Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 170127

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
927 PM EDT Wed Aug 16 2017

High pressure over the region will slowly move east off the
coast tonight. A warm front will move northward Thursday into
Thursday night followed by a cold front that should pass through
the region Friday night. Weak high pressure will build into the
area over the weekend.


Weak stalled front remains stretched west to east across the
middle of the forecast area. Isolated showers have been forming
in this zone, although a general dissipating trend is expected
to continue as nocturnal stability increases. Sharp inversion
around 14kft (below the freezing level) on the 00Z IAD sounding
explains why no lightning has been observed.

Overnight, warm advection will develop aloft, resulting in an
increase in mid-level clouds, but perhaps not until late. Thus
questions remain about low clouds and fog. The NAM has
definitely be the most pessimistic model in terms of both, while
the GFS only suggests some clouds between 5-7kft. Considered
SREF and NARRE probs (which are low for significantly reduced
visibility) and thus kept the flavor of the forecast similar,
with patchy fog in the typical prone locations.

Dew points suggest lows will bottom out in the mid 60s to low
70s. May need to adjust up a little, especially in areas of
thicker clouds and to the south of the stalled boundary.

Boundary will slowly move northward back across the area during
the day Thursday. PW`s will surge significantly, with values
likely to be above 2 inches by late afternoon. A weak shortwave
and vort max will also cross the region. However, cloud cover
will be abundant, limiting CAPE availability somewhat. The
result in soundings is a skinny CAPE profile, more suggestive of
a heavy rain/flash flooding threat than a severe threat, but
the latter can`t be completely ruled out given around 25 knots
of shear. The best overlap of shear and instability will be in
the northwestern quadrant of the area...with the highest chances
of storms in general near and west of the Blue Ridge. With the
clouds, temps should end up a little cooler than Wednesday, but
still a very warm and sticky day.


After the trough/max passes and insolation wanes, expect
precipitation to wind down during the evening, but with a very
moist environment, a few showers may linger. With the clouds and
high humidity, lows will stay very high, with 70s being
widespread. Any areas that see a little clearing might see some
patchy fog. Depending on the location of forcing/boundaries and
available instability aloft, there could be a continuation of
thunderstorms with locally heavy rain late in the night into
Friday morning due to an approaching low level jet. Current
model projections suggest this would be over the northeastern
quadrant of the area.

Friday the next cold front approaches. With more opportunity for
sun early in the day and cooling aloft, CAPE should be more
plentiful, with shear increasing to around 30 knots. This should
result in more of a severe threat versus flash flood threat,
though with PW`s still over an inch and a half, any cells that
train could still pose a flood risk. With more sun, highs
should approach if not reach 90 in the warmer spots.

Front passes across the region at night, with chance of precip
rapidly declining. However, warmth and moisture at low levels
will be slower to exit, with lows likely in the 70s near I-95,
cooler in the mountains.


A cold front will be stalling to our southeast as a weak high
pressure builds over our area on Saturday. Upper level energy
could bring some showers Saturday night, with dryness returning
on Sunday. High pressure moves offshore as the front slowly
pushes away from us and stall over the Carolinas Sunday into
Monday. Southerly flow will advect humidity into our area with
increasing temperatures into Monday. A pressure trough could
enhance afternoon showers and thunderstorms Monday into Tuesday.
A cold front will approach from the west on Wednesday with
additional showers and thunderstorms possible. High temperatures
will be in the mid to upper 80s... some 90s and 70s at higher


It looks like any remaining showers near a stalled front will
avoid the terminals. There certainly are a range of solutions
regarding cigs/vsby overnight. NAM has been the most pessimistic
with moisture overrunning with widespread IFR or lower, while
the GFS only suggest clouds between 5-7kft. Given low IFR probs
in the NARRE and SREF, have kept flavor of previous forecast
with MVFR at IAD, MRB, and CHO. One caveat to the forecast: if
low clouds do develop, they may take some time to lift Thursday
morning, until a warm front can lift to the north.

Main concern Thursday and Friday will be thunderstorms, mostly
during the afternoon and early evening hours, which could result
in brief intervals of IFR cigs and vis, along with gusty winds.

VFR conditions expected Saturday into Monday, with short
periods of sub-VFR conditions Saturday night and maybe Monday
afternoon due to possible showers and thunderstorms.


Sub-SCA at present, with variable wind directions due to a
stalled front as well as isolated showers which have produced
outflow boundaries. Southerly winds should develop Thursday
morning. An approaching cold front later Thursday into Friday
looks likely to bring southerly channeling to the bay and lower
Potomac. SCA raised starting late Thursday and likely will need
to be extended through Friday. Other marine concern will be
thunderstorms each of the next two days, with gusty winds a

Mainly dry conditions expected Saturday into Monday, with
periods of showers and thunderstorms possibly developing
Saturday night and Monday afternoon. Wind gusts are expected to
stay below the small craft advisory threshold.


Anomalies are around a half foot tonight, which will keep water
levels below flood stage. A return of southerly winds will
bring rising anomalies back to the region on Thursday, with
potential minor flooding at sensitive sites by Thursday
afternoon/evening and continuing into Friday.


MARINE...Small Craft Advisory from 4 PM Thursday to 6 AM EDT Friday for


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