Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 231901

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
301 PM EDT Sat Jul 23 2016

A weak cold front will dissipate as it drops south into the area
tonight. High pressure will then be in control of the area`s
weather Sunday before another front enters the area Monday into
Tuesday. This front may stall across the region during the middle
of next week.


The main impact for the remainder of the afternoon will be the
heat. A Heat Advisory remains in effect for the I-95 corridor
and portions of the VA Piedmont where 105 heat indices are most
likely. However, it is oppressive in most locations with
air temperatures well into the mid to upper 90s.

A few storms are beginning to fire in SW PA ahead of a weak front,
which is most notable for its dew point gradient. There may also
be some very weak mid-level energy per water vapor loop, but the
main forcing at this level is over New England. Winds are quite
variable in direction over the local area due to a trough along
the coast. So there could be some localized convergence as the
front drops south late this afternoon and evening into an
environment with >2000 J/kg MLCAPE and effective shear up to 35 kt.
The counteracting factors are westerly low level flow, warm
temperatures around 500 mb, and lack of large scale ascent. The
best chance for widely scattered convection appears to be across
the northern and eastern thirds of the area. Although expect most
updrafts will struggle, if a stronger storm can develop, it would
be capable of locally damaging wind gusts in a favorable DCAPE
environment. See SPC MCD 1372 for more.

Any storms which form should exit the SE corner of the CWA near or
shortly after midnight. Patchy ground fog is possible in the
typical rural areas, favored where storms track, and farther south
where lower dew points won`t reach. Lows will range from the upper
60s west to near 80 in the urban centers and along the Bay.


Ridging will build into the area on Sunday, which should help
suppress most thunderstorm development, although an isolated
storm will be possible in the higher terrain. Temperatures will
probably be quite similar to today, but maybe a degree or two
cooler with little thermal chance. However, the decaying front
will have introduced lower dewpoints to at least the northern half
of the area, so heat indices will be closer to 100 or a little
below. Central VA (i.e. Charlottesville area) will have to be
monitored, because dew points there may be similar if not higher
than today and could result in heat indices close to advisory

Sunday evening and night will have to be monitored as a weak
impulse may be riding the ridge across the Great Lakes into PA, in
addition to low level theta-e advection. Have maintained low POPs
across the north through the night to account for any showers or
storms which may propagate into the area. Otherwise still muggy.

Monday still has the potential to be one of the more impactful
days of the next few in terms of heat and thunderstorms although
much uncertainty remains. Have made little change to the going
forecast. A front will be approaching from the northwest late,
although the best forcing and shear will remain north of the area.
Some storms may propagate into the area from the NW, but scattered
storms could also develop in the hot and humid airmass, especially
with north and westward extent. A Marginal Risk for severe weather
is in place. Air temperatures will be in the mid to upper 90s,
with heat indices potentially exceeding 105. The chance of storms
will linger into Monday night.


Temperatures and humidity will remain quite elevated with no real
relief in sight. In addition, a stalled front draped over the area
will serve as a focus for waves of scattered showers and
thunderstorms. Any upper-level disturbances riding by in the flow
aloft could enhance this threat, but are difficult to pinpoint from
a timing perspective this far out.

It is especially important to remember heat safety during
particularly prolonged heat events such as the one currently getting
underway. Also, this type of heat and humidity do not usually go
quietly, so stay tuned to later forecast updates for any strong
thunderstorm potential during the upcoming week.


VFR conditions will largely dominate through the valid TAF period
with afternoon cu and winds generally less than 10 kt. Coverage
of storms late this afternoon and evening is still rather
uncertain, but have placed a VCTS in the Baltimore/Washington TAFs
where there may be a slightly better chance. Timing will likely be
21-01Z. Stronger storms could produce gusty winds. MVFR fog
possible late tonight at MRB/CHO.

Few concerns for Sunday-Sunday night with high pressure. Perhaps
predawn fog again at MRB/CHO. A front will approach late Monday
and could bring some scattered thunderstorms.

Generally westerly flow at or below 10 knots is expected
Tuesday, becoming increasingly erratic as a stalled front sets up
over the area. Sub-VFR is possible in any showers or thunderstorms
that develop.


Flow has become more S/SW this afternoon around 10 kt. A few
storms may cross the waters this evening which could contain
strong wind gusts. Light and variable flow expected again Sunday
and Sunday night. Brief surge of SW winds possible on the wider
waters during the evening.

Gradient winds will likely increase Monday ahead of a cold front. Do
not have high confidence on reaching Small Craft conditions, but its
within the realm of possibilities. Will continue to leave the
wording in the synopsis. Thunderstorm chances increase Monday as
well, which may have a greater risk of locally high winds.

Not much of a pressure gradient is expected over the waters
into the middle portion of the week, which should keep winds below
Small Craft Advisory thresholds.


Record highs and warm lows for the upcoming weekend...

DCA...102 in 2011...84 in 2011
BWI...102 in 2011...79 in 1978
IAD... 99 in 2011...76 in 2011

DCA...101 in 2010...84 in 2011
BWI...101 in 2010...82 in 2010
IAD... 99 in 2010...78 in 2010

DCA...100 in 1930...79 in 1965
BWI...100 in 2010...77 in 1887
IAD... 98 in 2010...76 in 1965


DC...Heat Advisory until 8 PM EDT this evening for DCZ001.
MD...Heat Advisory until 8 PM EDT this evening for MDZ011-013-014-
VA...Heat Advisory until 8 PM EDT this evening for VAZ050>057-502-


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