Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 150801

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
401 AM EDT Sat Jul 15 2017

A cold front will cross the region early today, with weak high
pressure gaining control of the weather through this weekend.


Surface front looks to be lingering across PA early this
morning, snaking to low pressure offshore. Cloud cover is likely
mitigating more widespread fog issues as dew points remain in
the lower 70s. However, expect patchy fog to be possible through
sunrise. A few straggling showers are also located near the
front, but they should be ending around the same time.

The front will get a kick south by an approaching shortwave
trough. Most of the CWA will be in northwest flow by late
morning. While it will be cooler than the past several days,
temperatures aloft will only be falling slowly. Thus expect DCA-
CHO and southeast will still be AOA 90F. Scattered cumulus
should also persist through the day.

Weak high pressure will be located west of the area tonight and
will provide tranquil conditions. There could be some patchy
fog, mainly in the western valleys. Lows will be in the 60s for
most areas.


Forecast has some minor wrinkles for Sunday and Sunday night as
the weak high moves offshore and return flow begins (especially
by night). Have kept the forecast dry through the daytime hours
due to capping and mid level dry air. However, some instability
will be returning to the mountains, so that would be the
favored area if a stray shower/storm were to form. Theta-e
advection begins in earnest overnight, with mid-level lapse
rates steepening as well. The question is whether there will be
any forcing in this layer. Troughing will be moving into the
eastern Great Lakes by that time, so it`s not out of the
question. Have allowed for some low end POPs for now.

A more defined chance for thunderstorms will come Monday as the
upper level trough continues to make slow eastward progression,
although it may be filling. Mid-level lapse rates will be
sufficiently steep for moderate instability to develop, though
deep layer shear will be on the weak side...around 20 kt. A few
stronger storms are within the realm of possibility though. The
highest chances for rain will be across northern parts of the
area. Showers and storms should diminish overnight.

Temperatures aren`t expected to stray too far from normal
Sunday through Monday as advection remains near neutral. Dew
points will rise though, especially on Monday.


An upper level trough will remain over the region Tuesday, with
attendant instability due to cooler air aloft and continued warm
southerly flow at the surface. This would result in a continued
chance of showers and t-storms with seasonable temps.

The lower end of the trough looks likely to cut off into a
closed low and sink southward away from our region later next
week, allowing heights to rise and warm air aloft to return.
This would likely reduce the shower/t-storm risk but also
increase temps back to above normal readings.

By the end of the week, another shortwave trough looks likely to
pass the region, possibly bringing a surface cold front with it
and another increasing risk of showers/t-storms.


Cold front remains to the north early this morning. Cloud cover
seems to be limiting fog somewhat, but have included MVFR vsby
through 12Z as temperatures are near the dew point and winds are
light. Front will push south this morning, with NW winds around
10 kt through the day with sct cumulus. There is at least some
minor potential for valley fog at MRB tonight.

Low probability of any aviation impacts Sunday and Sunday night
as weak high pressure moves east. Scattered to numerous
thunderstorms are expected Monday with an upper disturbance,
especially in the afternoon and evening.

Should be VFR through most of the week, except during any brief
intervals when showers/t-storms could affect the terminals on
Tuesday and again late in the week.


Thunderstorm threat has ended. SW flow will turn NW this
morning with cold frontal passage, but should remain below SCA
criteria. Light winds tonight and Sunday with high pressure,
then return southerly flow will begin Sunday night into Monday.
Will have to keep an eye on channeling potential, but otherwise
confidence is low on SCA conditions being reached.

After Monday, winds look likely to stay sub-SCA most of the
week. Main concern will be any need for special marine warnings
with t-storms, with highest risk being Tuesday and Friday.


Southerly flow developing across the region as high pressure
shifts offshore will start to build anomalies Sunday, with minor
flooding at high tide possible at our sensitive sites (most
likely Straits Point) by Sunday aft/eve. This would likely
continue into Monday.




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