Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 060903
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
403 AM EST Wed Dec 6 2017

.SYNOPSIS...
Weak high pressure will build into the area today and Thursday
in the wake of last night`s cold front. Low pressure will pass
to the east Friday and Friday night, then another low pressure
will cross the region Saturday and Saturday night. High
pressure will then return Sunday into Monday.

&&

.NEAR TERM /THROUGH THURSDAY/...
Cold front has just about cleared the CWA as of 4AM, with the
last showers still crossing southern Maryland. These may linger
for the next several hours before finally sliding east of the
bay by or shortly after sunset. Northwest winds have brought in
much chillier and drier air already, and will continue to do so
all day. Calendar day highs have already occurred, but some
modest warming is expected after temps bottom out in a few
hours, so not quite a purely non-diurnal trend... more like a
modified one. While some breaks in the clouds are expected,
southwest flow aloft will likely keep high clouds streaming
across, so true clearing is not really anticipated. Some sun
will be seen, however.  Temps will mostly stay in the 40s.

Tonight and Thursday, high pressure will continue to press in
from the west, but with the front stalling along the southeast
coast, we will continue to see high clouds streaming overhead.
Cold advection will push temps below freezing in most spots
tonight, but radiational cooling will not be exceptional with
the high clouds and some lingering breezes. Warming on Thursday
will also be modest with the high clouds, with temps likely to
return to the 40s. Otherwise, once the showers clear the CWA in
a few hours, it should be dry CWA-wide through sunset Thursday.

&&

.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
On Thursday night, the front which crossed our region overnight
will remain stalled just off the Southeast coast. High clouds
will continue to stream overhead while northerly winds will
continue to advect cold air across the region. Lows will
generally be below freezing.

An elongated wave of low pressure will ride northeast along the
stalled front Friday and Friday night as a shortwave moves in
from the west. Models continue to differ somewhat on the details
but most skirt southern Maryland with some light precipitation,
and given the antecedant air mass, odds are high that some snow
will at least mix in. Some guidance is considerably further
northwest, with the 0Z GGEM bringing precip well northwest of
I-95, but at this point this seems to be an outlier. EC is
closer to the middle of the road, bringing light precip right up
to I-95 but no further. This scenario could bring light
accumulations to southern Maryland, perhaps an inch, but warm
boundary layer would fight against much impact, and
accumulations further north and west are highly uncertain. Will
need to closely watch this system for any further adjustments in
track, but it must be noted that guidance has been wavering back
and forth between further NW 12Z runs and further SE 0Z runs, so
it has been hard to find much of a trend. Highs on Friday will
depend on the system so have gone with near 40 in much of the
CWA... these will be colder if it is cloudier with more precip,
but could be milder if it stays further east and gets a bit
sunnier. Lows Friday night will again be below freezing.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
There are two periods of interest in the long term, aside from
the overarching theme of persistent colder-than-normal
temperatures. 1) A strong clipper-like system Saturday night,
and 2) another upper trough/clipper Tuesday into Wednesday of
next week.

The first system will likely bring at least scattered snow
showers to parts of the area. There are several moving parts.
Some of the latest guidance suggests a weak offshore
low/baroclinic zone may linger into Saturday while the clipper
moves in from the west. This subtle increase in moisture lends
further support to the notion of snow showers Saturday night.
The most likely areas are along the western ridges of the
Allegheny Front (upslope), and northern Maryland/along the I-95
corridor in concert with juxtaposition of better mid/upper
forcing and moisture.

Reinforcing cold follows for Sunday, then the next system
pivots around the upper trough Tuesday into Wednesday of next
week.

&&

.AVIATION /09Z WEDNESDAY THROUGH SUNDAY/...
Generally VFR at the terminals through the TAF period and
probably through Friday night. Last showers are exiting at
present as cold front continues pushing east and other than a
gusty northwest wind today, VFR should be the rule. Winds will
diminish a bit this afternoon and tonight but may be a bit
reinvigorated on Thursday due to insolation. Winds will relax
again Thursday night and should remain relatively light Friday
and Friday night. Main concern Friday into Friday night is a
wave of low pressure passing to the southeast along a stalled
front. There is a chance it brings some snow to the region,
which could, at minimum, bring IFR cigs and vis, but as of this
writing, odds of precip reaching the terminals is below 50
percent.

Sub-VFR possible in any snow shower activity over the weekend.
Winds generally light and variable Saturday, become NW 10- 20
kts Sunday.

&&

.MARINE...
SCA for all waters through 6PM as cold high pressure slowly
builds in behind the cold front. We could touch gale over the
next few hours as pressure rises rapidly, but right now
uncertainty is high so will just continue to watch closely.
Winds probably subside on most waters tonight, though cold
advection over relatively warm bay waters might keep SCA gusts
going over the middle bay. Winds get a bit reinvigorated during
the day Thursday so have issued new SCA for the daylight hours.
After that, winds likely relax Thursday night and remain below
SCA through Friday night, though a passing coastal low pressure
could bring some rain and snow to the waters late Friday and
Friday night, which at minimum could reduce visibility
significantly. Odds of this happening are highest in the middle
Chesapeake Bay.

SCA gusts likely out of the NW Saturday night into Sunday
behind a strong clipper. Gales possible.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 PM EST this evening for
     ANZ530>543.
     Small Craft Advisory from 6 AM to 6 PM EST Thursday for
     ANZ530>534-538>541-543.

&&

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



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