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Area forecast discussion...updated 
National Weather Service Baltimore Maryland/Washington District of Columbia
946 PM EST Tuesday Nov 25 2014

Synopsis...
a low pressure system will continue to strengthen off the southeast
coast tonight...track up the Atlantic coast into Wednesday. The
low will quickly move off to our northeast Wednesday night. An
upper-level disturbance will pass through the area Thursday and
high pressure will build overhead Friday. The high will move off
the coast during the weekend.

&&

Near term /through Wednesday/...
late evening update... no headline changes and minimal
grid/forecast modifications for the incoming low pressure system. Since
this time yesterday evening...a relatively narrow rain band has
been set-up from the Gulf Coast northeastward toward the Virginia Tidewater. Over
the past several hours...more rain has been filling the regional
scope - across more of the southeast. A dynamic scenario is taking
shape as large- scale features from the Pacific northwest to the southeast
will begin to influence one large low pressure system.

The moist corridor over the deep south is growing and pushing middle
decks and cirrus debris further up the eastern Seaboard and into our
region. Clear skies over the western half of the County Warning Area earlier today has
allowed temperatures to drop much further than areas to the east. Many
shen valley observation already in the M-u30s or barely still into the 40s.
The relatively lower dewpoints also important for keeping temperatures
down...even as insulating cirrus debris builds overhead.

Overnight... a potent vorticity maximum scraping the Gulf Coast states will
be forced northeastward...turning the upper short shortwave to a negative
tilt overnight. This transition will occur because of a secondary
vorticity behind it - to the northwest - that will help dig the upper trough
even further south. The already moist and dynamic warm sector
stationed over the warmer Gulf Stream waters will strengthen the
large system further as it moves NE in the coming hours.

A surface high retreating to the NE will also filter colder/drier air
into the northern portions of the area...espec the shen valley heading
early morning hours. Temperatures across the area will large remain above
freezing well into the overnight and only begin to drop toward the
predawn hours. Light precipitation is expected to overspread the area
gradually into the early morning hrs/predawn...as the coastal low
tracks northeast toward Cape Hatteras.

Post-dawn Wednesday morning... the surface low will intensify as it tracks
northeast...and this will allow for moisture from the Atlantic and
Gulf of Mexico to get wrapped into the storm. Therefore...precipitation
will be moderate to locally heavy at times during the day
Wednesday. The track of the low and position of the surface high
suggests that there would be enough cold air for snow. There is
little cold air associated with this storm since the source region
from the high was originally over the central and Southern Plains
instead of Canada. There will be low-level dry air that filters
into this storm from the north and there will be frontogenetical
forcing that could lead to locally heavy precipitation...causing
dynamical cooling. These cooling mechanisms should be enough for
any mixed precipitation to change to snow for locations near the
Mason-Dixon line and also near and west of the Blue Ridge
mountains. The higher elevations are expected to get the most
snow. The heaviest snow is expected Wednesday morning through the
middle afternoon hours before precipitation tapers off as the low
departs. There should be decent moisture and lift with this
system...so several inches of snow are expected. A Winter Storm
Warning is in effect for these areas.

For the northern and western suburbs of Washington and Baltimore...
there will be enough warm air for precipitation to start out as
rain late tonight into Wednesday morning. However...colder air
will work its way in causing precipitation to mix with and perhaps
even change to all snow for a period before ending later Wednesday
afternoon. A Winter Weather Advisory is in effect for these
areas...but as of now it appears that most of the snow
accumulation would be on grassy surfaces with surface temperatures just
above freezing. For locations near and east of Interstate 95
mostly rain is expected. Snow may mix in for a period late
Wednesday morning into Wednesday afternoon before precipitation
ends...but temperatures will remain above freezing and this will
cause little or no snow accumulation.

Despite general model agreement in the timing/position of the
surface low...the bottom line is that the temperature profiles in
the lowest thousand feet of the atmosphere will be very sensitive
as to determining the type and amount of precipitation that
falls. There is a little more model spread in this area...and of
course these values will be marginal for snow with this system.
Wpc has noted that the NAM has both a cold bias and a bias for
generating too much precipitation in the cold air. Being that
models struggle in the boundary layer...confidence is not high on
the exact timing of changeover...although as noted...lower
elevations are looking like much more rain than snow. It is
conceivable the middle elevations may Switch Back and forth depending
on precipitation intensity...making for potential variability in
accumulations across short distances in these locations.

Precipitation will end from southwest to northeast later Wednesday
afternoon into Wednesday evening as the low moves away from the
area.

&&

Short term /Wednesday night through Thursday night/...
temperatures will dip below freezing across most areas so
any wet or slushy roads may become icy. An upper-level disturbance
will move into the region after midnight and this may trigger more
snow showers...especially across the Allegheny Highlands into
central Virginia. Light snow accumulations are possible across
these areas with temperatures close to freezing.

Upper-level trough/clipper system swings through the area
Thursday and could be accompanied by scattered snow showers or
flurries. Little to no accumulation is expected at this
time...except perhaps in a few spots over the western ridges.

&&

Long term /Friday through Tuesday/...
high pressure builds in for the end of the week accompanied by below
normal temperatures. As the high moves offshore over the
weekend...southerly flow sets up bringing temperatures back to near
normal.

A front will approach by late in the weekend...bringing another
chance of precipitation early next week. But the milder airmass that
will have built in over the weekend will still be in place early
next week so wintry precipitation chances appear negligible at this
time.

&&

Aviation /02z Wednesday through Sunday/...
VFR conditions will continue through this evening with west to northwest winds
generally under 10 knots.

Low pressure will move up the eastern Seaboard from late tonight
through Wednesday. Ceilings will continually lower...reaching IFR levels
(and below) after daybreak. Precipitation will spread from south to
north between 08-11z. At the terminals...precipitation will
initially be mostly rain...except snow could mix in at mrb.
Changeover to snow will occur early morning at mrb. Mix arrives at
iad...BWI and cho during middle to late morning. Changeover fully to
snow questionable at these sites...but if so...would happen midday
to early afternoon. Dca and mountain will be mostly rain for a majority
of the event...but snow could mix in past midday. Visibility in tafs would
be conservative if heavier snow bands develop...which would be most
likely at mrb. Winds will be northerly...with slightly higher
magnitudes near 15 knots for the eastern sites.

Conditions improve Wednesday evening...but snow showers may affect
cho during the late night hours. Sub-VFR possible in scattered snow
showers Thursday west/ west flow 10-20 kts. Mainly VFR likely Thursday night-
sun...west winds 10-15 kts becoming S Friday night 10 kts or less
through the weekend.

&&

Marine...
light northwest to north winds through this evening. Winds will increase after
midnight as low pressure moves up the coast from the south. Winds
will first reach small craft levels for the central and southern Maryland
Bay...and continue increasing northward through the morning. Current
wind projects remain below gale levels...but will be close over open
areas of the Bay. Winds are still forecast to remain below Small Craft Advisory
levels for the upper tidal Potomac. Precipitation with this system
will be mainly rain over the waters...but could mix with snow over
the upper part of the Bay by midday Wednesday. The Small Craft Advisory has been
extended until 10 PM for the middle/lower Maryland Bay and lower tidal
Potomac...but winds will overall show a diminishing trend
overnight.

A couple of snow showers or flurries could meander over the waters
in association with a clipper system Thursday. Westerly flow gusting
to Small Craft Advisory levels is expected late Thursday into
Friday...then southerly winds up to Small Craft Advisory levels is
forecast for Saturday.

&&

Tides/coastal flooding...
water levels are slightly above astro norms but have been showing
a slight decreasing trend today. Low pressure tracking up the
coast Wednesday may cause a rise in water levels...but the quick
speed of system and northerly winds suggest that tides will remain
below minor flooding levels.

&&

Climate...
climate...
records for earliest measurable snow already passed a few weeks
ago...but below is a list of daily maximum snowfall records specific
to the date of November 26th.

Dca 2.2" (1898)
BWI 4.5" (1898)
iad 1.1" (1978)

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...Winter Storm Warning from 3 am to 6 PM EST Wednesday for
mdz003-501-502.
Winter Storm Warning from 6 am to 6 PM EST Wednesday for
mdz004>006.
Winter Weather Advisory from 8 am to 6 PM EST Wednesday for
mdz007-009>011.
Virginia...Winter Storm Warning from 3 am to 6 PM EST Wednesday for
vaz025>031-036>040-503-504.
Winter Storm Warning from 6 am to 6 PM EST Wednesday for
vaz042-501.
Winter Weather Advisory from 8 am to 6 PM EST Wednesday for
vaz050>053-502.
WV...Winter Storm Warning from 3 am to 6 PM EST Wednesday for
wvz050>053-055-501>506.
Marine...Small Craft Advisory from 1 am to 10 PM EST Wednesday for
anz532>534-537-540>543.
Small Craft Advisory from 6 am to 6 PM EST Wednesday for
anz530-531-538-539.

&&

$$
Update...gms
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