Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 181412

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1012 AM EDT Thu May 18 2017

High pressure will remain centered over Bermuda through Friday.
A cold front will move south through the area late Friday into
Friday night before stalling to our south on Saturday. The
boundary will return north as a warm front Sunday before a
stronger cold front from the west passes through Monday into
Monday night. High pressure will briefly return Tuesday before
low pressure and its associated cold front impact the area
during the middle portion of next week.


Feeling more like July 18th out there than May 18th.
Temperatures at 10am were already in the mid 70s to around 80
across the entire area, which is a few degrees warmer than
yesterday. Dewpoints are in the 60s, and low-level flow should
be favorable for those to rise through the day. Tweaked the
forecast highs slightly, but not substantially, and they are
expected to be pretty close to yesterday. The exception might be
the bayshore communities and DCA, where the SW wind favors less
influence of the nearby cooler waters, so it might be a degree
or two warmer than yesterday there.

There is a line of weak echoes showing up on radar across SW PA
into E WV. A shower or two has formed along this line but
short-range models do not favor anything surviving across the
divide in the immediate future. However, the combination of
heat, humidity, the assistance of terrain, and what appears to
be some weak troughing near the surface (perhaps a lower-level
reflection of the same boundary showing up on radar?) will spark
isolated to scattered showers and storms this afternoon.
Coverage should be a bit higher than yesterday but it`s worth
noting that convective temps on both observed and model
soundings are 90+, so it may be a late start for anything not
terrain-induced. With limited shear and winds aloft, it remains
to be seen how well convection can sustain itself -- but if it
can, some gusty winds would be possible in those storms.

The best instability remains generally west of the metro areas,
so have left rain chances out further east. By the time anything
propagates into that area, things should be falling apart for
the evening.

A cold front will drop south into the Ohio Valley and
Pennsylvania tonight. The boundary should remain far enough away
from the area not to have any significant impact on the
weather. However...the unusual warmth and humidity will continue
due to a southwest flow.


The cold front will move into the area Friday afternoon and
evening before eventually passing off to our south late Friday
night. A few showers and thunderstorms are possible ahead of
the cold front Friday afternoon into Friday evening. A westerly
flow may limit coverage across northern portions of the CWA.
The best chance for thunderstorms are expected across central
Virginia into southern Maryland where a pressure trough may back
the surface winds a bit. Shear profiles will be weak...but
moderate to high amounts of instability combined with mid-level
dry air suggests that some thunderstorms will be capable of
producing gusty winds and hail. Confidence for severe
thunderstorms is too low to mention in the HWO at this
point...but this will have to be monitored during later shifts.

More unusually warm and humid conditions are expected...but it
may not be quite as hot as recent days since heights will be a
bit lower. Most areas will have max temps top off in the upper
80s to around 90 degrees.

The cold front will move off to the south late Friday night. A
north to northeast flow behind the boundary will allow for
noticeably cooler conditions.

The boundary will stall well to the south Saturday and an
easterly flow will cause much cooler conditions compared to
recent days. Max temps for most areas will be in the lower to
middle 70s. However...should low clouds develop from the marine
air then temps may be several degrees cooler than what is
forecasted...60s. Current thinking is dry advection should
displace influence from marine air during the day.

More easterly winds and cool conditions are expected Saturday
night. Low clouds may develop as well. Areas of drizzle are
possible overnight...but confidence is too low to mention in the
forecast yet.


Sunday the high offshore will move out as a cold front due Monday
approaches. Cool east winds will turn southerly, but with a fair
amount of cloud cover. The cold front on Monday will have decent
shear and energy with it, 0-6km shear 20-30kts on the GFS forecast
soundings. The limiting factor will be instability. And that will
largely depend on the timing of the front in relation to peak
heating, which remains uncertain at this time. After a brief respite
on Tue, next up is a storm system for Wed/Thu which is the result of
a large closed 500mb low diving into the eastern states from the
high plains.


VFR conditions are expected most of the time through Saturday. A
few showers and thunderstorms are possible this afternoon and
evening...especially across the western terminals. Even
across these areas coverage will be isolated to scattered.

A cold front will pass through the terminals later Friday
afternoon into Friday night. More showers and thunderstorms are
possible during this time. Coverage should remain isolated to
scattered with the most widespread coverage being across
central Virginia near KCHO. Brief SubVFR conditions and gusty
winds are possible in any thunderstorms today or Friday.

An onshore flow will develop behind the cold front for Saturday
and Sunday. There may be enough dry air for VFR conditions
Saturday...but the chances for low clouds will increase Saturday
night and Sunday. Confidence in timing of low clouds is low. Low
clouds may hang around for Sunday night as well. A cold front
will pass through the terminals later Sunday night through
Monday with showers and a few thunderstorms possible.


A south to southwest flow will continue through tonight as
Bermuda high remains in place. Have extended the Small Craft
Advisory to noon for the Maryland Chesapeake Bay south of North
Beach and the lower Tidal Potomac River south of Cobb Island as
gusts continue and are being shown by high-res gust guidance.

Although the wind field aloft is weak, the mixing layer will
increase this afternoon, especially in the open waters of the
lower Maryland Bay near the Virginia border. Guidance gives me
high enough confidence in continued gusts to keep the advisory
up all day down there, while dropping it further north and on
the Tidal Potomac.

More south to southwest winds are expected to channel up the
Bay and lower Tidal Potomac River tonight into Friday morning. A
Small Craft Advisory is in effect for the Maryland Chesapeake
Bay south of North Beach and the lower Tidal Potomac River south
of Cobb Island.

A cold front will approach the waters Friday before passing
through Friday night. Winds should remain below SCA criteria
Friday for most areas...but it will be close with gusts around
15 knots across the northern Chesapeake Bay closer to the
boundary. A pressure surge behind the cold front will likely
cause winds to gust past SCA criteria Friday night into
Saturday. An onshore flow will continue through Sunday before a
cold front passes through the waters Monday. Showers and a few
thunderstorms are possible later Sunday night through Monday.


Elevated water levels are expected through Friday...but south to
southwest winds should be relatively light. Therefore...minor
flooding is not expected.


Near record heat is expected to continue today. We were close to
the record warm lows this morning (68 at IAD, 70 at BWI), but
we will wait until later to make those official. Record highs
today should not be in jeopardy except at IAD.

Thursday (05/18) Record Warm Lows:
DCA: 72 in 2015
BWI: 68 in 1900 and 1896
IAD: 68 in 2015

Thursday (05/18) Record Highs:
DCA: 96 in 1877
BWI: 97 in 1962
IAD: 91 in 1987


MARINE...Small Craft Advisory until noon EDT today for ANZ533-537.
     Small Craft Advisory from 8 PM this evening to 10 AM EDT
     Friday for ANZ533-537.
     Small Craft Advisory until 10 AM EDT Friday for ANZ534-543.


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