Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 230759

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


High pressure remains off the southeast coast. A weak boundary
stretches from central Virginia westward to low pressure over
southern Illinois. An upper level trough of low pressure will
move over the northeastern US early in the week.



Another day, another challenging forecast... Surface analysis
reveals a weak frontal boundary extending from the northern neck
of Virginia...across southern WV back to southern IL. This has
provided the trigger for multiple mid summer MCS formation from
VA back to MO. This can be seen clearly on IR imagery.

While the storms of Saturday brought the Mid Atlantic a cooler
overnight the dewpoints remain in the low to mid 70s, and 88D
STP product estimates 2+" rainfall totals over the Highlands.
While the beam is scanning higher there, and thoughts of ice
contanimation enter the thought process a check of river/stream
gages shows rapid rises (now cresting/crested and well below
flood stage). But this area will need to be monitored today if
more heavy rain falls.

WHile convection moving into southern WV seems like it would be
on a track to potentially impact the southern fringes of our
forecast area the latest HRRR shows the cells tracking NE and
weakening during the morning hours. Models then seem to be in
agreement that more convection will develop this afternoon,
possibly focused over PA. Bottom line is there will be rainfall
across much of the forecast area today - question is where and
how much. For now am holding off on any thoughts of a flood
watch. Dayshift will no doubt reevaluate.

Next concern is severe potential. Model soundings are showing
CAPE in excess of 1000 this afternoon. Low level wind fields are
weak. The amount of CAPE we receive will be dependent upon the
amount of breaks in cloud/sunshine/heating the region sees. As
discussed above cloud cover could make heavy rainfall the
greater threat.

And there is still the heat threat. Although not as hot as
during the second half of the week temperatures still reach the
lower 90s...possibly mid 90s south of DC. This combined with the
extremely moist air mass will cause heat indices to exceed 100
in the I-95 and east area. Question becomes "will these reach
105?" We will ponder this further in the next hour or two.



SREF implies that the 00-06Z timeframe could be busy as far as
heavy rainfall, but it is also hinting that it will be maily
north of our foreast area. Am carrying likely PoPs across much
of the area this evening. We will remain in this 70+ dewpoint
air mass overnight. Lows therefore will again exceed 70 east of
the mountains.

The heat wave busting short wave will be tracking into the
northeastern US Monday, but relief will not be arriving until
Monday night and Tuesday. Highs Monday again in the low 90s
with the chance for isolated convection.

Tuesday`s temperatures will generally top out in the mid to
upper 80s. Dewpoints will fall back into the 60s.



Ridge of high pressure will be centered over New England Tuesday
night delivering lower humidity and noticeably cooler weather on low
level northeasterly flow. Lows in the 60s to near 70F.

The high will then quickly shift offshore by Wednesday, with winds
shifting from the northeast to southeast. There could be some
scattered showers/thunderstorms south/west nearest to the old
frontal boundary and across the higher terrain. High temperatures
likely to be a touch below normal with highs generally in the low to
mid 80s.

By Thursday/Friday, the flow will turn around to the southwest ahead
of an approaching frontal system. Temperatures/moisture will likely
spike back up ahead of the front, and along with that the chances
for showers/thunderstorms. Some uncertainty exists with regards to
the progressiveness of the front, and this will play into the
forecast for Saturday. If front slows or stalls, chances for
showers/thunderstorms may persist into the weekend. Highs
Thursday/Friday back up to near 90F.



VFR conditions will prevail through Monday...outside of
thunderstorm activity. Thunderstorms are possible this
afternoon and evening with gusty winds and reduced visibility in
the strongest storms.

VFR Tuesday.

Predominantly VFR expected Tuesday night through Thursday. However
there may be some patchy fog/low clouds both Wednesday and Thursday
mornings with low level marine flow. Chances for showers/thunderstorms
also increase on Thursday.



Winds will remain below SCA values through Monday. Thunderstorms
capable of producing gusty winds above Special Marine Warning
criteria are possible today.

No marine hazards Monday night or Tuesday.

Mainly sub-SCA winds expected Tuesday night through Wednesday,
generally northeasterly Tuesday night, turning southeasterly by
Wednesday. Winds may then approach SCA criteria Wednesday night into
Thursday as they increase out of the south/southwest.




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