Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 160750

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
350 AM EDT Wed Aug 16 2017

An area of high pressure will build into the region through
tonight. A warm front will move northward Thursday into
Thursday night followed by a cold front that should pass through
the region Friday night into early Saturday. Weak high pressure
will build into the area during the rest of the weekend.


A combination of light winds, lingering low level moisture, and
wet soils has led to the formation of dense fog and low
stratus late last night and early this morning. Therefore a
Dense Fog Advisory is in effect for much of the region through 9
AM for visibilities down to near zero in some locations.

Ridge of surface high pressure moving eastward through the Great
Lakes early this morning will then crest near the region this
afternoon. This will provide for mainly dry conditions and
partly- mostly sunny skies once the fog and low clouds break,
which may take until late morning (11 AM or so) in some
locations. There has been a couple of isolated showers this
morning as well towards the higher elevations, but the vast
majority of the area is not expected to see any measurable
rainfall. Temperatures will be slow to rise with clouds/fog
early, but should still reach the upper 80s to locally near 90F
by late in the afternoon.


For tonight, surface high will be slow to exit so another night
of light winds and mostly clear skies will be in store. We`ll
be one day removed from significant rainfall however, so while
areas of fog and stratus are likely again, coverage/density
should be less than this morning. Lows from the mid 60s to low

High pressure shifts offshore for Thursday allowing for the
return of southerly flow. As an approaching low pressure
system lifts through the Great Lakes, a warm front will lift
northward and into the region by Thursday afternoon. This and a
developing area of broad low level convergence will serve as a
focus for shower/thunderstorm development on Thursday. MLCAPE
south/west of the warm front is currently progged to be in the
1000-2000 J/KG range, which is quite sufficient for thunderstorm
development. Further examination of the modeled forecast
soundings would indicate potential for both an isolated severe
and isolated flood threat, with the presence of a low level
boundary, directional shear along the boundary, 0-6KM shear of
around 30 knots, and precipitable water values 2-2.25 inches.
Initial focus likely to be west of I-95, with propagation
eastward during the evening/overnight hours. Highs Thursday
should reach into the mid/upper 80s, with lows Thursday night in
the low to mid 70s.

The area of low pressure will push into southern Canada on Friday,
with its trailing cold front entering the Mid-Atlantic states
during the day. This will serve as another focus for scattered
showers/thunderstorms during the day Friday. A potential severe
threat will exist again on Friday, as temperatures and low
level moisture surge out ahead of the front, leading to the
development of 1500-2500 J/KG of MLCAPE, with 30-35 knots of
shear moving overhead. Highs upper 80s to around 90F. Convection
will wane Friday night.


Weak high pressure will build toward the area for Saturday and
Sunday. Most areas will remain dry with less humidity.
However...a pressure trough will likely be hung up near the
Chesapeake Bay into southern Maryland. Humidity may remain
across these areas along with an isolated shower/t-storm...but
much of the time will be dry.

High pressure will move offshore for Monday and Tuesday. Hotter
and more humid conditions are expected.


Widespread reductions in both ceilings and visibilities expected
this morning in fog and low stratus, with LIFR/IFR forecast at
all TAF sites. DCA has lowest risk of dense surface fog.
Conditions will gradually improve during the morning hours, with
VFR returning at most locations by 15z.

Potential for additional fog/stratus exists tonight, although
coverage/density should be less than this morning.

Next weather concerns will be the possibility of scattered
showers/thunderstorms both Thursday and Friday, where brief
reductions of both ceiling/visibility are possible.

High pressure should bring VFR conditions most of the time for
Saturday and Sunday.


Sub-SCA conditions expected through Thursday. South winds will
then be on the increase Thursday night and Friday ahead of a
frontal system, and Small Craft Advisory conditions are
possible. Isolated to scattered showers and thunderstorms are
also possible both Thursday and Friday, mainly in the
afternoon/evening, and Special Marine Warnings may be necessary.

A pressure trough will remain near the waters during the
weekend. Wind speeds should be below SCA criteria for the most
part since the gradient will be week. A return flow from the
south will develop early next week as high pressure moves


A light northerly flow will allow for anomalies to drop early
this morning. However...southerly winds will increase for
Thursday through Friday night ahead of a cold front. Minor tidal
flooding is possible near high tide Thursday night into Friday
morning and again Friday night into Saturday morning for
sensitive areas.


DC...Dense Fog Advisory until 9 AM EDT this morning for DCZ001.
MD...Dense Fog Advisory until 9 AM EDT this morning for MDZ003>006-
VA...Dense Fog Advisory until 9 AM EDT this morning for VAZ025>031-
WV...Dense Fog Advisory until 9 AM EDT this morning for WVZ051>053.


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