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Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
959 am EST Mon Nov 11 2019

Synopsis...
low pressure will develop along a cold front approaching from the
Ohio Valley today. This area of low pressure is expected to track
northeastward into New England through Tuesday, bringing the
associated cold front across the mid-Atlantic by Tuesday
afternoon. Arctic high pressure will follow Wednesday into
Thursday. A cold front will likely move from the Midwest to off
the East Coast Thursday into Friday. Low pressure may develop
offshore of Georgia and the Carolinas this weekend as another
area of Arctic high pressure builds over northern New England.

&&

Near term /through tonight/...
fairly solid cirrus shield envelops the area this morning, but
for the most part sun is still filtering through. In addition,
satellite suggests breaks to the west will work over the area
this afternoon. Thus the forecast highs near or above guidance
seem reasonable. Arriving 12z guidance suggests timing of
front/precip tonight is on track, too.

Previous discussion:

High pressure will remain offshore while a positively tilted
upper-level trough digs through the Great Lakes and Midwest.
Low pressure will develop over the Tennessee Valley today before
passing by to our northwest late this afternoon into this
evening. A southwest flow between the low to our northwest and
the high offshore will allow for mild conditions today. There
will also be some sunshine as well. Highs will top off well
into the 60s for most locations with some areas across central
Virginia near 70 degrees. There will be some high clouds due to
a southwest flow aloft ahead of the trough, but the cloud deck
should be thin.

The low will move off to the northeast tonight, but the cold
front associated with the low will move into the area from the
northwest after midnight. The upper-level trough will also slide
east, and a potent upper-level jet combined with the frontal
passage will be enough for some rain to break out around
midnight near the Potomac Highlands, and toward morning farther
east across the Shenandoah Valley and Metro areas. Much colder
air will begin to move into the area behind the front, and this
will cause rain to mix with and change to snow across the
Allegheny Highlands toward morning. The front is most likely to
be near the Blue Ridge and catoctin mountains by sunrise. Gusty
northwest winds will cause temps to fall sharply into the 30s
(20s in the mountains), but temps in the 40s and 50s will
persist ahead of the boundary through tonight.

&&

Short term /Tuesday through Wednesday night/...
through midday Tuesday...the cold front will pass through the
rest of the area Tuesday morning, likely reaching southern
Maryland by mid-morning. A gusty northwest flow behind the
boundary will cause temps across the rest of the area to fall
sharply into the 30s behind the frontal passage (with 20s in the
mountains). The upper-level trough axis will continue to slide
eastward toward our area. This will allow for a period of
precipitation behind the cold front that will last for a few
hours or so, mainly east of the Blue Ridge and catoctin
mountains. That precipitation will be rain to start. However, as
colder air works in that rain will likely mix with and perhaps
even change to a period of snow before ending across many
locations including the Washington and Baltimore Metro areas as
well as southern Maryland. West of the Blue Ridge and catoctin
mountains, the drier air should take over but it will be
blustery and cold. Snow showers are likely along and west of the
Allegheny Front, and some of those snow showers may spill east
of the Allegheny Front by midday.

The latest thinking is that with a positively tilted trough and
the very warm conditions preceding this system, that snow
accumulation (if any) would be light (coating) and confined
mainly to grassy surfaces.

Tuesday afternoon through Wednesday night...any
leftover precipitation across southern Maryland early Tuesday
afternoon will dissipate by the middle afternoon hours. Blustery
and very cold conditions for this time of year are expected
with temps in the 30s for many locations (20s in the mountains).
Snow showers are expected along and west of the Allegheny
Front. However, with some instability from strong cold
advection, some of those snow showers may spill east of the
Allegheny Highlands into the I-81 corridor as well as
northern/central Maryland, northern Virginia and the
Washington/Baltimore Metro areas. With some instability, and
temps at the top of the subsidence inversion within the
dendritic snow growth zone, a few bursts of heavier snow showers
cannot be ruled out. Right now the best chance for that to
occur will be across eastern WV, the northern Shenandoah Valley,
and northern Maryland. This is where most guidance is showing the
most instability from the mid-afternoon hours into the early
evening. Any heavier snow showers do appear that they will be
extremely localized with most areas remaining mainly dry.

For locations along and west of the Allegheny Front,
accumulating snow is expected with 1-3" most likely. A Winter
Weather Advisory may be needed for Tuesday morning through
Tuesday afternoon, but confidence was too low to issue at this
time since moisture will be limited in the afternoon. Will
continue to monitor.

High pressure will build toward the area Tuesday night. Any
leftover snow showers Tuesday evening should dissipate by the
overnight hours, except for right along/west of the Allegheny
Front. Very cold conditions are expected with min temps in the
teens and 20s for most areas and wind chills in the lower to
middle teens. Even colder conditions are expected in the
mountains and wind chills below zero are likely above 2500 feet.
A Wind Chill Advisory may be needed, especially along and west
of the Allegheny Front.

High pressure will build overhead Wednesday and Wednesday night.
Bitterly cold conditions will persist for this time of year,
but winds will gradually diminish later Wednesday and Wednesday
night.

&&

Long term /Thursday through Sunday/...
mid/upper troughing will dominate the eastern Continental U.S. Thursday
into the weekend. At the surface, high pressure will likely be
shifting offshore of southeastern New England Thursday. Southern
stream energy drifting off the southeast coast will probably
induce cyclogenesis offshore of Georgia and the Carolinas
heading into this weekend, but models (both deterministic and
ensemble means) have been rather inconsistent with the strength,
track and timing of this wave, and whether or not it gets
absorbed back into the mean upper flow pattern/northern stream
soon enough to bring precipitation to to the mid-Atlantic. Most
recent trends keep the system far enough offshore to keep much
of the weekend dry. Will need to keep a close eye on it, though,
since a reinforcing cold front/subsequent Arctic high will move
across/to the north of the region Friday into the weekend.

&&

Aviation /15z Monday through Friday/...
VFR conditions are expected through this evening. A period of
rain will develop late tonight into early Tuesday morning before
ending around midday. That rain will likely mix with and may
even change over to a period of snow. Any accumulation will be
light and confined mainly to grassy surfaces. Subvfr cigs/vsbys
are likely during this time. A sharp wind shift to the northwest
is expected behind the cold front early Tuesday morning. Gusts
around 20 to 30 knots are expected behind the boundary.

VFR conditions will return by Tuesday afternoon and continue
through Wednesday night. A snow shower is possible Tuesday
afternoon and evening. Winds will diminish some Tuesday night,
but some gusts around 20 knots are still possible before winds
diminish more later Wednesday and Wednesday night.

VFR expected Thu-fri; S flow Thu becomes northwest Thu night-Fri at or below 10
kts.

&&

Marine...
a southerly flow will gradually increase today as low pressure
passes by to the northwest. An Small Craft Advisory is in effect for middle
portions of the Bay and the lower tidal Potomac River later this
afternoon and tonight. A cold front will pass through the waters
Tuesday morning. A sharp wind shift to the northwest is
expected with gusts around 20 to 30 knots. An Small Craft Advisory is in effect
for all of the waters Tuesday. The Small Craft Advisory will likely be needed
Tuesday night and Wednesday for portions of the waters. High
pressure will build overhead Wednesday night.

Southerly flow Thursday flips to northwest by Friday as a dry
cold front crosses, but winds should remain generally below
Small Craft Advisory levels.

&&

Climate...
the Arctic air plunging southeast across our region later this
coming week will threaten a few records. Here are the current
record lows and record cold highs.

Site low 11/12 low 11/13 high 11/13 low 11/14
dca 24/1926 22/1911 31/1911 19/1920
BWI 18/1957 22/1911 32/1911 18/1986
iad 22/1988 21/1963 38/1996 13/1986

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...none.
Virginia...none.
WV...none.
Marine...Small Craft Advisory from 7 PM this evening to 6 PM EST
Tuesday for anz531>533-539>541.
Small Craft Advisory from 6 am to 6 PM EST Tuesday for anz530-
535-536-538-542.
Small Craft Advisory from 3 PM this afternoon to 6 PM EST
Tuesday for anz534-537-543.

&&

$$
Synopsis...bjl
near term...bjl/ads
short term...dhof

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