Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 171817

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

An area of high pressure will cross the southeastern United
States tonight, 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.


Visible satellite shows an awesome depiction of widespread wave
clouds this afternoon. As the upper low/trapped mid level
moisture exits to the northeast, clouds will quickly diminish
from southwest to northeast this evening.

An area of high pressure over the northeastern Gulf of Mexico
will slide across the Florida Peninsula and into the Atlantic
Ocean tonight. As this happens, surface ridging nosing northward
into the area will cause winds to go light across much of the
Shenandoah Valley and central Virginia, allowing temperatures to
drop. Dew points in the middle 20s suggest lows in the upper 20s
to around freezing are likely where skies remain clear and winds
do in fact go light. The gradient will remain elevated over
northeastern Maryland closer to the departing surface low moving
into the Canadian maritimes, but temperatures should still drop
to around freezing due to the cold airmass. Meanwhile, a
developing warm front over southwestern Virginia will cause an
increase in clouds across the southern and central Shenandoah
Valley after midnight. Considering the dry air and narrow window
(spatially and temporally) for clear skies and light winds,
believe physical frost formation is unlikely except in patches
tonight, so have only issued a Freeze Warning where I expect
temperatures to drop below freezing, with no Frost Advisory
surrounding it yet.


Wednesday should wind up pretty warm (60s to near 70). Went a
little above guidance with downsloping westerly flow and
skyrocketing 850 mb temps (+10-12 C by 00Z). 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 terrain.

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.


Breezy conditions will linger into this evening on west flow.
VFR deck (OVC050) should clear quickly early this evening. Winds
gradually shift to southerly AOB 10 kts Wednesday with VFR.

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.


MD...Freeze Warning from midnight tonight to 9 AM EDT Wednesday for
VA...Freeze Warning from midnight tonight to 9 AM EDT Wednesday for
WV...Freeze Warning from midnight tonight to 9 AM EDT Wednesday for
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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