Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 050900
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
400 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/...
Pattern changing trough and cold front is about to move through,
though we get a taste of spring before winter arrives. Some
places have radiated early this morning with clearer skies than
anticipated, and we also have some patchy fog out there. A
strong southerly jet is just above the surface, however, so as
we head through the morning, we should get enough mixing to
rapidly warm the surface. Temperatures will likely soar into the
60s across much of the region by afternoon. A few showers may
develop early this morning near the Bay with some warm advection
on the low level jet, but overall expect most of the day to be
dry near and east of I-95. Further west, showers with the
approaching front are likely to start encroaching on the area
early this afternoon, and will probably reach I-95 by rush hour,
making for a less than pleasant commute home. However, the front
will lag behind, so while it will likely be a wet evening
commute, it should still be mild.

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-existant, 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 /09Z TUESDAY THROUGH SATURDAY/...
Starting generally VFR aside from patchy fog this morning.
However, with a strong low level jet just above the surface,
low level wind shear is going to be an issue for the next
several hours. Wind shear should diminish later this morning as
jet weakens and low levels mix a bit, with patchy fog likely to
diminish as well due to the same process.

Otherwise, main concern will be sub-VFR cigs/vis as the day
wears on. This occur for a time this morning around BWI where
some showers could pop before dawn or shortly after, but for
most terminals, the main timeframe of concern is later this
afternoon and this evening as showers move across the region
ahead of the approaching cold front. MVFR cigs and vis look
likely and a period of IFR cigs is very possible as well, most
likely after sunset. 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 early this
morning, spreading north later this morning and up 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>533-
     537>542.
     Small Craft Advisory until 6 PM EST Wednesday for ANZ534-543.
     Small Craft Advisory from noon today to 6 PM EST Wednesday for
     ANZ535-536.

&&

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



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