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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
1121 am EST sun Jan 25 2015

Synopsis...
high pressure builds into the Middle Atlantic States briefly today.
Low pressure over Iowa will track across southern Virginia
tonight...strengthening as it moves up the coast Monday and Tuesday.
High pressure is then expected through midweek.

&&

Near term /through tonight/...
a couple of changes to the headlines in effect for the winter
storm tonight-Monday. The Winter Storm Warning remains in effect
for north central Maryland...and portions of Shenandoah Valley. Have
changed the areas under a Winter Storm Watch to a Winter Weather
Advisory...while also extending the advisory south to include District of Columbia
metropolitan and southwest down to Orange County. All products in effect
until 23z Monday. Have also issued a Winter Storm Watch for the
counties bordering the Chesapeake Bay for Monday night through
Tuesday to account for additional snow on the back side of the low
as it deepens and pushes to the northeast.

Brief high pressure brings a quiet first half of the day to the
area. Southerly flow will push temperatures into the 40s and into the
lower 50s around District of Columbia metropolitan.

Upper low will be tracking southward in the somewhat persistent western ridge/eastern
trough that North America has been seeing this winter. This is prognosticated
to track near the Virginia/NC border overnight and then deepen quickly
offshore Monday. This keeps the middle Atlantic in the cold air. While the
surface temperatures will not be much colder than normal the atmospheric column
will be subfrzg so precipitation will reach the grnd as snow. Every model we've
looked at has a bullseye of hviest snowfall over the nwnrn part of
the forecast area. Blv aftr mdngt the snow could come down at a rate
of over an inch/hour across western Maryland/eastern WV. This is expeceted to continue into
Monday morning b4 the moisture/energy translate toward NJ/NY.

&&

Short term /Monday/...
this multi-faceted low pressure system passage will be undergoing
the first transitional phase late sun into the predawn hours Monday.
While the first phase will have brought a fairly solid batch of
precipitation to the western sections of the area...the parent upper low will
be swinging around the middle tenn valley toward the southern Appalachians. The
trough axis phase shift from positive to neutral...then to negative
- all occurring on Monday and south of our area will change the
dynamics of the precipitation potential across the middle-Atlantic region.

While undergoing this dramatic shift during the daytime hours on
Monday...the forcing for snow bands across the County Warning Area will be of a much
smaller scale and duration than the clipper-like phase late Sun
night. Widespread light snow showers will likely continue throughout
the day...W/ pockets of moderate to heavy batches that move and
pivot across the area that add an extra challenge to forecasting
more precise storm total amounts.

The more confident locations for the heaviest snows will be the
Potomac Highlands over through the northern shen valley into the midday hours on
Monday. The next tier of counties to the south and east are coming to
light west/ recent model guidance as the possible locations for
warning criteria snow duration a portion of this event. One of the
issues is that the different transitions of this storm will cause
the event to last well into Tuesday afternoon - albeit dissipating by then.
Vector analysis shows brief and subtle banding potential in
different locations across the region...so while exact locations
are uncertain - the potential will be there for locally heavy snow
bands and quick additional accumulations. Another potential
limiting factor - albeit a relatively minor one - will be surface
temperatures across the southern Maryland/lower tier counties of the area Monday afternoon
as the coastal low gears-up and pulls in some warmer air just
inland.

&&

Long term /Monday night through Saturday/...
the next and larger impacting phase for the Atlantic coast will be
the developing and quickly strengthening coastal low as the parent
upper low approaches the Carolina coast. This will effectively cut
off a large measure of the forcing for the snows and rain /to our
south/ for the inland portions of the eastern Seaboard states
/outside the higher terrain/ and wrap-up the precipitation locally around
the coastal low. The western exiting edge of the comma head of precipitation
will be a sharp cut off centered over Delaware-Maryland-Virginia and uncertainty
exists to how far back inland it will reside into Maryland for
additional accumulations late Monday/early Tuesday.

The next weather concern for the extended is the cold...as a couple of
reinforcing waves of cold air advection swing over the area during the middle of
the week. Daily highs will maximum-out near the freezing mark and
overnight lows will drop well into the teens and 20s across the
region...some single digits over the Appalachians. High pressure briefly
builds in from late Wednesday into Thursday and a relative warm-up but only
into the u30s on Thursday afternoon.

Another clipper will be well on its way toward the area during
latter half of the week...moving across the region likely late Thursday
into Friday according to the recent GFS and Euro runs. Not a completely
similar solution between them but elements of the same look...except the
GFS holds a more compact upper wave that slides over the area but
keeps a large portion of the precipitation north of the Mason-Dixon line west/
the potential for some bands of light snow over the northern half of the
County Warning Area Thursday aftn/eve. Yet another clipper will be on the way after
that...but may hold off until early next week. High pressure in between
systems which may be little consolation as breezy and cold air
conditions will mark these days as well.

&&

Aviation /15z Sunday through Thursday/...
VFR conditions today with lowerign ceilings heading into this evening.
Winds light from the south becoming light/vrb in the afternoon as they back
to the east.

Sub-VFR conditions first half of the night...with IFR becoming likely
aftr 06z as the precipitation overspreads the region. LIFR conditiions possible as
early as 09z...and lasting through Monday morning with periods of
heavier snow. Precipitation type questionable as possible rain/snow mix or all
rain at kdca/kbwi/kcho at first...then becoming all snow aftr 12z.
Other terminals more likely only a brief period of rain/snow mix
becoming all snow overnight.

Ongoing precipitation Monday with snow...IFR or lower conditions likely.
Another round of heavier precipitation (all snow) Monday night into Tuesday keeps
IFR or lower conditions in place.

&&

Marine...
winds below Small Craft Advisory levels today but winds will be incrsg tonight as a low aprhs
the area. Small Craft Advisory in effect for the waters tonight.

SAC winds expeceted Monday...possibly reaching gale level on the Bay Monday night.

&&

Lwx watches/warnings/advisories...
District of Columbia...Winter Weather Advisory from 2 am to 6 PM EST Monday for
dcz001.
Maryland...Winter Storm Warning from 9 PM this evening to 6 PM EST Monday
for mdz003.
Winter Weather Advisory from 2 am to 6 PM EST Monday for
mdz013-014.
Winter Weather Advisory from 10 PM this evening to 6 PM EST
Monday for mdz004>006-011-503>508.
Winter Storm Warning from 6 PM this evening to 6 PM EST Monday
for mdz501-502.
Virginia...Winter Storm Warning from 9 PM this evening to 6 PM EST Monday
for vaz028.
Winter Weather Advisory from 2 am to 6 PM EST Monday for
vaz039-051>054-502.
Winter Weather Advisory from 10 PM this evening to 6 PM EST
Monday for vaz026-027-029>031-040-501-503>507.
WV...Winter Storm Warning from 9 PM this evening to 6 PM EST Monday
for wvz051>053.
Winter Weather Advisory from 10 PM this evening to 6 PM EST
Monday for wvz505-506.
Winter Storm Warning from 6 PM this evening to 6 PM EST Monday
for wvz050-055-501>504.
Marine...Small Craft Advisory from 9 PM this evening to 6 PM EST Monday
for anz530>543.

&&

$$

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