Area Forecast Discussion
Issued by NWS Sterling, VA
FXUS61 KLWX 231433
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1033 AM EDT Sat Jul 23 2016
A weak cold front will dissipate north of the area today. 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.
.NEAR TERM /THROUGH TONIGHT/...
Update: expanded Heat Advisory to Calvert and St. Marys Counties
as reworked temperature grids produce 105 heat index there.
Synoptic features across the eastern portion
of the United States early this morning are quite weak. The most
noticeable item is a trough axis across New England, which extends
southward along the Mid Atlantic coast. There is also a dewpoint
gradient across the eastern Great Lakes to New York City. These
two features will play a small role in today`s forecast.
The only clouds of note this morning are located in western MD, so
temperatures have been able to rise quickly to near 90 at 10 AM.
Deep mixing and a W/NW low level flow will support highs in the
mid-upper 90s, and am thinking a few spots (such as urban DC) have
a real shot at 100. While the westerly flow supports higher
temperatures, it also would dry dewpoints out to the mid-upper
60s. Have continued a middle-of-road solution to dew points in the
upper 60s-around 70, which yields a Heat Index on the threshold of
advisory criteria of 105. In terms of impact, the heat and sun
will be an issue for anyone outdoors.
For thunderstorm chances, the dewpoint boundary, which will be
sinking south late this afternoon and evening, may provide a zone
of weak forcing. But the westerly flow likely will act to suppress
anyactivity. While plenty of instability abounds, there is
little forcing to utilize that CAPE. In addition, the 12Z IAD
sounding shows a strong stable layer between 600-450 mb. With 500
mb forecasts changing little through the day, deep convection may
be thwarted. There is some signal in guidance that there may be a
slightly better chance east of I-95 this evening, perhaps due to
the boundary from the north interacting with bay breezes, along
with slightly lower heights.
Benign weather will be in place overnight, with perhaps patchy fog
in climo-favored locations. That will be especially true in the
higher dewpoint air if any storms do cross those areas.
.SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/...
High pressure will return for Sunday, with ridging in place aloft
too. Temperatures at 850 mb will still be with a degree or two of
22C, but dewpoints should be in or near the mid 60s, which means
that the heat index should remain under 100F for most. Temperatures
may be a degree or two cooler as well since downsloping is lacking,
further supporting the feeling that an Advisory won`t be warranted.
If any thunderstorms develop, they would be initiated by terrain
circulations, and have relegated low end PoPs to the mountains.
Monday looks to be the most active day of the three. First of all,
return flow stands a chance at pumping 70F dewpoint air north across
the forecast area once again. The airmass could certainly support
temperatures that rival Saturday, so headlines will once again be
possible. In terms of storms, shortwave energy tracking across the
northeastern states will drop a front southward, which would provide
forcing. While instability across the forecast area once again
remains ample, the better shear will be found north of us. Thus its
not certain if something more organized can develop, and if it
does would it survive the trip. Regardless, believe the setup
would support at least scattered convection. Given heat of the
airmass, storms could become strong assuming they can penetrate
the warm mid-levels. PoPs have been lowered from previous forecast
given these uncertainties, but still have 30-40% PoPs from midday
through the evening.
.LONG TERM /TUESDAY THROUGH FRIDAY/...
High pressure will continue to move into the Mid-West Tuesday while
a cold front will be located near the Mid-Atlantic region. Multiple
waves of low pressure are expected to move along the frontal
boundary and could potentially lead to an active weather pattern for
the region. Showers and thunderstorms will be possible Tue-Fri with
the best chance in the afternoon and evening. Warm and humid
conditions are expected to continue through the end of the work
.AVIATION /15Z SATURDAY THROUGH WEDNESDAY/...
VFR conditions will largely dominate through the valid TAF period.
The only exception may be localized fog at climo-favored
terminals... MRB/CHO. Even there am carrying only a couple hours of
MVFR. The only other risk today will be thunderstorms, which should
be isolated at best late this afternoon and evening. This risk
much too low to carry in a TAF at this time.
A similar setup carries through Sunday...VFR with a low end risk of
patchy fog. Thunderstorm chances even less than today.
Out of the next three days, the risk of storms highest on Monday.
Coverage would most likely still be scattered. Gusty winds/brief
restrictions a possibility.
SHRA/VCTS possible Tue-Wed.
Flow this weekend will generally be light and from the W/NW
today...10 kt or less. An isolated thunderstorm would be
possible late this afternoon-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 leave the wording in the
synopsis at this time. Thunderstorm chances increase Monday as
well, which may be the greater risk of locally high winds.
A cold front may linger near the waters Tue-Wed and showers and
thunderstorms are possible.
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-