Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 090833
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
333 AM EST Sat Dec 9 2017

.SYNOPSIS...
Low pressure will bring the first accumulating snow of the
season to much of the area through today. A cold front will
cross the region tonight. A couple clipper-like systems will
graze through the area next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Clouds continue to lower and thicken as snow develops aloft and
low levels saturate further. The going snowfall forecast
appears on track, albeit a bit slower. Due to the slower onset,
adjusted start time of WSW along PA border back a couple hours
(starting around daybreak), and nudged snowfall amounts to
tighten the gradient.

There has been a lot of convection developing offshore of the
Carolinas early this morning. Without a negatively-tilted upper
trough and strong easterly 850 mb flow advecting in moisture,
this convection may act to rob some precipitation (moisture used
up in the convection further south instead of producing snow
farther north). The latest HRRR/RAP subsequently have slightly
lower QPF amounts but it is still within tolerance with the
current forecast, so would rather stay the course for now and
monitor trends rather than make any substantial changes.

Northern and southern stream upper jets phase over the area
this morning resulting in widespread light to at times moderate
snow. A few bands of heavier snow are likely given the strong
dynamics aloft. Low level thermal profiles show just enough
warming for a little mix with sleet in parts of extreme southern
Maryland, but considering they`ve already had a few inches of
snow storm totals should still be in the 5 to 8 inch range.
Generally 3 to 4 inches is anticipated in the metro areas, with
1 to 3 inches as far west as I-81.

Overnight guidance hints at some lingering light snow/snow
showers until around nightfall in the I-95 corridor. Precipitation
should become more showery in nature by this time as well as
more isolated and sporadic. Western slopes of the Allegheny
Front likely see a couple inches of snow as the northern stream
wave/clipper moves across tonight. Held off on advisories for
now since timing/amounts are threshold (2-3 inches in 12-18
hours). Some dry air could cut down on totals over the ridges.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
Sunday looks dry, cold and breezy as an Arctic airmass moves
into the area. Highs struggling to get out of the 30s and wind
gusts up to 30 mph likely result in wind chills in the teens and
20s through the day.

Another shortwave pivots across overnight Sunday night. This
shortwave is much more pronounced in the NAM/GFS than the ECMWF,
but still believe there will be enough forcing with the
shortwave to get at least some scattered upslope snow showers.

Temperatures will be a couple degrees warmer on Monday with less
wind making it a bit more comfortable. All guidance then shows
another shortwave/clipper late Monday night into early Tuesday.
Warm air advection aloft over top cold air near the surface may
result in a period of mixed wintry precipitation late Monday
night into early Tuesday morning.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
By 12 UTC Tuesday, the trough axis would have swung through,
bringing cold air advection back to the area. Since temperature
profiles would be subfreezing throughout the column, any potential
precipitation would revert back to snow...or melt if boundary
later temperatures were warm enough. However, with downslope
flow, its unclear how much precip there would be (mtns aside).
Snow showers/squalls could jump the divide given sufficient
shortwave support, but that aspect hasn`t been resolved yet.
Will be maintaining a chance/slight chance of precip, with
greater probabilities closer to the mountains...and trough axis.

This would be a favorable pattern for several inches of upslope
snow showers. Will bump PoPs up to likely Tuesday. Potential
will gradually subside through Wednesday as low- level ridging
builds.

Heights will be moderating somewhat through the midweek, while
remaining troffy. Then another shortwave will drop into the trough
axis on Thursday. While agreeing on the overall concept, GFS and
ECMWF depicting completely different tracks for the vortmax and
surface reflection (western New York vs eastern Kentucky). That
would have a substantial impact on the forecast late Thu into
Fri. At this stage, believe its wise to have a chance of precip
to the forecast Thursday, lingering into Thursday night. Suspect
it primarily will be snow.

&&

.AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
IFR/possible LIFR likely by around 12z, through 18-20z in -SN.
Moderate to heavy bursts of snow possible at times. VFR returns
this evening. Light NE flow becomes NW 10-20 kts after midnight,
with gusts to 25+ knots likely Sunday.

VFR likely Monday, with sub-VFR possible late Monday night into
early Tuesday morning. Mixed wintry precipitation is possible
during this time. Winds will be light out of the south Monday
into Monday night.

Gusty winds on northwest flow will impact terminals Tuesday into
Wednesday; 30 kt winds possible. Ceiling will be low, but its
unclear if that will turn our as low VFR or high MVFR. There could
also be brief vsby restrictions in a potential passing snow shower.

&&

.MARINE...
SCA gusts expected out of the NE on the northwestern fringes of
low pressure passing over the western Atlantic. Gusts will peak
over the lower tidal Potomac River and lower Maryland portion of
the Chesapeake Bay today. A lull is expected this evening before
northwest flow kicks in after midnight. Solid SCA conditions are
expected after midnight through Sunday, and could linger into
Sunday night. Light winds are expected Monday before winds
increase again ahead of a clipper late Monday night.

Gusty northwest winds will impact the waters behind an upper level
disturbance. It will be high end Small Craft criteria if not low end
Gales. Have that possibility in the synopsis, and will add to the
Hazardous Outlook as well.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Winter Weather Advisory until 4 PM EST this afternoon for
     DCZ001.
MD...Winter Weather Advisory until 4 PM EST this afternoon for
     MDZ006-011-013-014-503>508.
     Winter Weather Advisory until 4 PM EST this afternoon for
     MDZ004-005.
     Winter Storm Warning until 4 PM EST this afternoon for
     MDZ016>018.
VA...Winter Weather Advisory until 4 PM EST this afternoon for
     VAZ025-026-029-030-036>040-050>054-501-502-506>508.
     Winter Weather Advisory until 4 PM EST this afternoon for
     VAZ505.
     Winter Storm Warning until 4 PM EST this afternoon for
     VAZ055>057.
WV...None.
MARINE...Small Craft Advisory from midnight tonight to 6 PM EST Sunday
     for ANZ530>533-535-536-538>542.
     Small Craft Advisory from 5 AM early this morning to 4 PM EST
     this afternoon for ANZ533-541-542.
     Small Craft Advisory until 6 PM EST Sunday for ANZ534-537-543.

&&

$$
SYNOPSIS...DHOF
NEAR TERM...DHOF
SHORT TERM...DHOF
LONG TERM...HTS
AVIATION...HTS/DHOF
MARINE...HTS/DHOF



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