Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 180210

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1010 PM EDT Tue Apr 17 2018

An area of high pressure will cross the southeastern United
States overnight, then move offshore Wednesday. An area of low
pressure will move west to east across the Great Lakes Wednesday
night into Thursday. High pressure will build in from the Ohio
Valley Friday into the weekend.


A surface ridge of high pressure will expand northward
overnight, allowing for surface temperatures to drop as winds
diminish or become calm. Temperatures will drop into the lower
30s to perhaps into the upper 20s in much of the eastern
Virginia Piedmont into northeast Virginia and northeast
Maryland. A freeze warning has been issued to alert of potential
harm to plants and sensitive vegetation. A developing warm
front over southwestern Virginia will cause an increase in
clouds across the southern and central Shenandoah Valley after
midnight. Frost formation could occur in parts of this region
due to the calm winds and clear skies, but the bigger concern is
for freezing temperatures to the north and east.


Wednesday should wind up pretty warm into the 60s to near 70
degrees. Clouds will increase through the day as a warm front
continues to develop over central/southwest Virginia and moves
northeastward, with a few sprinkles possible over the higher

24 hours after freezing temperatures, Wednesday night should
prove mild with clouds and mild downsloping breezes. Lows in
the middle 40s to lower 50s are expected. Most showers should be
confined to the Allegheny Highlands as low pressure passes to
the north, with very little moisture return east of the
mountains on westerly flow.

Cold advection begins by about midday Thursday as low pressure
to the north departs across southern New England. Downsloping
flow should result in mild temperatures at the surface, though,
especially in the I-95 corridor. The cooling temperatures should
cause precipitation to change to snow showers over the Allegheny
Highlands with a coating to an inch or two of snow expected.

Cooler temperatures return Thursday night with lows in the 30s.


At the start of the period Friday, a departing area of low pressure
will be located over the Canadian Maritimes. In the wake of
that low, breezy northwesterly winds will continue ushering
colder air into the region. Other than a few lingering upslope
snow showers along the Allegheny Front, no precipitation is

High pressure centered over the Great Lakes and Ohio Valley
will build in for the weekend, maintaining cool and dry
conditions over the area. Winds will weaken and shift from light
northerly to light easterly as high pressure slides off to our

By Sunday Night, models have a cut-off upper level low and
associated surface low tracking through the Lower Mississippi
Valley. As that system moves east on Monday, models keep it
suppressed off to our south as it interacts with a southeastward
translating, positively tilted trough located over the Mid-Atlantic.
As a result, it looks like the precipitation associated with that
low will stay south of our area on Monday.


VFR conditions should clear through the evening. Winds will
shift to the south at around 10 knots by Wednesday. VFR
conditions once again Wednesday.

Low pressure will pass north of the area Wednesday night into
Thursday. Widespread showers are not expected to reach east of
the mountains, so no restrictions are expected. BKN VFR deck
likely as the low passes, with gusty W flow (G 30-35 kts)
expected in its wake.

VFR conditions are expected Friday through the weekend. A gusty
northwest wind on Friday should diminish by Saturday.


SCA gusts linger until at around midnight for all waters, then
begin to retreat east across the open waters of the Chesapeake
Bay as the gradient relaxes from west to east. Winds should be
below SCA criteria by daybreak. A few gusts around 20 knots are
possible during the day, but mixing down gusts in warmer air
over cooler water makes this more uncertain.

Small Craft Advisories will become more likely late Wednesday
night as the gradient increases further after low pressure
passes over Pennsylvania, with gale conditions likely
(especially in any scattered shower or sprinkle activity) on

Small Craft Advisories may be needed Thursday night into Friday
as departing low pressure moves into the Canadian Maritimes.
Winds should be light over the weekend.


Drier air advecting into the region Thursday on gusty west winds
could result in an enhanced risk of fire spread Thursday,
especially if fuel moistures decrease appreciably by then.


Due to the mixed fresh/tidal input, the forecast is more
uncertain than normal for the SW DC Waterfront, but minor
flooding could be observed around midnight around the high
tide cycle.


Millville will be close to flood stage and we will be monitoring it
overnight. Shepherdstown will be having minor flooding into
Wednesday morning.


MD...Freeze Warning until 9 AM EDT Wednesday for MDZ003>006-011-
VA...Freeze Warning until 9 AM EDT Wednesday for VAZ026>031-038>040-
WV...Freeze Warning until 9 AM EDT Wednesday for WVZ051>053.
MARINE...Small Craft Advisory until 6 AM EDT Wednesday for ANZ530>534-
     Small Craft Advisory until midnight EDT tonight for ANZ535-536-


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