Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 161450

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

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.


Patchy fog is holding tough in parts of central Maryland (near
BWI to Annapolis) this morning. A few breaks of sun are being
observed across central VA at the moment, but any breaks should
quickly fill in again as the upper trough/cold front approach
from the west. Light snow was observed along the MD/PA border
(1/2" in Manchester MD) associated with upper jet dynamics this
morning, but that light snow has since lifted north. Snow is now
confined to NW MD and is generally light.

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. BWI dropped to 1/2 mile in FZFG with CIGS of 100
feet, but believe this will slowly lift through noon. 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 kts.

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


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 until 10 PM EST this evening for
     Winter Weather Advisory until 10 PM EST this evening for
     Winter Weather Advisory from 4 PM this afternoon to 7 AM EST
     Wednesday for MDZ004>006-507.
VA...Winter Weather Advisory until 10 PM EST this evening for
WV...Winter Weather Advisory until 10 PM EST this evening for
     Winter Weather Advisory until 10 PM EST this evening for
MARINE...Small Craft Advisory from 7 AM to 6 PM EST Wednesday for


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