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Office: LWX

FXUS61 KLWX 151451
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
951 AM EST Fri Dec 15 2017

.SYNOPSIS...
Low pressure will develop along the North Carolina coast today,
moving northeastward and out to sea tonight. High pressure will
return for Saturday before weak low pressure impacts the area
early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Latest analysis shows low pressure developing off the North
Carolina coast and an upper-level trough swinging through the
Ohio Valley toward our area.

The upper-level trough will pass through the area today. A
strong jetmax aloft that is associated with the upper-level
trough will swing through late this morning through this
afternoon. Our area will be within the left exit region of the
jetmax and that will enhance lift across our area which will
also lead to the development of a coastal low off the North
Carolina coast. This system is progressive...and that is due to
energy digging into the Pacific Northwest and northern Rockies.
This means that the system will not phase in time over our area.

However...the jetmax is potent and this means that precipitation
will develop overhead as it moves through and it will continue
to develop and blossom as it is moving off to the north and
east. Temperatures are cold enough for the p-type to be snow
across our area. The best lift will be across the Washington and
Baltimore metropolitan areas this afternoon into southern and
central Maryland as well as northern Virginia. These areas have
the best chance for accumulating snow...and again with this
system developing overhead and moving northeast...the farther
northeast you go the better chance for accumulating snow.

Most likely snow accumulations are less than an inch south and
west of the Potomac River across northern Virginia to around and
inch near Washington DC...to 1-2 inches near Baltimore and
northeastern Maryland. Farther west across central Virginia into
the Shenandoah Valley...a brief period of light snow is possilbe
late this morning through early afternoon but any accumulation
will be light.

A couple things to point out. First...temperatures were cold
overnight and plenty of cloud cover will prevent temps from
rising too much today. Even for areas that get above
freezing...if it snows temperatures will drop below freezing due
to evaporative cooling. Therefore...any snow that does occur
will stick on paved surfaces and this will cause any untreated
surfaces to become slippery. Slippery untreated surfaces will
affect the evening commute for the Washington and Baltimore
Metropolitan areas even if snow ends before. Second...there is
a tight gradient overhead between no or little snow vs
accumulating snow and that is setting up near the Potomac River.
Therefore...any slight change in the gradient will have a
significant impact on the forecast. This lowers confidence a
bit.

The snow will exit the area between 1 and 3 pm across central
Virginia and the Shenandoah Valley and between 3 and 6 pm for
most other areas. Snow could hold on another hour or two near
Baltimore and extreme northeastern Maryland. High pressure will
build overhead tonight...bringing dry conditions with temps
below freezing. Slick spots that develop on untreated surfaces
will last throughout the night.

&&

.SHORT TERM /SATURDAY/...
High pressure builds in across the area on Saturday and Saturday
night with dry conditions expected. Temperatures will top out
during the day on Saturday in the upper 30s to middle 40s, with
overnight lows in the 20s to near freezing in the metro area.

High pressure will reside to our south on Sunday, resulting in
southerly flow across the region. This will generate moderating
temperatures in the mid to upper 40s. Conditions will remain dry
during the day with high clouds beginning to stream overhead
ahead of an approaching shortwave trough that will cross the
region Sunday night and early Monday morning. There is a slight
increase in precipitation as a result, but moisture looks
limited and with marginal temperatures, most favored
precipitation will be light rain showers across our northern
zones. Lows overnight Sunday will bottom out in the mid to upper
30s.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
East coast 500 mb ridging will be the dominant feature for the
extended forecast. Consequently, as shortwave energy traverses the
CONUS Tuesday, the primary precip type will be rain. And, since the
northern and southern streams will remain unphased, it's looking
like a lighter QPF amount for the DC/Baltimore metro. Will be
carrying slight chance PoPs for most, chance for the mountains.

High pressure and mild conditions will prevail for the balance of
the week.

&&

.AVIATION /15Z FRIDAY THROUGH TUESDAY/...
Predominate VFR conditions are expected through Sunday. A period
of light snow is expected across our eastern terminals this
afternoon, which will result in IFR periods for IAD/DCA/BWI/MTN.
High pressure builds in to the area Saturday and Sunday with
light to moderate westerly winds of 10 knots or less.

VFR will likely prevail Mon-Tue. There will be an impulse moving
through, so precip and flight restrictions will be possible, but
confidence low.

&&

.MARINE...
Low pressure will develop to our south today and move out to sea
tonight. A Small Craft Advisory is in effect for the Maryland
Chesapeake Bay south of Drum Point and the lower Tidal Potomac
River...closer to the gradient. High pressure will build to the
south for tonight through Saturday...but a tightening gradient
will result in SCA winds for portions of the waters. The
gradient will subside Saturday night and Sunday.

No hazards anticipated Monday into Tuesday. Winds will increase
thereafter, but any flags may hold off til beyond current forecast
period.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Winter Weather Advisory from 1 PM this afternoon to 7 PM EST
     this evening for DCZ001.
MD...Winter Weather Advisory from 1 PM this afternoon to 7 PM EST
     this evening for MDZ004>006-011-013-014-503>508.
VA...Winter Weather Advisory from 1 PM this afternoon to 7 PM EST
     this evening for VAZ054.
WV...None.
MARINE...Small Craft Advisory from 9 PM this evening to 7 PM EST
     Saturday for ANZ530>533-538>541.
     Small Craft Advisory from 6 AM to 7 PM EST Saturday for ANZ542.
     Small Craft Advisory from 3 PM this afternoon to 7 PM EST
     Saturday for ANZ534-537-543.

&&

$$
SYNOPSIS...BKF
NEAR TERM...BJL/BKF
SHORT TERM...BKF
LONG TERM...HTS
AVIATION...BJL/BKF/HTS
MARINE...BJL/BKF/HTS