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FXUS61 KLWX 240035
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
835 PM EDT Sun Jul 23 2017
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.
Have had a couple of waves of showers/thunderstorms so far this
afternoon/early evening...the most prolific of which dropped
between 3-5 inches (radar estimated) across northeast Maryland.
The forecast area presently in a lull radar-wise, but that
likely won`t be the end of the precip potential tonight.
Mesoscale analysis suggests 1000-1500 J/kg MLCAPE still present
across much of Maryland and adjacent Virginia. The 00Z LWX RAOB
confirms this. Effective shear, fortunately, a bit less than
this afternoon, but still around 20-25 kt. A cap evident around
500 mb on the sounding.
As a weak surface boundary drops toward the forecast area
tonight, subtle height falls likely will erode the inhibition,
making it possible for another round of convection...unrelated
to the activity currently in the upper Ohio Valley. RAP/HRRR
composite suggests that showers/thunderstorms will develop
somewhere near the Blue Ridge/Catoctins just after dark and
track toward Baltimore and Washington toward midnight. After
that, details diverge. But given recent rains (and ongoing
flooding) and precipitable waters at or above 2 inches in the
metros...will be keeping the Flash Flood Watch in place through
the early overnight.
Would still not rule out localized severe weather. However, if
will take a more vigorous updraft to collapse and produce
downburst winds. Some CAMs suggesting thats possible...even
after midnight. Otherwise, there will a decreasing trend between
midnight and sunrise. Will be utilizing the latest obs and
composite meso models to fine tune temperature forecasts.

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 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.
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.
VFR conditions will prevail through Monday...outside of
thunderstorm activity. Another wave of showers and thunderstorms
will be possible late this evening into the overnight hours.
Flight restrictions will be brief, vsby more than ceiling,
related to the rainfall. There may be some gusty winds, but
confidence in details low.
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
Winds should remain below Small Craft Advisory criteria through
Monday. That said... some mesoscale models suggest 20 kt gusts
possible up the Bay in southerly channeling tonight. Will be
taking a closer look at that shortly. In addition, thunderstorms
capable of producing gusty winds above Special Marine Warning
criteria will be possible tonight.
A couple of thunderstorms will be possible on Monday as well.
No marine hazards Monday night or Tuesday.
Other than thunderstorms (most likely on Friday), no widespread
marine threats are anticipated Wed through Friday.
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-

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