Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 151400
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1000 AM EDT Sat Jul 15 2017

.SYNOPSIS...
A cold front will push east of the region today, with weak high
pressure gaining control of the weather through this weekend.
An upper level trough will affect the weather early next week.
Another cold front may approach the region by the end of the
week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Mixing from northwest flow combined with a shortwave moving
across the US/Canadian border assisting in driving cold front
east of the forecast area this morning. A couple of showers have
stubbornly hung on across northeast Maryland. Have had to put
20% PoPs back into the forecast. Still believe that building
subsidence and advection of drier air make precip a limited,
near-term possibility.

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. Latest LAMP concurs, and no
adjustments made to temperature forecast. 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.

&&

.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
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.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
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.

&&

.AVIATION /14Z SATURDAY THROUGH WEDNESDAY/...
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.

&&

.MARINE...
Northwest flow overspreading the waters this morning. The
leading edge achieving full mixing, which is able to tap winds
close to 20 kt in gusts. Have added a Small Craft Advisory where
this threat resides (Maryland portion of the Bay below Sandy
Point and tidal Potomac below the Rte 301 bridge). Am using 3pm
as an end time for now. Threat will be diminishing through the
day (best mixing only at leading edge), so Advisory may be able
to be cancelled early.

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.

&&

.TIDES/COASTAL FLOODING...
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.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory until 4 PM EDT this afternoon for
     ANZ532>534-536-537-541.

&&

$$
SYNOPSIS...RCM/HTS
NEAR TERM...HTS/ADS
SHORT TERM...ADS
LONG TERM...RCM
AVIATION...ADS/RCM
MARINE...HTS/ADS/RCM
TIDES/COASTAL FLOODING...RCM



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