Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 171612

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1212 PM EDT Wed Aug 17 2016

A weak cold front will move through the area tonight and move
south of the area Thursday and Friday before dissipating. Another
cold front will cross the area early next week. High pressure
returns for the middle of next week.


Low pressure is located near the Maine coast this morning with an
associated cold front stretching south into the Ohio Valley.
A vort max is located over northern Kentucky and will be
responsible for an increase in showers and storms late this
afternoon and evening. The main concern is a heavy rain/flash
flooding threat, with a secondary severe threat this afternoon and
evening. A Flash Flood Watch has been issued for the central part
of the CWA including Baltimore and Washington.

The cold front will stall across the Mid-Atlantic region this
afternoon/evening. Warm and humid conditions are expected ahead of
the cold front with max temps in the mid 80s across the Potomac
Highlands to the low and mid 90s across the Washington DC metro
and south. Based on trends, have bumped up temperatures a degree
or two. Heat indices around 100 or slightly higher are possible
from DC to the south.

While some storms will begin to form over the terrain this
afternoon, the best forcing will not arrive until late afternoon
or evening. 12Z IAD sounding also shows a small cap that will need
to be overcome. There may be just enough overlap in shear and
instability for there to be a threat of some severe storms.

There will be large moisture flux along the boundary as the wave
arrives from the west this evening along with strong synoptic
forcing. In an environment of high PWAT, heavy rain rates will be
possible, and some training or regenerative processes could occur
along the front. This corridor may align from the central
Shenandoah Valley to DC, where the HRRR time lagged ensemble is
showing maximum probabilities for heavier rain. Latest guidance
continues to support this trend. However convective trends could
warrant expansion of the watch.

Showers and thunderstorms will sag southeast overnight as the
shortwave trough moves across the region.


Light northerly flow and subsidence behind the shortwave trough
Thursday will lead to temperatures below 90 degrees. Some
instability will be present Thursday afternoon and an isolated
shower or thunderstorm can`t be ruled out mainly across the
southern half of the outlook area. Little forcing across the area
will keep most places dry.

High pressure will be across the region Thursday night into Friday
and dry conditions are expected with temps reaching the low 90s/80s
in the mtns Friday.


Global models show warm moist srly flow returning to the area Sat
in advance of a deepening trough over ncntrl US and cntrl Canada.
Temperatures rise back into the low 90s Sat and Sun with
increasing risk of showers and t-storms. Numerous to widespread
showers expected Sun-Sun night as cdfnt crosses the area. Don`t
think there will be a flood threat as front looks fairly
progressive. By Mon night, high pressure begins to build from the
west and front is far enough east off the coast for dry weather.
Temperatures return back to normal or slightly below normal with
readings in the mid 80s. Dry wx much of next week under high
pressure and zonal flow aloft.


SHRA/TSRA possible mainly from late this afternoon through the
evening as wave of low pressure approaches from the west. There`s
potential for a longer period of heavy rain/restrictions in the DC
area early this evening. Sub- VFR conditions expected in activity.
A gusty wind threat will also be present. SHRA/VCTS will taper off
into early Thurs morning.

Mostly dry conditions expected Thurs however isolated SHRA/VCTS are
possible. VFR conditions expected Thur-Friday night.

Scattered t-storms possible Sat afternoon which may produce brief
flying restrictions. A cold front moves through the area late Sun
with more widespread convective activity.


Winds have become generally westerly this morning and should
remain less than 15 kt today. Strong storms with gusty winds will
approach the waters possibly late this afternoon, but more likely
this evening.

The front will stall or only slowly move to the south tonight.
The weak gradient will eliminate the threat of small craft
conditions. A few thunderstorms may redevelop Thursday.

No marine hazards expected Thursday through Friday night.

Winds strengthen some Sat but should remain below SCA.


DC...Flash Flood Watch from 4 PM EDT this afternoon through late
     tonight for DCZ001.
MD...Flash Flood Watch from 4 PM EDT this afternoon through late
     tonight for MDZ011-013-014-503>506.
VA...Flash Flood Watch from 4 PM EDT this afternoon through late
     tonight for VAZ026>031-038>040-050>054-501-502-505>507.


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