Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 201812

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
212 PM EDT Sun Aug 20 2017


High pressure over the area will move offshore tonight through
Monday. A cold front will approach the area Tuesday before
passing through Wednesday. High pressure will build in behind
the boundary for Thursday through Saturday.



High pressure is overhead this afternoon. A cu field is developing
along the ridge lines out west. High pressure will slowly move
eastward today and a light southerly flow will begin mainly across
the VA Highlands, central Shenandoah Valley and central foothills.
This will increase dewpoints some and iso showers and
thunderstorms are possible in this area this afternoon/evening.
High pressure will continue to move eastward and off the coast
tonight, and southerly flow will increase moisture into the
region overnight. Although any shower/thunderstorm activity will
likely be diurnally driven, waning early this evening, some
activity may continue to move eastward possibly reaching I-95 by
early Monday morning. Many NCAR ensemble members show this.

Moisture advection will continue Monday and low stratus will likely
be across much of the region Monday morning. Daytime mixing will
help break up low status by late morning and breaks of sunshine are
expected early Monday afternoon. We are concerned with the activity
across IA/MO this afternoon as it will track eastward in the
westerly flow aloft and be near our outlook area Monday afternoon.
Increased instability and shear will lead to an environment
conducive for showers and thunderstorms to form across the region.
There should be a lee trough in place and coupled with a crossing
disturbance some storms may become strong to severe.



Convection will likely be ongoing Monday night mainly near I-95
and east. Little change to the environment overnight into
Tuesday so leftover boundaries may cause a pop-up shower and
thunderstorm into Tuesday morning.

Warm and humid conditions will continue Tuesday as a cold front
crosses the Ohio Valley. Showers and thunderstorms are
possible...especially late Tuesday into Tuesday night across
northern and central portions of the CWA closer to the cold front.
Some storms may be strong to severe given the instability and
strengthening shear profiles.



Most of the guidance is in decent agreement that the cold front
won`t clear southern parts of the CWA (CHO to southern MD) by
Wednesday afternoon, which means another round of scattered showers
and thunderstorms will be possible for these areas. With the upper
trough diving down/0-6 km shear still modestly high (especially for
August), can`t rule out a few strong to isolated severe storms for
this corridor Wednesday afternoon and early evening, as well.

A rather strong area of high pressure for this time of year (~1025
mb) will drop across the Great Lakes Wednesday night into Thursday
which should push the front well south. The high pressure center
will gradually weaken as it drifts just north of the area Friday
into the weekend, which should result in below average temperatures
and humidity with mainly clear and dry weather conditions.



Light flow AOB 10 kts today gradually veers from W to SE by
evening. LAMP/MET guidance wants to bring in widespread IFR
vsbys but given climo/synoptic setup believe this is over done.
MVFR vsbys/cigs psbl CHO/MRB included in TAF, maybe IAD/BWI/MTN
by daybreak but not confident enough to put in TAF at these
sites. May be VCSH near CHO this aftn 20-23z but again
confidence too low/coverage too sparse for TAF.

Brief break in clouds (from bkn to sct) between 14-18z Tue, then
back to bkn E of Blue Ridge (all terminals except MRB) w/ pop up
shower or tstm seeming plausible 18-00z timeframe. Too sparse this
far out for mention in TAF just yet. Brief vsby restrictions psbl.

Tstm psbl Wed PM near CHO, should stay S of DCA/IAD unless
front slows its southward progress. Otherwise mainly VFR. Generally
NW flow AOB 10 kts.



High pressure will build over the waters today before shifting
toward the Mid-Atlantic Coast tonight. The high will move offshore
Monday through Monday night and a cold front will approach the
waters Tuesday through Tuesday night. The boundary will pass through
Wednesday and high pressure will build over the waters for late next
week. A Small Craft Advisory may be needed for portions of the
waters Monday night through Wednesday night.

Isolated thunderstorms are possible over the waters late tonight
through Monday morning. Scattered thunderstorms are possible Monday
afternoon and evening. Some storms may produce gusty
winds...especially later Monday into Monday evening. A few showers
and thunderstorms are possible later Tuesday into Wednesday ahead of
a cold front as well...and some of these storms have the potential
to be strong to locally severe.

Northerly pressure surge/channeling behind cold frontal
passage Wednesday into Wednesday night will likely require the
issuance of Small Craft Advisories.



Minor coastal flooding is possible at Annapolis Tuesday and
Wednesday morning.




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