Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 210144

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
944 PM EDT Mon Aug 20 2018

A ridge of high pressure stretches across the northeastern
United States while a stationary boundary remains stalled near
the Virginia and North Carolina border. Low pressure will pass
northwest of the area Tuesday, lifting the boundary to our
south northward as a warm front. A cold front will then push
eastward on Wednesday, with high pressure building in from the
west through the end of the week.


A stationary boundary resides along the Virginia/North Carolina
border this evening as a ridge of high pressure stretches
from the New England states southwestward into our region. This
ridge will begin retreating northeastward overnight as the
boundary to our south begins to lift northward as a warm front
Tuesday morning. With a moist low level easterly flow over the
region tonight, low clouds and patchy fog will be the main
weather features through the overnight period. The 00z IAD upper
air sounding sampled the bulk of the moisture (1.49" PWAT) in
the lowest 10kft of the atmosphere, with substantial dry air
above there as the flow turns more northwesterly.

Increasing scattered showers are expected towards daybreak,
with a rumble of thunder not out of the question either as the
boundary to our south starts migrating northward and forcing
aloft improves. Overnight lows will be seasonable ranging from
the middle 60s to near 70 degrees. No changes needed with the
ongoing forecast package.


The warm front will gradually lift northward during the day
Tuesday. Low clouds and a few showers/drizzle will likely start
the morning for much of the region prior to warm frontal
passage. By the mid-afternoon hours, the warm front is expected
to have made some progress northeastward, although the extent
of which is still in question. At the same time, an upper trough
and cold front will be approaching from the west, with passage
during the evening and overnight hours. MLCAPE values on the
warm side of the boundary are progged to reach into the
1000-1500 J/KG range, with shear values of 30-40 knots atop it.
This will provide for an environment where strong to locally
severe thunderstorms are possible, and the Storm Prediction
Center has continued to highlight portions of the region in a
Slight Risk for severe thunderstorms, although the extent of the
event is still uncertain given instability questions and frontal
timing. With deep moisture, noted by forecast precipitable
water values in excess of 2", heavy rain and flooding is also a
concern, although storm motion won`t be all that slow. Highs
Tuesday will be in the low to mid 80s, with lows in the 60s and

A secondary front will cross the region on Wednesday, with
additional chances for scattered showers and isolated
thunderstorms. Highs Wednesday will be in the 80s. Much cooler
and drier air will push in behind the front for Wednesday night,
with lows in the 50s and 60s.


Thursday into Friday, an upper level ridge will build over the
region from the west. Surface high pressure will also move into
our area starting Thursday from the midwest. Skies will be
mostly clear with light west to northwesterly winds. Daytime
temperatures will be mild and below average in the mid to upper
70s on Thursday and the upper 70s to mid 80s on Friday.

Saturday through Sunday, a weak upper level shortwave trough will
approach the region from the west and move through over the weekend.
A weak frontal boundary associated with the shortwave will attempt
to move through late Saturday into Sunday. The 00Z Euro and the 12Z
GFS both agree that this front will be mostly dry leading to little
to no precipitation occurring over the weekend. The best chance for
precipitation will be near the mountains. Day time temperatures
for the weekend will hover in the upper 70s to mid 80s. Winds
will be light out of the south.


MVFR/IFR CIGs and MVFR VIS is likely at the terminals through
the overnight period as a moist low level easterly flow engulfs
the region. Scattered showers will increase towards daybreak
Tuesday, with an isolated thunderstorm not out of the question,
however the main driver for flight restrictions will be the low
ceilings and any fog/drizzle development, especially at MRB/CHO.
Expect light east southeasterly winds at less than 10 knots to
prevail overnight.

A warm front will lift northward Tuesday, followed by a cold
front Tuesday night. In between, ceilings will likely lift, but
showers and thunderstorms will become likely, some of which
could be strong with gusty winds. A secondary front will cross
on Wednesday and may produce some showers as it crosses.

On Thursday and Friday, winds will be light out of the west to
northwest with mostly clear skies. VFR conditions expected.


Predominate sub-SCA winds are expected overnight, however isolated
wind gusts upwards of 20 knots will be possible through
midnight along the lower Tidal Potomac and middle Chesapeake Bay
before tapering off a bit for the rest of the overnight period.

A warm front will lift north Tuesday and flow will turn
southerly. Wind speeds through the day may be marginal for SCA,
but the strongest winds are likely Tuesday evening and Tuesday
night, as an initial front passes across. A Small Craft Advisory
has been issued for this period. Showers and thunderstorms will
also become increasingly likely during the afternoon and
evening and could contain gusty winds.

A cold front will cross the area early Wednesday, and winds
don`t look particularly strong behind it. However, a secondary
cold front will pass late in the day with potential for a few
showers and storms. In its wake, SCA conditions will be

On Thursday and Friday, winds will be light out of the
northwest with no marine weather hazards expected.


Light onshore flow will persist through Tuesday morning before
turning more southerly as a warm front lifts northward Tuesday
afternoon. Minor coastal flooding is possible tonight through
Tuesday night, especially at the climatologically favored sites.
A Coastal Flood Advisory continues for St. Mary`s county for
tonight`s high tide cycle and additional advisories for
Baltimore, Annapolis, and the District looking more likely early
Tuesday morning.


MD...Coastal Flood Advisory until 2 AM EDT Tuesday for MDZ017.
MARINE...Small Craft Advisory from 6 PM Tuesday to 6 AM EDT Wednesday
     for ANZ532>534-537-540>543.


TIDES/COASTAL FLOODING...MM/BKF is the U.S. government's official web portal to all federal, state and local government web resources and services.