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Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia
936 PM EDT Tuesday Sep 1 2015

Synopsis...
high pressure will remain centered to the west through tonight.
An upper-level disturbance along with its associated surface
trough will pass through the region Wednesday. Weak high pressure
will build overhead Thursday before a backdoor cold front moves
into the area late Friday into the weekend.

&&

Near term /through Wednesday/...
surface high pressure is centered over West Virginia this evening with
a weak low off Cape Hatteras. Aloft...ridging is centered along
the eastern Seaboard with a cutoff low near the Indiana/Ohio
border. Most convection has diminished at this hour but a few
stray showers remain west of the Blue Ridge. These should continue
to gradually wane.

Light to calm winds and elevated dew points will result in another
night where fog is possible. The uncertainty factor tonight will
be the approach of clouds from the west as the weakening low
approaches. As a result...the Virginia Piedmont may once again be the
favored location. There also appears to be some fairly thick haze
trapped under the inversion...which is also resulting minor
visibility reductions. Lows will only fall to the middle 60s to
lower 70s.

The closed low is forecast to open and dive southeast Wednesday as
ridging builds over the plains. The track isn/T much different
than the overnight forecast but is still farther south than
previous days. While the upper level trough and weak surface
reflection will be a source of lift...appears the main vorticity maximum
will just clip the southwestern County Warning Area. Therefore have focused
highest probability of precipitation there. It is quite possible that District of Columbia...and especially
Baltimore...see little to no activity. May see showers develop
before typical diurnal timing depending on timing of forcing.
Instability and shear are not very impressive...so do not have much of
a severe concern. Highs may be dependent on cloud cover...but
upper 80s to lower 90s are still supported across much of the
area...and if clouds hold off it could very well be just as hot on
Wednesday as it was on Tuesday.

&&

Short term /Wednesday night through Friday/...
the trough aloft will be east of the area by around 06z...so have
the lingering probability of precipitation in the southeast County Warning Area ending by then. There
should be some clearing...especially in northern areas. However
with little change to the low level air mass...it will again be
muggy with lows in the middle 60s to lower 70s...and fog will again
be possible.

Thursday...weakening upper level short wave will be south and east of County Warning Area
leaving US in region of height rises and subsidence. Another in a
string of days with temperatures 5-10 degrees above normal...with
highs in u80s/l90s. Moderate humidity with dewpoints in m60s...but
little chance of meaningful precipitation...as lapse rates remain poor and
there is little forcing for ascent. As such...expect only isolated
afternoon activity along/west of Blue Ridge...with most areas
remaining dry most of the time.

Friday...after several days of temperatures in the u80s/l90s...there
could be a little relief by Friday...as weak back door cold front
nudges into the area. Current frontal and thunderstorm timing occurs
early enough that daytime highs might be a few degrees lower than
previous days...generally in M/u80s...though still above normal.
Heat and humidity remain in place ahead of the front and middle-level
temperatures cool slightly...which will lead to at least low-end shower and
thunderstorm chances across the entire area as lapse rates steepen
ever so slightly.

&&

Long term /Friday night through Tuesday/...
upper level disturbance in addition to a will bring the chance for
scattered showers and thunderstorms Friday night into Sat... mainly west of
the Blue Ridge and in the southern part of our County Warning Area. After a
backdoor cold front slips down the middle-Atlantic coast...high
press builds in on Sat through Tuesday bringing drier conditions.
Approaching cold front may bring showers and thunderstorms on Tuesday afternoon.
Temperatures will be near normal during the weekend...becoming above normal
Monday and Tuesday.

&&

Aviation /02z Wednesday through Sunday/...
maintained overall forecast consistency in regards to fog
overnight. Cloud cover is the main uncertainty and potential
limiting factor. Have not included br at iad for now...but ground
fog is a possibility. Trough of low pressure passes through region
Wednesday...bringing a little better probability of storms...especially
in the afternoon. Cho will be the main target...with much more
uncertainty in coverage with NE extent (especially Baltimore
area). Any impacts should be short- lived. May be some fog again
in prone locations Wednesday night. After light/vrb winds through
tonight...a more northwest component should be obtained Wednesday...but still
generally less than 10 knots.

VFR conditions expected Thursday under high pressure. Afternoon and
evening shower/thunderstorm chances return Friday with brief periods
of sub-VFR possible near any activity.

VFR conditions expected Friday night and Sat. Sub VFR conditions
possible due to scattered showers and thunderstorms.

&&

Marine...
onshore component to winds this afternoon...perhaps becoming SW
tonight. Winds become west to northwest Wednesday. Speeds remain under 10 knots most
of the time through Wednesday night. Isolated to scattered storms a
little better possibility on Wednesday...more so over the
southern waters.

Winds expected to remain sub-Small Craft Advisory Thursday and Friday. Thunderstorm
chances increase along the waters Friday as front moves into the
region.

Sub-Small Craft Advisory conditions expected for Friday night into Sat. Some showers
and thunderstorms possible over the waters.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...none.
Virginia...none.
WV...none.
Marine...none.

&&

$$
Synopsis...ads/rcm
near term...ads/rcm
short term...ads/mse
long term...imr
aviation...ads/imr/mse/rcm
marine...ads/imr/mse

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