Area Forecast Discussion
Issued by NWS Sterling, VA

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000
FXUS61 KLWX 232121
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
521 PM EDT Sun Jul 23 2017

.SYNOPSIS...
High pressure remains off the southeast coast. A weak boundary
will remain near the area through Monday before stalling south
and east of the area Tuesday. An upper level trough of low
pressure will move over the northeastern US early in the week.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...

Update...
Initial showers/thunderstorms tracked to the Delmarva by mid
afternoon. However, an additional impulse behind that fed upon
1000-1500 J/kg MLCAPE air with around 30 kt of effective
shear...blossoming into another cluster of thunderstorms
tracking across Maryland. While there has been forward
movement, there have also been repeated cells. Further, RAP/HRRR
both hinting at another round of showers/thunderstorms possible
in the 03-06 UTC timeframe. Have therefore issued a Flash FLood
Watch between Washington and Baltimore, including Baltimore
metro, until the early overnight hours.

Would not rule out local severe weather this evening. Otherwise,
it appears as though hydro may be the bigger concern tonight.
CAM suggestions are that there will be a break in the early
evening, and then this activity will wane after midnight, with
little precip during the overnight hours.


Previous Discussion...
MCV which was mentioned in the morning AFD update has been the
primary player thus far as it has traversed across the southern
half of the forecast area. Recent radar/satellite trends
continue to show an area of showers and a few thunderstorms
increasing in intensity as it moves east of DC toward the
eastern shore of MD. Expect heavy rain and gusty winds with this
activity as it pushes east...over the next hour or so.

Forcing for ascent becomes far more nebulous behind MCV...with
some subsidence noted in satellite imagery. However, area still
remains very moist...so am expecting at least isolated-to-widely
scattered redevelopment through the evening (perhaps into the
overnight) where airmass can recover. Once such location may be
across central VA this afternoon...where full sun has returned
and visible satellite imagery shows some agitated cumulus
developing.

While severe threat has lessened as destabilization was
hampered by cloud cover...scattered strong winds with isolated
pockets of damaging wind are still possible in the strongest
storms. Additionally, PWATs 1.75-2 inches coupled with recent
rainfall continue to heighten concerns for localized flooding.
Currently believe the threat this afternoon is too localized
and tied to meso-beta influences to warrant a Watch. However,
repeated rounds of rainfall over the same area (especially as we
continue into tonight) could increase hydro concerns locally.
Evening shift will continue to monitor this.

As expected...cloud cover has helped to keep temperatures a few
degrees cooler than yesterday...which have kept heat indices AOB
100F.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH TUESDAY/...
Frontal boundary will finally try to push through the area
Monday. Most guidance continues to be stubborn with convective
coverage...though believe at least isolated showers and storms
are possible as the front encounters a warm and unstable
airmass. The strongest storms would be capable of gusty winds
and locally heavy rainfall.

Some improvement from the heat Tuesday...as front stalls to the
S/E of the area. High temperatures will generally top out in
the mid to upper 80s. Dewpoints will fall back into the
60s...which should feel more pleasant after the extended stretch
of hot and humid weather.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
Wednesday should be the pick day of the week with high pressure
centered over New England, and extending down over our region. That
should keep temperatures slightly below normal, and humidity in
reasonable check with dewpoints in the 60s. By the time we get to
Thu, the high has slipped off to the east and southwest flow returns
with a rebound in temps and humidity. Late in the week, probably
Friday, our next cold front is slated to push through from the
northwest. Current indications are that it will stall just offshore
with an area of low pressure forming along it off the Delmarva Sat
and Sun.

&&

.AVIATION /21Z SUNDAY THROUGH FRIDAY/...
VFR conditions will prevail through Monday...outside of
isolated thunderstorm activity. Thunderstorms are possible this
afternoon and evening with gusty winds and reduced visibility
in the strongest storms. An isolated storm is also possible
Monday...though most sites should remain dry. VFR returns
Tuesday.

VFR conditions expected Wed and Thu. Lowered vsbys and cigs are
possible on Friday from the expected cold front with its showers and
thunderstorms.

&&

.MARINE...
Winds will remain below SCA values through Monday. However, thunderstorms
capable of producing gusty winds above Special Marine Warning
criteria are possible today and perhaps on Monday.

No marine hazards Monday night or Tuesday.

Other than thunderstorms (most likely on Friday), no widespread
marine threats are anticipated Wed through Friday.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...Flash Flood Watch until 3 AM EDT Monday for DCZ001.
MD...Flash Flood Watch until 3 AM EDT Monday for MDZ004>006-011-013-
     014-503>508.
VA...None.
WV...None.
MARINE...None.

&&

$$
SYNOPSIS...MSE
NEAR TERM...MSE/HTS
SHORT TERM...MSE
LONG TERM...CAS/MSE
AVIATION...CAS/MSE
MARINE...CAS/MSE



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