Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 170127

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
927 PM EDT Tue May 16 2017

High pressure will settle over Bermuda tonight and persist
through Saturday. A cold front will cross from the north Friday
night. The cold front will stall over the southern Mid-Atlantic
Saturday, before returning north as a warm front Sunday. A
stronger cold front looks to move east through the area Monday


As of 9pm, 1020+mb surface high pressure center is drifting
well east of Cape Hatteras. This high will drift east to
Bermuda through tonight and persist there at least through
Saturday. A little thin cirrus is crossing the area this
evening, with perhaps a thicker batch after midnight. Otherwise
a tranquil night is expected. Already seeing winds decouple,
which should lead to another night with large spread between
rural valleys and urban/nearshore areas. It will be warmer than
last night though, with a range from the lower 50s to the mid
60s. With rising dew points, some patchy fog may form,
especially in the western river valleys.

Continued southerly flow will allow for a warming
trend. 925mb temps were 13C this morning and are expected to be
23C Wednesday morning. This will allow for low to mid 90s max
temps Wednesday. Heat index 2-4 degrees above the air
temperature can be expected through the afternoon with dewpoints
in the mid 60s.

Terrain and a weak pressure trough warrants isolated to widely
scattered thunderstorms for the northern Shenandoah Valley and
NE to northern MD Wednesday late afternoon into Wednesday night.
CAMs have a decent signal for convective initiation in this
area with around 2000 J/kg of SBCAPE, but mid level dry air/poor
lapse rates and rising heights will likely limit coverage and
updraft strength. This threat will wane during the evening, with
thicker cirrus possible overnight. Min temps will be 5-10
degrees higher Wednesday night.


925mb temps around 22C Thursday morning and increasing clouds
from the west in the afternoon should mean a bit lower max
temps for Thursday except for areas NE of Baltimore which will
be about 90F both days. Chances for thunderstorms west of the
Blue Ridge with weak troughing ahead of a low over the Great


A cold front will drop in from the north Friday afternoon and
evening. Although timing is ideal for convection and the airmass
will be quite unstable, shear is expected to be weak (around 20
knots or so). Convergence is rather weak as well (westerly flow
largely parallel to boundary). Would expect scattered to perhaps
numerous showers and thunderstorms developing during the heat of the
day and congealing into a bit of a messy cluster as it slowly
progresses southeastward (convergence a little better across
southern parts of the CWA). Given a few thousand J/kg of CAPE, a
couple stronger storms can`t be ruled out, but a more widespread
and/or organized severe threat appears low at the moment.

The front drops south to near the VA/NC border for Saturday before
gradually returning north as a warm front Sunday (slower progression
noted in the guidance over the past 24 hours). This will likely lead
to daytime highs in the 70s with periods of overcast given moist
easterly onshore flow.

A sharper upper-level trough and associated surface cold front
approach the area Monday, though there are still timing
discrepancies as to exactly when it crosses. A bit more organized
convective threat could evolve in this scenario given better
dynamics/flow aloft but remains to be seen. Timing of the cold front
and limited heating due to cloud debris from upstream convection
during the previous days could tamper with instability.

The front may linger near or over the area into Tuesday of next week
as it bumps into strong Bermuda high pressure offshore.


VFR conditions prevail through Thursday. Patchy fog possible
late tonight with MVFR vsbys in fog prone areas just before
sunrise. MOS guidance/forecast soundings suggest the main risk
is at MRB.

A south flow will will continue to be 5-10 kt through
Thursday, perhaps gusting to around 20 kt in the afternoon at
favored southerly wind locations like CHO and IAD. A bay breeze
(SE) could affect MTN. Isolated afternoon thunderstorms (both
Wed and Thu) may form in the vicinity of MRB.

A cold front will approach Friday before passing through Friday
night. A better chance for thunderstorms is expected during
this time. Brief sub-VFR psbl. Low cigs or fog psbl Sat AM. W
flow 10 kts Fri shifts N Fri night then E Sat.


High pressure will continue to drift from Cape Hatteras this
evening and set up near Bermuda tonight through Saturday. A
south to southwest flow will slowly increase through the
midweek. Southerly channeling may cause winds to gust into SCA
criteria Wednesday night into Thursday and again Thursday night
into Friday. Afternoon wind gusts along the shore may also
approach 18-20 kt but confidence is low for these marginal

Gusty showers and thunderstorms are possible over the waters
Friday afternoon and evening as a cold front drops through. SCA
conditions are possible Friday night into Saturday as easterly flow
strengthens between the front departing to the south and high
pressure wedging in from the northeast.


Unusually hot conditions are expected Wednesday and Thursday.
Record heat is possible during this time. Below are the record
highs and high min temps for Wednesday through Thursday night...
May 17th and 18th.

Wednesday Record Highs:
DCA: 92 in 1974.
IAD: 89 in 1986.
BWI: 93 in 1896.

Thursday Record High Minimums:
DCA: 72 in 2015.
IAD: 68 in 2015.
BWI: 68 in 1900.

Thursday Record Highs:
DCA: 96 in 1877.
IAD: 91 in 1987.
BWI: 97 in 1962.




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