Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 141943
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
243 PM EST Sun Jan 14 2018

.SYNOPSIS...
Canadian high pressure will settle over the eastern U.S. through
tonight before sliding to our east Monday. A clipper system
will bring a cold front through the area Tue night into
Wednesday. Low pressure will develop offshore late Wednesday
into Thursday before high pressure settles to our south late in
the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Canadian high pressure will continue to build toward the area
this afternoon...bringing dry and cold conditions despite
sunshine.

High pressure will remain overhead tonight. Light winds and dry
air will provide a good setup for radiational cooling. There
will be some high clouds ahead of an upper-level trough over the
Great Lakes into the southeastern CONUS. This may prevent
radiational cooling to some extent...but current thinking is
that the cloud deck will be thin. Therefore...min temps are
expected to range from the single digits in colder valleys and
rural areas to the teens across most other locations.

&&

.SHORT TERM /MONDAY THROUGH TUESDAY NIGHT/...
A potent northern stream system will cause upper-level low
pressure to dig into the northern Plains and western Great Lakes
Monday. This will continue to carve out an upper-level trough to
our west. Shortwave energy and a jetmax well ahead of the
upper-level low will pass through our area Monday while high
pressure slides just to our east. The jetmax will have decent
forcing with it since our area will be in its left-exit region.
This will back the low-level flow...resulting in isentropic lift
across much of the area. Since there is plenty of dry air in
place most locations should end up dry with just some clouds.
However...flurries and perhaps even a period of very light snow
cannot be ruled out across eastern areas where there will be a
little more moisture to work with. The best chance for a period
of very light snow will be near the Bay from mid-morning through
the afternoon and it could coat the ground. Confidence in this
is low at the this time since the deeper moisture is expected to
remain well off to the east. The current forecast mentions a
chance for flurries but this will have to be monitored closely.
Chilly conditions will continue with highs in the upper 20s and
lower 30s near Washington and Baltimore.

The shortwave energy and jetmax will move off to the north
Monday night. A cold front will approach from the Ohio Valley
and a southerly flow will allow for relatively milder conditions
compared to Sunday night...but it will still be cold.

A potent upper-level low will slowly pass through the eastern
Great Lakes Tuesday through Tuesday night. This system will be
strong enough for surface low pressure to develop over the same
area. The upper-level trough axis associated with this system
will swing closer towards neutral tilt during this time. A
strengthening jet will cause a band of precipitation to develop
Tuesday and it will slowly move south and east into our area.
Exact timing remains uncertain due to divergence in the
guidance. For now...the timing of snow is a blend between the
faster GFS and the slower ECMWF/NAM which is close to the GEFS
which lies in between. This means that the best chance for
accumulating snow Tuesday morning will be in the Potomac
Highlands...but it may make a run south and east toward the I-95
Corridor by Tuesday evening. There is a better chance for snow
later Tuesday night near and south/east of the I-95 corridor.

&&

.LONG TERM /WEDNESDAY THROUGH SUNDAY/...
Light snow may be ongoing Wednesday as an upper trough and jet
max aloft move across the area and induce cyclogenesis off of
the eastern US coastline. Much of the precipitation tied to the
coastal low now appears to remain offshore, however upper level
forcing and jet dynamics still favor a period of snow
Wednesday. Some accumulation is likely, although timing and
amounts are still uncertain as non-trivial model differences
still exist.

Very cold and blustery conditions will then again follow behind the
departing system for later Wednesday, Wednesday night, and lasting
into Thursday morning. High pressure will then build south of the
region from Thursday through Saturday, which will promote southerly
flow and moderating temperatures over the Mid-Atlantic. A fast
moving upper level shortwave will race across the northern tier of
the country and into the northeast on Friday, and this may bring a
chance of some light precipitation.

By next Sunday, a low pressure system will be developing in the
central/southern Plains states and eject northeastward towards the
Great Lakes. This will further increase warm air advection and
southerly flow across the area.

Temperatures during the extended period will start quite cold with
highs in the 20s on Wednesday, and lows in the teens Wednesday
night. A gradual moderation is expected for the end of the week and
weekend with 30s for highs Thursday, 40s Friday, and then likely
breaking into the 50s by Saturday and Sunday.

&&

.AVIATION /19Z SUNDAY THROUGH FRIDAY/...
VFR conditions are expected through tonight. Warm advection
behind high pressure sliding east Monday will cause lower clouds
to develop across the eastern terminals. MVFR conditions are
likely for KBWI and KMTN...KDCA and possible for KIAD. A few
flurries are possible Monday across the same terminals. MVFR
cigs may continue Monday night across the eastern terminals.

A cold front along with low pressure will cause a band of snow
to develop Tuesday morning to the north and west. The snow will
slowly slide south and east...impacting the terminals later
Tuesday and Tuesday night. The snow may hang around for much of
Wednesday as well...especially across the eastern terminals.
Exact timing is uncertain but confidence has increased for a
period of accumulating snow during this time.

Primarily VFR will return for Wednesday night through Friday,
although some light precipitation is also possible Friday. Gusty
northwest winds are also expected to develop Wednesday into
Thursday.

&&

.MARINE...
A north to northwest flow will continue through the rest of the
afternoon. Wind speeds will gradually decrease during this time.
A Small Craft Advisory remains in effect until 5 pm.

High pressure will remain over the waters tonight...but a weak
pressure surge will cause northerly winds to channel down the
Bay/lower Tidal Potomac River. A Small Craft Advisory remains
in effect for the lower Tidal Potomac River and Maryland
Chesapeake Bay south of Smith Point. Weak high pressure will
remain just offshore Monday through Tuesday night. Winds should
remain below SCA criteria during this time.

Sub-SCA conditions are likely Wednesday into Thursday behind a
departing low offshore. Another period of SCA conditions are
possible on Friday with a weak system.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 5 PM EST this afternoon for
     ANZ530>533-535-536-538>542.
     Small Craft Advisory until 6 AM EST Monday for ANZ534-537-543.

&&

$$
SYNOPSIS...BJL
NEAR TERM...BJL
SHORT TERM...BJL
LONG TERM...BJL/MM
AVIATION...BJL/MM
MARINE...BJL/MM



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