Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 051443
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
943 AM EST Tue Dec 5 2017

.SYNOPSIS...
High pressure will move further offshore today as a strong cold
front approaches from the west. The front will move through the
area tonight. Weak high pressure returns to the area for
Wednesday and Thursday. Low pressure will pass to the east
Friday and Friday night, then another low pressure may cross
the region Saturday and Saturday night.

&&

.NEAR TERM /THROUGH TONIGHT/...
Cold front is located back across the Ohio Valley this morning,
but will continue moving eastward towards the region through
the day today. Southwest flow out ahead of the front is ushering
in milder and more moist air, and we`re already seeing some
showers this morning west of the Blue Ridge. These will
gradually increase in coverage as the main rain band moves into
the area, and should be approaching the I-95 corridor by late in
the day (around the afternoon/evening rush hour). Also
contending with low stratus and some patchy fog/drizzle this
morning directly along the I-95 corridor as well, and we should
see gradual lifting of the lower clouds and reduced visibilities
heading into the afternoon as southwest flow increases. High
temperatures are expected to reach the 50s/60s, warmest in
central VA.

Showers continue until around the time of frontal passage or
perhaps just after it tonight, with timing looking to be just
after midnight near I-95. As it crosses the region, winds turn
NW and temps will rapidly drop. Since most precip should be
ahead of the front, think any snow even in the highest
elevations will be minimal if not non-existent, but allowed for
mixing at the end in the higher terrain of WV and the Blue
Ridge. Temps will drop into the 40s by morning most areas, with
30s colder spots, and sub-freezing mountains.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Much colder but relatively tranquil weather is expected during
the short term as high pressure settles to our southwest but
extends its influence northeastward across our region. Any
showers lingering in southern Maryland early Wednesday should
exit fairly quickly, with clouds breaking for some sun and a
continued gusty northwest wind advecting much colder air across
the region. While some temperature rise is expected on
Wednesday, it will be very modest, less than 10 degrees for
sure, perhaps less than 5.

Clouds will not completely clear the region as the front will be
stalled to our southeast Wednesday night through Thursday, and
upper level winds will likely continue to carry high clouds
northward. However, odds of any precip look quite low Wednesday
afternoon through Thursday evening. Lows will mostly be below
freezing, but not particularly cold for December, on Wednesday
and Thursday night, with limited radiational cooling expected
due to continued NW flow and some cloud cover. Highs Thursday
will be similar to Wednesday... 40s in much of the area.

By late Thursday night, a developing low pressure along the
stalled front to our southeast may start to spread some rain or
snow into southern Maryland towards morning. Rest of the region
should stay dry.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
An upper trough elongated from NE to SW across the eastern U.S.
will continue to deepen on Friday. A shortwave embedded within
this trough will round the base and eject eastward through the
Tennessee Valley toward the Carolinas through the day. This
shortwave energy is expected to be more strung out as opposed to
compact, resulting in a surface low that develops east of Cape
Hatteras and then moves well offshore. Still, the right entrance
region of a strengthening 250 mb jet and PVA ahead of the
shortwave will likely bring some precipitation over southeastern
parts of the CWA, with lower (but non-zero) chances further
west.

Perhaps a more important player locally will be a stout upper
low pivoting around the trough Saturday. 500 mb temperatures
plummet to below -30 C suggesting enough instability for squalls
as the upper low pivots through. Models differ on the surface
reflection and placement/extent of precipitation, but localized
bursts of snow would be possible in this set up. It should be
noted, however, that even over the last 24 hours models have
shifted on strength and timing, lending to uncertainty in the
all-important finer scale details. For now it appears the best
window for snow showers or squalls would be late Saturday into
early Sunday, but this is subject to change.

Timing of individual shortwaves rounding the upper trough
becomes much more uncertain by early next week, but a consensus
of the latest guidance would suggest the next spoke would round
the bicycle wheel Tuesday into Wednesday.

&&

.AVIATION /15Z TUESDAY THROUGH SATURDAY/...
IFR being observed this morning at DCA/BWI/MTN in low stratus
and some light drizzle/fog. Expecting gradual improvement by
late morning and early afternoon with conditions rising to MVFR
and then perhaps to VFR by later this afternoon. Elsewhere at
IAD/MRB/CHO VFR is expected to continue until rain moves in from
the west later today. Low level wind shear is also a concern
this morning with FL020 winds about 40 knots. Periods of sub-
VFR conditions will then overspread the region in rain for the
late afternoon and evening.

Conditions should improve after cold frontal passage late
tonight, with VFR returning by Wednesday morning and continuing
through Thursday night. Southerly winds which may gust 20-25
knots at times today will shift northwest and remain gusty late
tonight and Wednesday, with some diminishing in wind speed
Wednesday night and Thursday.

Mainly VFR Friday with light north flow. Sub-VFR possible in
snow showers over the weekend (most likely late Saturday into
early Sunday). Winds may tend to be light and variable during
this time with the region between low pressure well to the east
and a clipper-like system approaching from the west.

&&

.MARINE...
Southerly flow ahead of an approaching strong cold front will
result in small craft advisory conditions overspreading the
waters by this afternoon, starting in the middle bay this
morning, spreading north up the bay the Potomac this afternoon.
There may be lulls this evening but winds likely pick back up
overnight behind the strong cold front, with small craft
advisory conditions expected to continue on all waters through
midday Wednesday. They may relax as the afternoon wears on but
extended SCA until 6PM for the time being. Winds look like they
may be lighter with a weak high pressure ridge over the region
Wednesday night and Thursday but a few stray gusts up to 20
knots can`t be completely ruled out.

Winds should be relatively light Friday into Saturday as the
region becomes sandwiched between low pressure well to the east
and a clipper-like system approaching from the west.

&&

.TIDES/COASTAL FLOODING...
Water levels are expected to rise today as surface winds
strengthen. Minor coastal flooding is possible at times of high
tides this afternoon and tonight. Water levels should decline on
Wednesday with northwest winds returning.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 6 PM EST Wednesday for ANZ530>534-
     537>543.
     Small Craft Advisory until 6 PM EST Wednesday for ANZ535-536.

&&

$$
SYNOPSIS...RCM
NEAR TERM...MM/RCM
SHORT TERM...RCM
LONG TERM...DHOF
AVIATION...MM/RCM/DHOF
MARINE...MM/RCM/DHOF
TIDES/COASTAL FLOODING...MM/RCM/DHOF



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