Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 181327

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
927 AM EDT Sun Jun 18 2017

A very warm and humid air mass will remain in place
through Monday until a cold front clears the area Monday night.
Weak high pressure will build over the area for the middle of
the week.


High pressure remains over the Atlantic while a cold front
tracks through the Midwest. A southerly flow between these
systems will usher in hot and humid conditions today. A bkn
stratocu deck trapped underneath the nocturnal inversion for
locations along and east of the Blue Ridge Mountains will
continue to mix out late this morning. Increasing sunshine will
cause max temps to range from the 70s and 80s in the mountains
to the lower and middle 90s across most locations. The hottest
conditions are expected along the Interstate 95 urban
corridor...with heat and humidity causing heat indices around
100 to 105 degrees. Will continue with the heat advisory for
Washington DC...Arlington County and the city of Alexandria as
well as Falls Church. Heat indices are expected to be closer to
105 degrees due to the urban heat island. Will continue to watch
for Baltimore...but confidence is a bit lower since low clouds
may take a bit longer to mix out so temps may be a couple
degrees cooler.

The heat and humidity will lead to an unstable atmosphere this
afternoon. NAM/GFS bufkit soundings are indicating around
1000-1500 J/KG of MLCAPE. However...there will not be much in
the way of a lifting mechanism. Terrain circulation may be
cancelled out by a strengthening low-level flow.
Therefore...convection may have a tough time developing. Will
continue with isolated to widely scattered showers and
thunderstorms this afternoon since there will be little in the
way of inhibition but any organized convection seems unlikely at
this time due to the lack of a strong lifting mechanism.

Any convection should dissipate around sunset due to the loss
of daytime heating. However...additional showers and
thunderstorms may propagate into western portions of the CWA
overnight. Confidence is low due to the unfavorable timing...but
with the warm and humid conditions overnight there will be some
instability. Also...shear profiles will be strengthening as a
cold front and its associated upper-level trough approach from
the west. A few severe thunderstorms cannot be ruled out
overnight...mainly west of the Blue Ridge Mountains but again
confidence is low at this time.


An upper level trough will be moving across the eastern Great
Lks Mon pushing a cdfnt into the area. As cdfnt intercepts a
plume of very high PWATs aoa 2.0 inches, expect widespread
showers and t-storms to develop and last into the evening except
in southern MD where they will likely last through much of the
night. While shear will be on the increase through the day
(0-6km values of 30-35kt) which would support organized
convection, amount of instability available still remains
questionable given thick clouds are likely to arrive early and
there might be showers already by 12Z. 00Z EPS indicate
probabilities of CAPE greater than 1000 J/kg well below 50%.
While individual storm cell motion is progged at 30-35 kt,
unidirectional wind profiles suggest some training/repeated
convective activity is likely causing some concerns for flash
flooding especially given the very high PWATs. Still believe the
primary threat will be flash flooding and a flash flood watch
may be required for the I-95 urban corridor of DC and Baltimore.

Cdfnt clears the area late Mon night with showers ending in most
areas except perhaps southern MD. Drier and more seasonable
Tue-Tue night.


Low humidity but seasonable temperatures for Wednesday. A departing
broad upper trough will allow for some potential for instability
convection. By Thursday and Friday zonal flow will replace the
trough. Surface features will be fairly benign...but given the low
level flow, some amount of tropical moisture could make it up here
from the Southeast during the late week. Next weekend looks to be
our next cold front, although the timing of which day is uncertain
given the model spread. Dewpoint temps and humidity are expected to
increase day by day Wednesday thru Saturday.


MVFR cigs should mix out late this morning. A popup shower or
thunderstorm is possible this afternoon and early evening...but
coverage will be isolated to widely scattered. Additional
showers and thunderstorms are possible after midnight...but
confidence remains low. Areas of low clouds are possible late
tonight into Monday morning as well.

A cold front will approach the terminals Monday before passing
through Monday night. Showers and thunderstorms are possible.
Thunderstorms may be severe...especially during the afternoon
and evening hours across the eastern terminals. Weak high
pressure may return for Tuesday behind the cold front...but a
popup shower/thunderstorm cannot be ruled out.

Other than possible showers and thunderstorms at times, no
significant aviation concerns Wed through Fri.


Solid SCA winds today through Mon under strengthening
srly winds. T-storms on Monday may require special marine
warnings. Winds begin to diminish late Mon night.

Other than thunderstorms, no significant marine concerns Wed through


DC...Heat Advisory until 6 PM EDT this evening for DCZ001.
VA...Heat Advisory until 6 PM EDT this evening for VAZ054.
MARINE...Small Craft Advisory until 6 PM EDT Monday for ANZ530>543.


MARINE...BJL/CAS/LFR is the U.S. government's official web portal to all federal, state and local government web resources and services.