Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 221857

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
257 PM EDT Sun Oct 22 2017

.SYNOPSIS...High pressure will push eastward into the northwest
Atlantic through Monday. Low pressure will move up the
Appalachians Monday night into Tuesday. Upper troughing will
remain over the region through the second half of the week.



Mostly clear this evening, but low clouds and areas of fog are
expected to develop late tonight as low-level moisture increases
in SSE flow. Warmer tonight due to more clouds and SSE winds.



Low clouds mix out during the late morning, but mid-high clouds
thicken through the day with showers developing over the
Appalachians and spreading east Mon night as moisture deepens in
response to strenghtening low-level jet. Models indicate a thin
convective line will move quickly across the area with potential
for gusty winds of 35-40 mph as strong 850mb low-level jet of
50-60 kts mixes down with the showers. The pre-frontal convective
line should be east of the Chesapeake Bay by 18Z Tue but light
showers will remain possible Tue-Tue night as actual cdfnt
moves through and upper trough remains still to the west.
Rainfall amounts of 0.5 to 0.75 inches are expected except
higher over the Appalachians and Blue Ridge mountains. Given
progressive nature of front and dry antecedent conditions, no
flooding is anticipated.



The cold front will be well off the eastern seaboard 12Z
Wednesday as weak high pressure builds. Moderate 850 hPa cold
advection on Wednesday afternoon in NW flow should bring a fair
amount of strato-cumulus cloudiness across the area which will hold
down temperatures during the afternoon. 12Z/GFS shows 850 hPa
temperatures falling to at or just below 0 degC overnight Wed into
Thursday, but quickly start to rebound by 12Z Thursday. Along
upslope mountain areas Wednesday night, there could be some a few
showers, with temps fall aloft to below freezing...some wet snow
could mix with rain after midnight.

Thursday...high pressure builds across the southeast and into our
region...and will be in control with dry weather and seasonal
temperatures. Morning temperatures will probably be the coldest of
the week...but nothing unseasonably cold.

Friday...the center of the high will move off the southeast coast
during the day...allowing a return of warmer readings to commence on
Friday under clear skies.

Weekend...Saturday...the center of high will be over New
England...and temperatures will once again rebound to above seasonal
normals. Dry weather is expected on Saturday. By Sunday, more
uncertainty in the forecast associated with the timing of a cold
front extending from a low centered over the northern Great Lakes,
that is expected to cross the area. The new 12Z GFS is faster
brining in the colder air in on Sunday compared to the slower
00Z/ECMWF ensemble guidance. Looks like this is the next best chance
for widespread wetting precipitation.



Widespread low clouds (MVFR cigs) and areas of 2-4sm fog are
expected to develop late tonight as low-level moisture advects
in SSE flow. Dense fog is possible at KBWI and KMTN. Showers
expected to overspread the area quickly late Mon afternoon and
early evening as cdfnt approaches. Gusty winds will accompany
these showers with gusts up to 40 mph possible. LLWS is also
possible Mon night. Convective line will move east of the
terminals by 15Z Tue with flying conditions improving.

Expect VFR conditions through the extended. Possibly on Friday
there could be locally IFR visibilities in early morning fog at
IAD/MRB terminals.


.MARINE...Winds are expected to strengthen some this evening but
especially Mon afternoon and night. Have issued a gale warning
for much of the Chesapeake Bay and lower Potomac for Mon night in
anticipation of a convective line to move across the area when a
850-mb jet of 50+kt is expected to spread the area.

Small craft winds possible Wednesday. As high pressure builds in on
Thursday, expect winds/waves to remain below small craft advisory
levels for the remainder of the week.



Water levels are expected to rise rapidly Mon night as strong
srly low-level flow develops with minor coastal flooding
expected at most of the sensitive locations.


MARINE...Small Craft Advisory from 11 AM to 6 PM EDT Monday for
     Gale Warning from 6 PM Monday to 6 AM EDT Tuesday for
     Small Craft Advisory from 11 AM Monday to 6 AM EDT Tuesday for
     Small Craft Advisory from 6 PM Monday to 6 AM EDT Tuesday for


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