Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 180746

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
346 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.


Bermuda high will remain in place through tonight. More hot and
humid conditions are expected along with sunshine. Max temps
will range from upper 70s and 80s along the ridge the
upper 80s and lower 90s in the Shenandoah Valley...and lower 90s
across most other locations. Temps will hold in the 80s along
the shore of the Chesapeake Bay.

The heat and humidity will once again lead to an unstable
atmosphere. 500mb heights will be just a bit lower compared to
yesterday as the upper-level ridge axis slides to our east.
Moisture will be a bit deeper compared to yesterday as well.
Therefore...a few showers and thunderstorms are expected to
develop later this afternoon into this evening. Coverage will
remain isolated to widely scattered due to a capping inversion
and weak lifting mechanisms. The best chance for thunderstorms
to develop will be across northern Maryland into the Shenandoah
Valley where a weak pressure trough will develop. This activity
will propagate toward the Washington/Baltimore Metropolitan
areas toward this evening. However...latest forecast keeps
thunderstorms west of the I-95 Corridor due to a stronger
capping inversion.

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 widely

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. Will continue with a Small Craft
Advisory early this morning across the Maryland Chesapeake Bay
south of North Beach and the lower Tidal Potomac River south of
Cobb Island. The gradient is a bit stronger across these areas
and gusts around 18 knots are expected. will be

The wind field aloft will decrease today...but the mixing layer
will increase. Capped wind gusts around 15 knots for now due to
the relatively cooler waters...but it will be close to SCA
criteria. 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.


Record heat did occur Wednesday. Information on that is below.
Near record heat is expected to continue today. The records
for today...May 18th are listed below as well.

Wednesday`s (5/17) High and Record Highs:
DCA: Today: 91F. Record remains 92 in 1974, 1896 and 1877
BWI: Today: 93F. Ties record with 1896
IAD: Today: 92F. Beats former record 89 in 1986

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 10 AM EDT this morning for ANZ533-
     Small Craft Advisory from 8 PM this evening to 10 AM EDT
     Friday for ANZ533-534-537-543.


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