Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 221357

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
957 AM EDT Fri Jul 22 2016

High pressure has moved off the coast. A system will pass north of
the region today, followed by a weak front moving in on Saturday.
High pressure will then be in control of the area`s weather
Sunday before another front enters the area Monday into Tuesday.


Large upper ridge remains centered across the Central Plains this
morning, with the region sitting in north/northwest flow aloft on
the northeastern periphery of the ridge. Storms have developed on
the downwind side of this ridge, with the main forecast problem
for today being the ongoing MCS in NW PA. The definite outcome of
this system is the extensive cirrus shield overspreading the area.
Although filtered sunshine is making it through, the clouds could
knock a degree or two off today`s highs, especially NW. Still
would expect low to mid 90s for most areas with heat indices up to

The second question is precip chances. The eastern side of the MCS
is stratifying as expected as it moves into an unfavorable
environment. Can`t rule out a dying shower making it to northern
MD around noon, so have sped up the timing of POPs. Meanwhile
storms have been redeveloping on its back side in response to
strong theta-e advection. This will be the section to monitor. In
addition, an upper level disturbance over southern Canada will race
eastward and into the northeastern US at the same time a weak
frontal boundary pushes southward into the eastern Great Lakes and
Northeast to provide additional forcing. Ahead of these features,
the low level moisture difference can be noted between the IAD and
PIT. Modifying the IAD sounding for this moisture advection and
warming BL, MLCAPEs could exceed 1500 J/kg this afternoon. Am
unsure of extent of the suppressive effects of the upstream clouds
and precipitation, but theta-e advection may be enough for
additional showers and storms, especially northwestern parts of
the area. Latest RAP/HRRR are hinting at this although run-to-run
variability has been high due to the MCS. Have already made one
forecast update to bump POPs in the NW, but may need another.

Shear profiles are not impressive, with around 20
knots of 0-6KM shear, but any storm during the day has the
potential to be pulse severe and any ongoing cold-pool driven
convection later this evening has potential to become strong to
locally severe.

Any convective activity will wane after midnight. Lows tonight
from the upper 60s to upper 70s.


Remnant frontal boundary will then push southward into the Mid-
Atlantic on Saturday. By this time, will be primarily a
moisture/dew point boundary. Current indications are that the
front will move to about the DC metro by midday Saturday,
separating mid 60s dew points to the north across PA/northern MD
from low 70s DC southward. With MLCAPE values progged to reach
1500-2000+ J/KG and with 25-30 knots of 0-6KM shear, any
convective activity that can fire Saturday afternoon along this
boundary has the potential to become strong to locally severe.

In addition, highs will range from about 90-97F, and when combined
with increasing humidity, especially on southern side of boundary,
heat indices are expected to reach from 100-105F. Thus a Heat
Advisory may be needed for Saturday. Convection will once again
wane Saturday night with lows in the 70s.

Boundary will wash out for Sunday, but dew points not expected to
recover past the mid 60s. Thus, should be looking at a mostly
sunny and mainly dry day, although the region will still be
entrenched in NW flow aloft, so will still need to monitor for any
MCS activity upstream. Highs will reach 90-97F again, but with
lower dewpoints, heat indices will be a little lower than
Saturday, generally maxing out about 100F. Temperatures still
warm Sunday night again with lows in the 70s.


Monday may end up being both the hottest and the stormiest day of
the stretch. Low pressure tracking across southern Canada will drop
a weak frontal boundary toward the area. Higher dewpoints will be
drawn north in advance of the front, while 850 mb temps remain in
the 20-22C range. This temperature/dewpoint combo may yield heat
index values approaching 105. The inherent instability, lower
heights, and focusing mechanism would be the necessary ingredients
for thunderstorm development. It remains to be seen how it all comes

The synoptic pattern doesn`t look much different for the middle of
next week. The westerlies will remain in retreat up in Canada,
permitting broad ridging to set up shop across the continental US.
The core of this ridge will remain west of the Appalachians, though,
meaning the local area won`t be seeing the worst of the heat.
Daytime temperatures should still be in the 90s. However, drier air
may attempt to filter in; that will be dictated by where the front
stalls out. At this point, its not a forgone conclusion that the
front will clear the area, so precip chances will linger in the
extended...with reduced confidence.


Primarily VFR through Sunday. Will have to monitor thunderstorms
to the NW and what develops this afternoon, but confidence not
high enough to put a mention in the TAFs now. MRB would be best
candidate for storms, but can`t rule out DC/Baltimore. Isolated
to scattered showers/thunderstorms again Saturday. Some patchy
light fog also possible each morning, but again coverage not
expected to be widespread. Southwest winds 5-10 knots today turn
west-northwesterly Saturday before becoming light and variable

Scattered thunderstorm chances Monday, especially afternoon and
evening. This looks to be the most likely day to see coverage, but
specific impacts still questionable.

Front will either be over or south of the terminals by Tuesday.
Another round of storms possible, but confidence lower than Monday
due to positional uncertainties.


Generally sub-SCA S/SW winds through this afternoon. The isolated
20 kt gusts from early this morning should be subsiding. Later
this evening, will see an increase in speeds across the central
Chesapeake and lower tidal Potomac again, and a Small Craft
Advisory has been issued. This may need to be expanded by a few
zones, and the higher winds could start by mid-afternoon. These
winds will dissipate Saturday morning with sub-SCA winds expected
Saturday into Sunday.

Gradient winds should increase Monday ahead of front. It is unclear
how significant that will be. Of greater...but not high...certainty
will be the development of thunderstorms, which may contain local
strong wind gusts. That front will either be over or south of the
mid Bay and lower Potomac by Tuesday.


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

DCA...103 in 1926...83 in 2011
BWI...106 in 2011...81 in 2011
IAD...105 in 2011...76 in 2011

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


MARINE...Small Craft Advisory from 6 PM this evening to 6 AM EDT
     Saturday for ANZ534-537-543.


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