Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 110722

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
322 AM EDT Wed Oct 11 2017

.SYNOPSIS...A frontal zone will remain nearly stationary across
the Mid-Atlantic through this evening. An area of low pressure
will track along the front tonight and push it to our south and
east late tonight into Thursday. High pressure will build north
and east of the area later Friday through Saturday before
moving offshore Sunday. A stronger cold front will pass through
the area early next week.



The stalled front has become a lot easier to locate overnight thanks
to modest instability and isentropic lift triggering a line of
showers and thunderstorms located just south of Fredericksburg and
Charlottesville...or just south of the forecast area. Since the air
mass contains anomalously high precipitable water, rainfall
rates have been impressive...locally exceeding 2 in/hr per dual-
pol estimates. While instability will be easing slightly toward
dawn, the zone of best lift...and the front...will both be
lifting north to the aformentioned cities. Therefore, anticipate
an uptick of precipitation across central Virginia toward dawn.

Meanwhile, the shortwave trough will be passing eastbound just south
of the Great Lakes today. So, not only will there be isentropic
lift, but also ascent due to positive vorticity advection and the
right rear quadrant of the upper jet. As such, a line of showers
should overspread the region today. Anticipate that modest
instability will remain, especially south of the boundary...which
should stall somewhere close to the Potomac River. Rainfall should
continue to be efficient, with rates moderate to locally heavy. And,
instability will regenerate too with daytime heating, so have added
at least a slight chance of thunder. Guidance sugesting that central
Virginia should see MUCAPEs exceeding 1000 j/kg, so have a higher
order (chance) thunder there.

The trough axis/occluded front and triple point low should cross the
area this evening. Precipitation chances will diminish. However,
ample low level moisture will remain along with easterly winds.
Believe any overnight precipitation will be either light rain or,
more likely, drizzle.



The pattern through the end of the week (Thursday and Friday) will
be similar, marked by east/northeasterly flow around building high
pressure over New England. Model guidance exhibiting a pattern of
cold air damming, with light precipitation lingering along the Mid
Altantic. Forecast will be for ample clouds, chance of rain, areas
of drizzle, and patchy fog. The mountains will be preferred for
precipitation due to upslope, but chances reside just about
everywhere. Temperatures will be substantially cooler, likely
struggling to reach 70 degrees during the day, and staying near the
dewpoints (in the 50s) at night.



A strong mid-upper level ridge will remain anchored to the
south on Saturday with a return to above normal temperatures on
Sat after a couple of cool days Thu and Fri. The ridge will be
pushed to the east Sunday as a shortwave-trough and associated
cdfnt move across the Great Lks region Sun night. There could be
a few showers Sun night with the frontal passage but the
westerly sfc wind trajectories ahead of the front and lack of
instability at night suggest any rainfall will be very light at
best. The trof axis will move across the area Mon night with a
taste of fall weather middle of next week as sfc high pressure
builds in from the northern Rockies.



TAFs will be a challenge for the morning, as conditions will be
deteriorating as showers overspread the area. In the big picture,
flight conditions will decline from VFR to IFR. The challenge will
be to pinpoint when those threshold crossings will occur.

DCA has already reached IFR, and CHO looks like its about to. Am
uncertain whether DCA will remain constant after daybreak, as the
source causing the showers still far removed. Am more confident with
respect to CHO; believe that IFR will be in place by the morning
push. TAFs have prevailing IFR after the morning push, during the
midday hours, just ahead of a band of showers and perhaps a
thunderstorm. Caution that there likely will be an error of several
hours, and anticipate updates as these trends become more certain.

Flight conditions likely won`t improve once the showers pass east of
the terminals tonight. Instead low clouds and some fog will linger
due to east flow. MVFR-IFR likely to linger for a majority of the
time through the remainder of the week with a real taste of fall
weather for the middle of next week.

Cdfnt will be crossing the area late Sun night and early Mon
with only a few showers but no flying restrictions anticipated
at this time.



Flow across the waters light from the east at this time, but winds
should be increasing this afternoon and tonight. A Small Craft
Advisory in effect for the Bay and lower tidal Potomac after
midnight, and have extended it through Thursday. It likely will need
to be extended further.

Small craft advzy conditions likely Sun into Tue with a small
chance of gale conditions Mon night due to cold air advection
over warm water.



Water levels close to normal early this morning. Believe that water
will increase as prolonged east winds will push water into the Bay.
Issues may arise as soon as tonight or Thursday near St George
Island, although have a conservative forecast of Thursday evening
there. Complications will become more widespread on Friday.


MARINE...Small Craft Advisory from midnight tonight to 6 PM EDT
     Thursday for ANZ530>534-537-539>541-543.


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