Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 160904

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
404 AM EST Tue Jan 16 2018

.SYNOPSIS...A cold front will approach the area today before
passing through tonight into Wednesday. Low pressure will
develop along this boundary offshore later Wednesday before high
pressure settles to our south Thursday. High pressure will
remain to our south through the weekend.


.NEAR TERM /THROUGH WEDNESDAY/...Cdfnt will cross the
Appalachians later today and slowly progress ewd reaching the
I-81 corridor by 00Z Wed. Snow today will fall mainly over the
upslope areas of the Appalachians behind the front/wind shift.
Some snow could also affect nrn MD counties along the Mason-
Dixon line but this is not a done deal as some models and
ensemble keep this area dry through much of the day.

Tonight and the first half of Wed is when the majority of
precip will fall as heights fall in response to approaching
upper level trof and forcing for upward vertical motion
increases. Deterministic models and their respective ensembles
depict precip breaking up as it crosses the mtns with a
secondary area of snow over the VA piedmont late tonight into
Wed. Ensemble QPF is generally 0.1 to 0.2 inches along and south
of the I-64 and west of the I-81 corridors. A big minima or hole
in the precip is shown over nrn VA and central MD. Even with
these small amounts of QPF, the very cold temperatures will
result in SLRs in the order of 15 to 1 and even a little bit of snow
potentially causing moderate impacts given sfc temps expected in
the mid 20s. It is possible that additional advisories may be
needed for the Charlottesville and Fredericksburg areas, but
since precip is not expected to begin until late second period
or perhaps closer to 12Z Wed decided to hold off on any
advisories. Also, our snow probs indicate only a 50% prob of
snow exceeding 2 inches and 80% confidence is needed. For the
metro areas, the prob of exceeding one inch is about 50-60%
which would require an advzy given snow is expected to impact
the Wed morning commute, but this is again late second or even
third period event.



Snow will begin to taper off quickly in the afternoon Wed with
snow ending by 7 PM Wed if not sooner. Clearing and turning
blustery Wed night and Thu. Wind and wind chill advisories may
be needed for the mtns.



Surface high pressure will build over the southeastern states on
Friday while a shortwave trough swings eastward. Very little
moisture will be associated with this trough, and thus only some
increase in mid-level clouds can be expected. Temperatures will
begin to increase above climatology due to warm air advection.

The surface high will move off the southeast coast and broader
ridging aloft will develop over the southeastern CONUS through the
weekend. Meanwhile, low pressure will be developing in the lee of
the Rockies and lifting toward the Great Lakes. A second low will be
crossing well to the north in Canada. Overall, temperatures should
continue their above normal trend. It`s worth noting that the 00Z
ECMWF sends a backdoor cold front southward from the northern low
and thus prevents the really warm air from working into the area on
Sunday. For example, there is a 10 degree spread in the forecast
high temperature at Baltimore between the ECMWF and GFS MOS. It
looks like the weekend will be dry in most places, though a few
showers extending along the warm front could clip northwestern parts
of the CWA.

As the low lifts north of the Great Lakes Monday, the trailing cold
front will approach the area. There`s fairly good model agreement
with the front crossing Monday afternoon and evening, though of
course timing could change some at this range. Showers will become
likely ahead of the front. Rain will be exiting with the front
anyway, but even so, flow will be zonal behind the front and cold
air limited. Only the upslope areas of the Allegheny Front may see a
changeover to snow showers briefly before ending.



IFR cigs across eastern terminals this morning lifting to VFR
this afternoon. Light snow developing tonight first at KMRB then
spreading slowly eastward overnight with MVFR/IFR conditions
expected at all terminals. Snow begins to taper off by midday
Wed with conditions improving rapidly and winds picking up from
the NW. Blustery Wed night and Thu with wind gusts well over 20

No aviation impacts are expected Friday and Saturday as high
pressure builds south of the area.


.MARINE...Light and variable winds through this evening, then
winds strengthening late tonight into Wed with SCA conditions
expected Wed into Thu. Risk of gales appears low at this time
for late Wed night and Thu.

A weak disturbance will be passing Friday, which could kick up
westerly winds a little, but it`s uncertain at this time if they
will reach SCA levels. High pressure will slide south of the area
Friday night into Saturday with a light southwest flow developing.


MD...Winter Weather Advisory from 7 AM this morning to 10 PM EST
     this evening for MDZ501.
     Winter Weather Advisory from 10 AM this morning to 10 PM EST
     this evening for MDZ003-502.
     Winter Weather Advisory from 4 PM this afternoon to 7 AM EST
     Wednesday for MDZ004>006-507.
VA...Winter Weather Advisory from 7 AM this morning to 10 PM EST
     this evening for VAZ503.
WV...Winter Weather Advisory from 7 AM this morning to 10 PM EST
     this evening for WVZ501-503-505.
     Winter Weather Advisory from 10 AM this morning to 10 PM EST
     this evening for WVZ050>053-055-502-504.


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