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fxus61 klwx 160128 
afdlwx

Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
828 PM EST sun Dec 15 2019

Synopsis...
high pressure will be overhead tonight, then move offshore
Monday. Low pressure will pass south of the area Tuesday and
high pressure will build over the region for the end of the
week.

&&

Near term /through Monday/...
a nearly stationary boundary will remain draped across southern
Virginia into the Tennessee Valley and Gulf Coast states. A
nearly zonal (west-southwest flow) continues at the mid and
upper-levels of the atmosphere between upper-level high
pressure off the southeast coast and the polar jet over Canada.
Our area lies in between Artic air over the northern plains and
subtropical air over the Gulf Coast states. A series of upper-
level disturbances will ride along the boundary overnight into
Monday, and this will cause the low-level flow to turn to the
southwest. Warm and moist air will overrun the surface colder
air in place, causing precipitation to develop overnight.
Precipitation development will be enhanced by the right entrance
of an upper-level jet to our northeast and mid-level
frontogenetical forcing.

Precipitation is expected to develop over the Appalachian
Mountains, then into northern Virginia, northern Maryland and eastern
West Virginia after midnight, before eventually filling in
farther south and east toward morning over the Washington and
Baltimore Metro areas into central Virginia. The p-type forecast
will be quite tricky through Monday. At first, the cold air
should be deep enough for snow to develop late tonight for most
areas, but precipitation will be a wintry mix farther south
toward the Washington Metro area into central Virginia. Warm air
will gradually work its way into the area from south to north,
causing precipitation to change to rain over the Washington
Metro area into central Virginia Monday morning. Farther north
and west, snow will change to sleet and freezing rain for a
period Monday morning before changing to rain across most areas
Monday afternoon. However, freezing rain is expected to continue
across the Allegheny Highlands of Maryland and West Virginia.
Freezing rain is also likely to continue for most of Monday over
the higher elevations near the Mason-Dixon line into the Potomac
Highlands.

The wintry precipitation will impact the morning commute,
especially northwest of Interstate 95. Snow covered and slippery
roads are expected across these areas. A Winter Weather Advisory
is in effect late tonight into Monday for the farther northern
and western suburbs of Washington/Baltimore and for locations
near/west of the Blue Ridge and catoctin mountains. A Winter
Storm Warning is in effect for western Allegheny and western
Mineral counties where several inches of snow are expected
before changing to ice from freezing rain.

There may be a brief lull in the precipitation later Monday
morning, but left likely to categorical pops due to
overrunning. A stronger disturbance will approach Monday
afternoon toward Monday evening, causing steadier precipitation
to break out once again. During this time, most areas will be
rain, but freezing rain is expected across western Maryland and
western Mineral County in WV. Freezing rain may also hang on for
the higher elevations near the Mason-Dixon line into the
Potomac Highlands. Winter headlines may have to be adjusted near
the Mason-Dixon line into the Allegheny and Potomac Highlands
due to the possibility of freezing rain. Confidence is too low
to do so at this time.

&&

Short term /Monday night through Tuesday night/...
the stronger wave of low pressure will continue to approach
Monday night before passing through Tuesday morning. Steady
precipitation is expected across much of the area during this
time, with the heavier precipitation amounts most likely across
the northwestern half of the County Warning Area (wash/balt Metro areas north
and west). Temps should be warm enough for rain across most
areas, but there cold air will be tough to scour out near the
Mason- Dixon line and the Allegheny and Potomac Highlands since
winds will be light underneath the inversion. Therefore, pockets
of freezing rain is possible. Confidence for freezing rain is
highest in western Allegany and western Mineral counties Monday
night. The Winter Storm Warning may need to be extended across
these areas. Elsewhere, winter weather headlines may be needed
near the Mason-Dixon line, but confidence is low at this time.

The low will move off to the northeast Tuesday afternoon and
drier air will slowly filter in from the northwest.
Precipitation will diminish for most areas, but a few showers
may persist. Snow showers are possible along/west of the
Allegheny Front.

High pressure will approach Tuesday night, bringing blustery and
cold conditions, but it will be dry for most areas.

&&

Long term /Wednesday through Sunday/...
cold front will be moving away from our region on Wednesday as a
shortwave trough pushes east into our region. Northwest flow will be
settling over US and temperatures will become below normal. High
pressure will build over our area behind the front and remain in
control through Friday. A southern stream system may affect US
by the weekend.

High temperatures during the long term period will be in the 30s
and 40s increasing gradually into the weekend. Low
temperatures will be in the teens and 20s Wednesday night,
increasing to the 20s and 30s into the weekend.

&&

Aviation /01z Monday through Friday/...
snow will develop around midnight at kmrb and snow/sleet will
develop toward morning across the other terminals. Precipitation
will change to rain Monday morning, but freezing rain is
expected around kmrb. IFR conditions are expected to develop
late tonight into Monday morning. IFR and LIFR conditions are
most likely late Monday into Tuesday morning. Conditions
improving Tue afternoon with frontal passage but with
strengthening winds.

VFR but breezy conditions are expected Wednesday into Wednesday
night. VFR conditions continue Thursday and Friday.

&&

Marine...
an Small Craft Advisory is in effect for the middle portion of the Bay and lower
tidal Potomac River until midnight. Winds will diminish
overnight. Winds should remain below Small Craft Advisory conditions through 06z
Tue and strengthen again late Mon night and Tue. Risk of gales
has diminished for Tuesday.

Breezy conditions on Wednesday into Thursday will need a Small Craft
Advisory. Winds will be receding later on Thursday and remain below
criteria into Friday.



&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...Winter Storm Warning until 7 PM EST Monday for mdz501.
Winter Weather Advisory from midnight tonight to 1 PM EST
Monday for mdz003>006-503-505-507.
Winter Weather Advisory until 1 PM EST Monday for mdz502.
Virginia...Winter Weather Advisory until 1 PM EST Monday for vaz025-026-
503-504-508.
Winter Weather Advisory from midnight tonight to 1 PM EST
Monday for vaz027>031-040-501-505-507.
WV...Winter Storm Warning until 7 PM EST Monday for wvz503.
Winter Weather Advisory from midnight tonight to 1 PM EST
Monday for wvz052-053.
Winter Weather Advisory until 1 PM EST Monday for wvz050-051-
055-501-502-504>506.
Marine...Small Craft Advisory until midnight EST tonight for anz531>534-
537-539>541-543.

&&

$$
Synopsis...lfr
near term...bjl/lfr

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