Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 181400
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1000 AM EDT Tue Jul 18 2017

.SYNOPSIS...
An upper level trough will affect the weather today. High
pressure will dominate Wednesday and Thursday. A cold front will
approach from the north at the end of the week, but will most
likely stall before reaching the area.

&&

.NEAR TERM /THROUGH TONIGHT/...
Upper level trough that moved overhead yesterday will remain in
place for today, providing additional chances for scattered
afternoon showers/thunderstorms. Not much change in the air mass
from yesterday, so nature of development likely to be similar to
yesterday, developing off of subtle low level boundaries and
terrain circulations. 12z IAD sounding and SPC mesoanalysis does
depict mid levels a bit warmer with mid level lapse rates not
quite as steep as yesterday, reporting 6.5 C/km this morning.
That being said, RNK sounding this morning still showing in
excess of 7 C/km. This combined with surface temperatures near
or slightly above 90F and dew points near 70F should allow for
development of near 2000 J/kg of MLCAPE again by this afternoon.
Thus another pulse thunderstorm day with potential for isolated
severe exists, with perhaps the highest chance in our southwest
towards the Blue Ridge/Shenandoah Valley where steeper lapse
rates exist. SPC has placed this area in a Marginal Risk today.
In addition, once again, there is very little shear/steering
flow, so heavy rain and isolated flooding is again a
possibility.

As instability wanes tonight, it should become completely rain
free by midnight or soon thereafter as the atmosphere
re-stabilizes, since the synoptic forcing which could keep
storms going through the night will remain weak. Lows will be
warm again with 70s common.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
The trough which has been affecting us will slide eastward a bit
and weaken somewhat on Wednesday, which should allow several
things to change a bit. The first is that with reduced
instability thanks to warming aloft, we should see a noticeable
decline in thunderstorm coverage, though isolated storms are
still possible. The second is that with more sun and warmer
temps aloft, surface temps will also likely increase a bit. The
third is that with a bit more of a westerly flow at the surface
in the wake of the system, dew points may in fact decline a
little (though not much). Heat indices will approach advisory
criteria in parts of the area, so will maintain mention in HWO
for now, but right now expecting most places to fall short.

On Thursday a weak ridge crosses the area, resulting in another
slight increase on high temps, though with a stronger westerly
flow progged in advance of a cold front dropping down from
Canada, dew points may see a more noticeable decline, so we once
again expect heat indices to approach, but perhaps not exceed,
criteria. Will leave mention in HWO for time being though. Storm
coverage, if any, again looks isolated, and by this time will
likely be mainly terrain forced if any form at all. By Thursday
night, the approaching front may start sending a few storms into
the northwestern portion of the CWA, but otherwise, effects from
the front should be limited. With the southwest flow keeping
warm air moving in with diminished radiational decoupling, it
should be among the warmest nights of the year, with temps
staying well into the 70s, and perhaps 80s in the cities.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
A particularly hot stretch is expected for days 4-7 across the
Mid- Atlantic. Deep high pressure remains centered over the Mid-
South states through this time. 850mb temps around 20C are
expected over the Mid-Atlantic through this time. Just like last
week, the only impediment to a notable heat wave is organized
thunderstorm/MCS activity (and associated clouds) that will
round the high. The high center positioned near Memphis allows
streamlines from the southern Great Lakes to arc to the central
Mid-Atlantic. This is of note as last week activity was
generally regulated to the northern Mid- Atlantic (with the
central Atlantic receiving quite a bit of debris clouds from
Midwest activity the night before).

Scattered terrain driven convective thunderstorms looks
possible each day Friday through the weekend with a slight
height decrease through this time per guidance.

Max temps look higher this week compared to last as the heat is
advected from the west instead of the south. This should limit
dewpoints somewhat and allow for a higher ambient temperature.
Risk for 105F heat index (heat advisory minimum criterion east
of the Blue Ridge) is possible Thursday through Monday. Will
blend ER Superblend with MEX guidance for max temps Friday-
Monday.

&&

.AVIATION /14Z TUESDAY THROUGH SATURDAY/...
VFR overall through the next few days. We still have the
continued caveats of patchy fog over the next few mornings and
scattered thunderstorms this afternoon, though coverage should
drop to isolated on Wednesday and Thursday. Winds mostly 10
knots or less.

Organized thunderstorms are possible at times Friday through
Monday. Otherwise VFR with light westerly flow from high
pressure centered over the Mid-South states.

&&

.MARINE...
Mostly sub-SCA over the next few days, with main concern being
gusty winds from any thunderstorm which develops near or over
the waters during the afternoon and evening hours. Highest risk
of this is today and Wednesday, with a lower chance Thursday.

Generally light Wly flow Friday to Monday with high pressure
centered to our west. Organized thunderstorms are possible
through this time.

&&

.TIDES/COASTAL FLOODING...
South flow will maintain elevated water levels through
Wednesday night. Water levels are expected to remain just below
minor coastal flood thresholds during the afternoon/evening
preferred high tide.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...None.

&&

$$
SYNOPSIS...RCM
NEAR TERM...MM/RCM
SHORT TERM...RCM
LONG TERM...BAJ
AVIATION...BAJ/MM/RCM
MARINE...BAJ/MM/RCM
TIDES/COASTAL FLOODING...BAJ



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