Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 081507 AAA

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
1007 AM EST Fri Dec 8 2017

Weak surface ridging weakens today. A low pressure system will
impact the area tonight through Saturday. High pressure will
return Sunday into Monday.



06Z GFS/GEFS continued the westward trend that began yesterday
afternoon and continued last night. New snow probs show
potentially higher snow totals especially across our southeast
counties. We`ll not be making any headline adjustments or
changes to the expected or most likely amounts at this time
pending 12Z guidance but if fcst remains consistent or models
trend any further west we would need advisories to the Blue
Ridge mtns and warnings possibly for King George, St. Marys and
Calvert counties.

Previous discussion...

A ridge of high pressure at the surface will weaken through the
day today as southern stream low pressure develops off the
southeast U.S. coast and northern stream low pressure approaches
from the Great Lakes. An overcast day is anticipated as abundant
mid and high level clouds pour in ahead of a buckling upper
level trough. Temperatures should be about 10 degrees cooler
than yesterday as a result of the clouds and low December sun
angle: upper 30s to around 40 east of the mountains and upper
20s to lower 30s over the higher terrain.


The first widespread accumulating snowfall of the season seems
inevitable late tonight into Saturday for much of the area.
This is a significant change from the most likely scenario from
24 hours ago. A Winter Weather Advisory has been issued for the
I-95 corridor to account for this. What changed is 1) the
southern stream surface low ejecting off the southeast U.S.
coast appears a little slower and stronger, allowing the
northern stream upper trough to catch up and interact/partially
phase with it as it passes; and 2) the approaching northern
stream trough takes on a neutral (as opposed to positive) tilt,
placing the area more squarely in the favorable right entrance
region of an amplifying upper jet (quite strong, 200-knot max at
250 mb Friday night over northern New England). Therefore, what
was a lower (but non- zero) probability solution this time
yesterday now appears to be the most likely scenario.

Right entrance region jet dynamics coupled with mid-level PVA
and frontogenesis would suggest some banded snow is possible.
Snowfall events driven by strong jet dynamics tend to
overperform, so have upped QPF/snowfall amounts, with 2-4" for
the I-95 corridor, and measurable back to the I-81 corridor.
This is consistent with the latest GFS/Euro trends as well as
ensembles which are coming into better agreement. But this is
still subject to change. A sharp cutoff on the northwest edge of
the precipitation is possible, with locally higher amounts
likely in any banded precipitation (most likely east of the Blue
Ridge). Boundary layer temperatures may be a couple degrees
above freezing through much of the event, but strong dynamics
likely overcome this. Also, the sun angle is at about its lowest
and least impactful point of the year, so even though most snow
will fall during the daytime hours Saturday, it should still
accumulate fairly well.

As the surface low pulls away, snow tapers off west to east
Saturday afternoon and evening, though some flurries or a couple
snow showers could spill over the mountains ahead of the
northern stream trough. A decent round of upslope snow showers
appears likely along and west of the Allegheny Front Saturday
night, but subsidence behind the slightly stronger low offshore
may result in less appreciable snow showers east of the
mountains than previously thought. Of note, cold air pouring in
on strong northwest flow behind the low will likely cause any
residual moisture from snow that fell to re-freeze Saturday

Dry, cold and breezy conditions are expected Sunday as an
Arctic airmass plummets into the region.


The 500 mb trough axis pulls out Monday only to be reinforced
Tuesday-Wednesday. Such a pattern would likely favor upslope snow
showers; however, depending upon placement and timing of
forcing, flurries or even scattered snow showers would be
possible elsewhere too. These will be the considerations going
forward. Will have highest Tuesday PoPs (50%) for the Potomac
Highlands, and maintain 20-30 percent chance elsewhere. Will
also need to monitor for warm advection aloft Monday night,
which may trigger a mixed precipitation situation at onset.

Temperatures will be at or below normal through the extended
forecast, with the coldest period Tuesday night into Wednesday
in the wake of the trough axis.


VFR through much of the day, then IFR increasingly likely for
I-95 corridor terminals by daybreak Saturday through early
Saturday afternoon due to snow. Most likely accumulations range
from around an inch near the Blue Ridge to 2-4 inches in the
I-95 corridor. Locally higher amounts/briefly lower restrictions
are possible in banded snow (mainly a factor east of the Blue
Ridge). Accumulating snow is possible as far west at MRB. VFR
should return by Saturday evening.

Winds will be light out of the N/NE AOB 10 kts through
Saturday, then increase markedly out of the NW Saturday night
into Sunday 10-20 kts w/ gust to 30 kts.

Flight restrictions will once again be possible by Tuesday due
to scattered snow showers...affecting cigs and perhaps vsbys.


Winds will be relatively light through early Saturday as the
region is sandwiched between low pressure approaching from the
northwest and another of low pressure off to the southeast. As
these lows interact then move away Saturday night, gusty
northwest winds take hold and SCA will likely be needed, with
gales possible into Sunday before winds diminish.

The next impulse will come on Tuesday. The system will be potent
enough to anticipate an increase in winds to SCA thresholds.
Gale force winds appear likely Wednesday.


DC...Winter Weather Advisory from midnight tonight to 4 PM EST
     Saturday for DCZ001.
MD...Winter Weather Advisory from midnight tonight to 4 PM EST
     Saturday for MDZ006-011-013-014-016>018-503>508.
VA...Winter Weather Advisory from midnight tonight to 4 PM EST
     Saturday for VAZ052>057-506.


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