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fxus61 klwx 171920 

Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
320 PM EDT Thu Aug 17 2017

a warm front will exit the area to the northeast tonight. A cold
front will approach from the west Friday before crossing the
region Friday night. Surface high pressure will build in
Saturday, but an upper level disturbance will cross the region
Saturday night. A warm front will advance back north across the
region early next week, followed by a strong cold front in the
middle of the week.


Near term /until 6 am Friday morning/...
warm front is slowly advancing back north across the region and
has currently cleared nearly all of the County Warning Area and Metro. Scattered
storms have developed near and east of I-95, mainly in response
to the passage of a weak vort Max aloft. This convection is
supported by decent cape (close to 1500 j/kg mixed layer) and
modest shear (20-30 knots). However, a weak cap remains aloft.
This cap should diminish as the day progresses, so still expect
storm coverage to persist if not expand westward, towards the
forcing provided by the terrain to the west, with individual
storms moving northeastward over time. Given high pw's now
approaching 2 inches, the main risks today are isolated
microbursts and localized flash flooding, with the outside risk
of a tornado if a storm can get strong and organized enough and
tap into the decent directional shear in the vicinity of the
warm front.

Overnight, we naturally lose the surface cape, but another weak
shortwave looks to move across the region, which may keep
generating isolated to scattered storms. Given diminishing
surface cape, think coverage will diminish, but perhaps not end
entirely overnight. Where skies briefly clear, we could get some
patchy fog again. Lows will stay very warm with the high dew
points, with 70s common.


Short term /6 am Friday morning through Saturday night/...
patchy fog will be a concern early Friday, but not expecting
widespread dense fog, and it should break fairly quickly once
the sun rises.

Probably the biggest forecast concern is the threat of severe
weather on Friday afternoon. While it is certainly not the best
set up, very high pw's in excess of 2 inches (potentially above
2.25 inches) and surface temps perhaps into the low 90s with
surface dew points into the mid 70s and shear of 25-35 knots,
and the surface cold front moving into it, will likely get some
sort of line of storms going by early afternoon, which will then
slowly push east across the region. Best time in the cities
looks like mid-late afternoon, with storms lingering well into
the evening east of I-95 into southern Maryland. Training storms could
easily produce flash flooding as well. Storm Prediction Center has outlooked the
region for marginal severe threat, while wpc has outlooked the
region for slight risk of excessive rainfall.

Front slides in later Friday night, with some drying and
cooling. Lows will be a bit cooler than tonight, with some areas
returning to the 60s. Surface high presses into the region
Saturday and Saturday night, but a trough aloft may result in
isolated storms late in the day into the evening. For now have
kept probability of precipitation mainly northwest of the cities, but these may need to be
expanded south and east if confidence in storms grows. Highs
Saturday will be in the 80s. Temps will cool a bit more at
night, with 60s more common.


Long term /Sunday through Thursday/...
high pressure will keep dry conditions on Sunday as it slowly
moves offshore Sunday night into Monday. Southerly flow settles
in advecting moisture into our region Monday and Tuesday. A
pressure trough at the surface could trigger showers and
thunderstorms Monday and Tuesday afternoon and evening. A cold
front will approach from the northwest on Wednesday and move
across on Wednesday night. High pressure returns on Thursday.
High temperatures during this period will be in the mid to upper
80s in northern Maryland... to low 90s in central Virginia at
times... and 70s at higher elevations.


Aviation /19z Thursday through Tuesday/...
VFR overall through this evening, but scattered thunderstorms
are a concern at all terminals until at least late this evening,
given multiple shortwaves. Late tonight concern shifts to low
clouds and fog potential, which while not as severe as Wednesday
morning, is likely to reduce cigs and vis to MVFR if not high
end IFR. We should see improvement Friday just in time for the
next round of thunderstorms on Friday afternoon. This activity
will be more organized with the cold front, with a better severe
threat, along with a better chance of low cig and vis with
storms. Generally should improve to VFR by Saturday morning,
with a much lower (though not zero) risk of storms late Saturday
as an upper level trough crosses the region behind the surface

VFR conditions expected Sunday into Tuesday, with short periods
of sub-VFR conditions mainly Monday and Tuesday afternoon due
to possible showers and thunderstorms.


southerly channelling ongoing, with Small Craft Advisory in effect over Middle
Bay and lower Potomac. This will continue through the night and
Friday, ending Friday night as the cold front slides southward
into the area. Otherwise, main concern will be for thunderstorms
which may produce gusty winds this afternoon and evening, and
again Friday afternoon/evening. Strongest storms likely to be
tomorrow, but even today any storm may produce gusts to 34
knots. Winds diminish and storms end Saturday morning, but a few
isolated storms can't be ruled out late Saturday as an upper
level disturbance moves acoss the region.

Mainly dry conditions expected Sunday into Tuesday, with
periods of showers and thunderstorms possibly developing Monday
and Tuesday afternoon. Wind gusts are expected to stay below the
Small Craft Advisory threshold.


Tides/coastal flooding...
southerly winds are increasing anomalies and we currently expect
Annapolis to hit minor tidal flooding thresholds overnight, thus
we've issued the advisory for it. Think SW Waterfront may also
hit early Friday, but will let next shift re-evaluate before
adding advisory there. Straits Point will also be close, but
think the anomalies will not quite be there for the overnight
cycle. With continued south winds, additional advisories may be
needed Friday before the front bring a northwest flow and allows
anomalies to drop Saturday.


Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...coastal flood advisory from 11 PM this evening to 8 am EDT
Friday for mdz014.
Marine...Small Craft Advisory until 1 am EDT Saturday for anz533-534-


near term...rcm
short term...rcm

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