Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 170927 AAA
AFDLWX

Area Forecast Discussion...UPDATED
National Weather Service Baltimore MD/Washington DC
527 AM EDT Wed May 17 2017

.SYNOPSIS...
High pressure will remain near Bermuda through Friday. A cold
front will pass through the area late Friday into Friday night
before stalling well 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 may briefly return for Tuesday.

&&

.NEAR TERM /THROUGH TONIGHT/...
...Updated to include section below on elevated Tides this
morning...

High pressure will remain near Bermuda through tonight. An
upper-level ridge will also build overhead during this time. The
subsidence from the upper-level ridge combined with a south to
southwest flow around the Bermuda high will usher in unusually
hot and humid conditions for this time of year. Max temps today
are expected to range from the 70s and 80s along the ridge tops
to the lower 90s across most other locations. Guidance is in
good agreement that near record heat will take place this
afternoon. See climate section below for details. Plenty of
sunshine is expected today due to the upper-level ridge building
overhead.

However...the heat and humidity will lead to an unstable
atmosphere. Isolated showers and thunderstorms are possible
later this afternoon into this evening. Best chance for this to
occur will be along a weak pressure trough across northern
Maryland into northern Virginia and eastern West Virginia.
Rising heights associated with the upper-level ridge will keep
convection isolated at best across these areas and much of the
time will be dry. Elsewhere...no significant lifting mechanism
and a capping inversion should keep dry conditions in place.

A south to southwest flow will continue around the Bermuda high
tonight. Unusually warm conditions are expected with min temps
ranging from the upper 50s and lower 60s in the Allegheny and
Potomac Highlands...to the middle and upper 60s in the suburbs
of Washington and Baltimore...to the lower and middle 70s in
downtown Washington and Baltimore. Min temps this warm are more
comparable to July and August instead of the middle of May.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
Bermuda high will remain in place for Thursday. More hot and
humid conditions are expected along with sunshine. Max temps
will be in the lower 90s across most locations. Cannot rule out
an isolated thunderstorm...but current thinking is that a
capping inversion will keep most places dry. Did allow for the
slight chance of thunderstorms Thursday afternoon near the
higher terrain of the Blue Ridge Mountains and Potomac Highlands
in central Virginia. Terrain circulation may provide the lift
needed across these areas but confidence is low.

A cold front will drop through the Ohio Valley into and
Pennsylvania Thursday night. The boundary should remain far
enough away from our area to have any significant impact. A
southwest flow ahead of the cold front will allow for more
unusually warm and humid conditions.

The cold front will move into the area Friday afternoon before
eventually passing off to our south Friday night. A few showers
and thunderstorms are possible ahead of the cold front.
Confidence is lower in thunderstorms at this time since there
will be a downsloping westerly flow ahead of the cold front.
This may limit coverage of convection. The best chance for
thunderstorms will be across central and southern portions of
the CWA where a pressure trough may back the surface winds a
bit. More unusually warm and humid conditions are expected...but
it may not be quite as hot as today or Thursday due to lower
heights. Max temps well into the 80s are expected for most areas
and some locations will top off near 90...especially across the
Washington Metropolitan area into central Virginia. Noticeably
cooler conditions are expected behind the boundary later Friday
night due to a north to northeast flow.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
High pressure over southeast Canada will build southward along the
eastern slopes of the Appalachian Mountains Saturday and Saturday
night. There is a slight chance of rain showers in the Potomac
Highlands and portions of the Shenandoah Valley. Elsewhere will be
cooler and dry.

As the high pressure weakens and moves offshore of southern New
England Sunday and Sunday night, humidity will increase across the
region with a southerly flow. A chance for rain showers and perhaps
a rumble of thunder lingers in the Potomac Highlands.

A cold front will move across the region Monday bringing increased
chances of thunderstorms that could produce heavy downpours.

Dry air returns with a separate area of high pressure building in
from the west Monday night through Tuesday.

&&

.AVIATION /10Z WEDNESDAY THROUGH SUNDAY/...
VFR conditions are expected through Thursday night along with
south to southwest winds. An isolated thunderstorm cannot be
ruled out near KMRB later this afternoon into this evening but
confidence is low. A cold front will pass through later Friday
into Friday night. A few showers and thunderstorms are possible
ahead of the boundary. Winds will shift to the north and
northeast behind the boundary Friday night.

High pressure behind a cold front may bring VFR conditions
Saturday. The boundary will return north as a warm front
Saturday night through Sunday. SubVFR conditions are possible
during this time.

&&

.MARINE...
Bermuda high will setup through Thursday. A south to southwest
flow is expected during this time. The wind field aloft will
strengthen a bit later this afternoon. This may cause wind gusts
to approach SCA criteria for a brief period...but confidence is
too low for a Small Craft Advisory since the mixing layer will
be decreasing during this time. However...southerly winds are
expected to channel up the middle portion of the Bay and lower
Tidal Potomac River tonight into Thursday. A Small Craft
Advisory is in effect for these areas during this time.

The wind field aloft will decrease a bit later Thursday...but
the mixing layer will increase. Winds may approach SCA criteria
during this time...but confidence is too low for an advisory due
to the relatively cooler waters. More southerly channeling is
possible Thursday night into Friday morning.

A cold front will approach the waters Friday before passing
through Friday night. A Small Craft Advisory will likely be
needed for the waters during this time.

An easterly flow is expected behind a cold front Saturday. The
boundary will return north as a warm front Sunday. A Small Craft
Advisory may be needed for portions of the waters during this
time.

&&

.TIDES/COASTAL FLOODING...
A southerly flow has caused tidal anomalies around three
quarters of a foot above normal. This will cause water levels to
reach caution stage at sensitive locations such as Annapolis and
Washington DC with the tide cycle this morning. Will continue to
monitor anomalies...but as of now light winds should prevent
minor flooding with the high tide cycle this morning.
However...it will be close.

&&

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

Today`s 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.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 6 PM this evening to 10 AM EDT
     Thursday for ANZ532>534-537-540-541-543.

&&

$$
SYNOPSIS...BJL
NEAR TERM...BJL
SHORT TERM...BJL
LONG TERM...KLW
AVIATION...BJL/KLW
MARINE...BJL/KLW
TIDES/COASTAL FLOODING...BJL
CLIMATE...BJL



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