated with the advancing cold front
Monday will result in 0.25-0.5 of rainfall through Monday.
Clearing will occur by Monday night with snow showers ending along
the Allegheny Front. High pressure will move overhead Monday
night.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
Guidance is in somewhat better agreement through the long term
period of the forecast than it was last night. We start out on
Tuesday with a weak area of high pressure over the region which
looks to keep the weather dry with near normal temperatures. It
looks like Tuesday night stays mostly dry ahead of an approaching
cold front. On Wednesday however, latest guidance is notably in
better agreement regarding an approaching shortwave and vort max
helping to initiate cyclogenesis along the front as it crosses the
area, which could lead to a period of mixed precipitation or snow.
Still a long way out but we continue to watch this potential
closely. By Thursday, this system is long gone and we end up with
arctic high pressure and temps struggling to reach the freezing
point even in the warm spots. The arctic high settles right
overhead on Friday with continued very cold conditions.

&&

.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
VFR conditions expected through tonight. NW wind gusts to 20 kt
will be possible (except CHO) today. -SN may move into MRB late
tonight into Sun morning and may result in vsby restrictions
however confidence is low. -SN will move out of MRB by Sunday
afternoon and VFR conditions expected Sunday at all terminals.
Sub-vfr conditions expected Sunday night into Monday as moisture
advection occurs. -SN/IP/FZRA is possible at MRB during this time.
-SN/IP possible at other terminals however activity should become
-RA early Monday. RA expected at all terminals Monday. VFR
conditions expected Monday night- Tuesday. Sub VFR conditions
possible Wednesday as a storm may try to bring rain or snow to the
region again just ahead of the next arctic surge.

&&

.MARINE...
Enough pressure gradient remains across northern portions of the
area for SCA conditions to continue today. Expanded the SCA one
tier of zones south, including the Potomac near DC. Even though
the SCA was expired for the remainder of the waters, winds will be
on the threshold, so an SCA may need to be reissued based on obs.
Lighter winds expected with high pressure Saturday night into
early Sunday. Increasing southerly flow ahead of the next storm
system looks to warrant another SCA by Sunday evening.

SCA level winds expected Monday...dropping off Tuesday before
possibly redeveloping Wednesday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 PM EST this evening for
     ANZ530>533-535-538>541.

&&

$$
SYNOPSIS...RCM
NEAR TERM...ADS/HSK
SHORT TERM...HSK
LONG TERM...RCM
AVIATION...ADS/HSK/RCM
MARINE...ADS/HSK/RCM



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