Area Forecast Discussion

FXUS61 KLWX 091457
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
957 AM EST Fri Dec 9 2016
High pressure builds east from the Great Plains through Saturday
before moving off the Mid-Atlantic coast Sunday morning. A cold
front crosses the area Monday followed by a reinforcing cold front
Wednesday. High pressure builds back into the area for the latter
half of next week.
Morning surface analysis shows 1040+ mb high pressure extending
east of the continental divide. 12Z IAD 850 mb temperature of -11C
indicates today's chilly forecast is on track. CAA mountain wave
stratocumulus is generally west of the Blue Ridge, but may expand
a little through the day. The 36 MPH wind gust at IAD at 9 AM
should be the upper end of what's expected today, with gusts
only gradually subsiding this evening. Observations from the
Highlands show less than an inch of snow through 7 AM, so sub-
advisory amounts over the 30 hour period seem reasonable.
Previous valid discussion follows: 
Cold and blustery today...with tight pressure gradient remaining
as large area of high pressure over the central CONUS gradually
builds towards the area. Even with lots of sun...most places
remain AOB 40F the entire day. Gusty winds up to 25-30 mph will
create winds chills in the teens through midday...even by
afternoon wind chills remain AOB 30F.
Westerly component through a moist layer will continue to promote
off and on snow showers across upslope areas through tonight.
Generally less than 3 inches expected through 7 AM Saturday
before any activity begins to wane.
High pressure builds overhead Saturday through early
Sunday...thereby relaxing the pressure gradient. Perhaps a few
degrees colder than Friday as CAA continues...with many places
failing to get above 40F despite sunshine. However, diminishing
winds by afternoon will offer some relief.
High pressure moves offshore Sunday with a southerly flow
developing on its backside. This should help to nudge temperatures
up a few degrees. Though, increasing cloud cover ahead of next
system will temper any appreciable warming.
Precipitation may move into NW/N portions of area
Sunday...thermal profiles during this time would suggest mainly
snow initially...with a transition to a snow/rain mix by
afternoon. Any wintry precipitation will be fighting warm air
advection aloft (with warm nose developing near 850 hPa) and at
surface Sunday into Monday morning. Eventually most places will
transition to all rain...but there could be a brief period of
freezing rain near the Mason-Dixon line early Monday morning.
Timing of precip over DC/Balt metros suggests activity should
remain of the liquid variety.
Storm system passing well to the north will bring warmer air north
on Monday, allowing temps to rise into the 50s in many areas and
causing precipitation to fall in the form of rain. High temps may
be underdone in fact, but will wait til we get a little closer to
push us above model guidance averages. Right now the rain looks
like it will be mostly in the morning, with potential for a
mostly dry, breezy but relatively mild afternoon. 
High pressure pushes overhead Monday night with cold advection
bringing temps back down into the 30s. Tuesday will start out dry
with high pressure overhead, however some guidance develops a wave
of low pressure to the south which may brush the region later
in the day. Disagreement grows as we head into Wednesday, with
some guidance showing a stronger and warmer system coming in from
the southwest, while others show only a weak disturbance and rapid
cold advection already underway. Bottom line is there is a chance
of precipitation and it could be snow, but it would seem slightly
more likely that it either is mostly rain for I-95, or very little
falls whatsoever. Still a long way out and a lot can change.
Either way, very strong cold advection takes place behind that
system so that we are probably seeing well below normal temps by
Thursday. Temps may in fact struggle to rise above freezing while
lows will be well below freezing. Northwest flow will mean that
the only precip will likely be upslope flow on the Allegany Front.
VFR continues through much of Sunday for most terminals. Cigs
FL040-060 most likely at MRB today. By Sunday, MRB could have
sub-VFR in rain/snow showers, but confidence in precipitation
coverage is low. Sub-VFR then possible at all sites late Sunday
into Monday as precipitation (possibly a mix) spreads across much
of the area. Strong NW winds with gusts up to 20-30 kts continue
through this evening before moderating back below 15 kts
Sub VFR with potential IFR Monday as storm system with rain
showers moves across the area. Conditions improve back to VFR
Tuesday for a time before next system potentially arrives midweek.
Solid SCA conditions continue through much of tonight. Based on
strong gradient, did not think gusts would immediately subside
with sunset, so extended SCA headlines across all waters through
9 PM. SCA then will taper off for the upper tidal Potomac...and
then the upper Chesapeake and inlets at midnight. Headlines clear
rest of waters at 6 AM Saturday as pressure gradient relaxes and
winds fall under SCA criteria with high pressure building
overhead. Will peruse latest data to see if headline needs to be
extended into Saturday morning.
Marginal SCA conditions could develop Sunday with southerly flow
ahead of approaching cold front. SCA may continue Monday as the
front passes. They may diminish briefly Tuesday under a very
transitory bubble of high pressure.
MARINE...Small Craft Advisory until 6 AM EST Saturday for ANZ531>534-
     Small Craft Advisory until midnight EST tonight for ANZ530-536-
     Small Craft Advisory until 9 PM EST this evening for ANZ535.