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Area forecast discussion
National Weather Service Baltimore Maryland/Washington District of Columbia
339 am EDT sun Sep 22 2019

Synopsis...
high pressure will be situated off Cape Hatteras with mostly
dry conditions and above normal temperatures expected. The high
will drift further off the southeast coast by Monday as a cold
front approaches from the Ohio Valley. In the wake of the
frontal passage, high pressure will return to the region once
again through the middle of the week.

&&

Near term /through tonight/...
high pressure analyzed just off Cape Hatteras early this
morning. Still have some mid-level clouds crossing the area,
mostly east of the Blue Ridge, which has prevented fog
development thus far. Patchy fog is evident in the Potomac
Highlands/northern Shenandoah Valley/Martinsburg-Hagerstown area
where skies have cleared and ground is moist from yesterday's
rain.

Any fog should erode soon after sunrise. Still have some mid-
level energy passing by atop the ridge across the southeastern
US today; this will impact Pennsylvania more than
Maryland/Virginia, but have increased afternoon cloud cover in
diurnal heating/instability. Have backed off previous
precipitation forecast, but wouldn't rule out a sprinkle in the
Potomac Highlands.

Temperatures at 850 mb should be 1-2 degrees c warmer today vs
yesterday (close to 18c). Since temperatures reached 90 degrees
at a number of sites yesterday, today should be another 90
degree day. Forecast will be higher than most ensemble/MOS
guidance, perhaps most closely resembling European model (ecmwf).

With high pressure in the western Atlantic and southwest flow
across the region tonight, the main forecast point should be the
temperatures. Since dewpoints will be in the mid 60s,
anticipate a more mild night. Lows in the 60s except around 70
along I-95/western shore of the Bay.

&&

Short term /Monday through Tuesday night/...
a shortwave/trough axis across the Great Lakes Monday morning
will transition toward Quebec by Monday night, sending a cold
front toward the area. Guidance has slowed the progression of
the front, although there is spread in this evolution. While
most of the forcing will remain confined north of the area,
believe there will be just enough of a baroclinic zone to
support at least scattered showers. Pop forecast kept closer to
mav (gfs), which offers some continuity with previous forecast.
Instability meager but present. Opted to keep at least a slight
chance of thunder too. Best timing will be late afternoon into
early evening (west-east), although this won't be a moisture-
fed system.

Once again, went above guidance for high temperatures, as 850 mb
temps still at 18c. It should be another 90 degree day,
especially east of the mountains. Cold advection and dry air
should effectively offer some cooling by Monday night. Stuck
with MOS suggestion of mid 50s-mid 60s.

By Tuesday, area back to the same pattern we've been in
recently...high pressure and dry conditions. Kept temperatures
close to guidance, although suspect these may be too cool once
again given dry pattern.

&&

Long term /Wednesday through Saturday/...
the second half of the week and into the weekend will continue
the theme of above normal temperatures and dry conditions.

Surface high pressure will slide offshore on Wednesday, leading to
dry conditions and developing southerly flow. Temperatures will
likely top out in the low 80s for much of the region Wednesday
afternoon.

On Thursday, a moisture-starved cold front will make its way across
the region, with the possibility of a few showers, but the chances
of meaningful precipitation are low. Highs will be in the 80s.

By Friday and Saturday, heights will rise again as a very large
upper ridge builds over the southeastern United States, with 500mb
heights progged 594-596dm over the southeast. This will provide for
warm to potentially hot weather and dry conditions. Both 22/00z
European model (ecmwf) and GFS show 850mb temperatures from 18-20c by Saturday which
indicate potential for 90f+ readings, although more conservative
blended guidance at this point give mid to upper 80s. Will show near
80f on Friday behind the surface front and near 90f Saturday.

&&

Aviation /07z Sunday through Thursday/...
VFR conditions expected through sunrise at all but mrb. Have had
LIFR/vlifr in fog due to wet ground from yesterday's showers.
Starting to see some fluctuations, but will not commit to
changing this forecast through dawn unless metars exhibit
consistency. Any restrictions will erode after sunrise.

VFR conditions will primarily prevail through Tuesday. Could be
some showers and maybe a thunderstorm Monday afternoon-evening
with a cold front, but confidence in significant areal coverage
rather low. Even where they do form, restrictions should be
limited/brief.

VFR expected Wednesday and Thursday. A weakening frontal boundary
will cross the region Thursday, but with little to no sensible
impact expected aside from a wind shift and a few isolated showers.

&&

Marine...
winds light early this morning, and should become onshore this
afternoon. As air heats today and flow becomes south, setup
will be favorable for channeling winds tonight. Have issued a
Small Craft Advisory tonight on the Main Channel of the Bay
below Sandy Point, including Tangier Sound.

Pressure gradient will increase Monday ahead of a cold front
Monday night. Will at least have some flow. It's unclear whether
there will be additional Small Craft Advisory. The best chance would come Monday
night-Tuesday morning in better mixing behind frontal passage.

Largely sub-Small Craft Advisory conditions are expected Wednesday and Thursday as
high pressure Wednesday gives way to a weakening frontal boundary
crossing the waters Thursday.
&&

Tides/coastal flooding...
water levels running around a half foot or less above
astronomical normals. No issues expected today. South flow
should increase on the Bay tonight. That may lead to increased
departures. Guidance suggesting that tides at Annapolis will
peak near the minor threshold. Parked tide forecast just under
the line, but some ensemble members suggest an advisory may be
required.

&&

Lwx watches/warnings/advisories...
District of Columbia...none.
Maryland...none.
Virginia...none.
WV...none.
Marine...Small Craft Advisory from 10 PM this evening to 6 am EDT
Monday for anz532>534-543.

&&

$$
Synopsis...heights
near term...heights
short term...heights

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