Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 221031

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
631 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. As is typical in these
patterns, a large convective system (MCS) is ongoing early this
morning across the Midwest, but is expected to remain west of our
area as it weakens this morning. That being said, will likely see
some debris clouds in the form of cirrus streaming overhead this

Through today, will see an upper level disturbance over southern
Canada race eastward and into the northeastern US at the same time
a weak frontal boundary pushes southward into the eastern Great
Lakes and Northeast. This will be the primary focus for any
additional shower/thunderstorm development today and into this
evening, and actually already seeing showers/thunderstorms blossom
near Lake Erie this morning. While the majority of the convective
activity for much of today should remain north of the Mason Dixon
line where greater forcing and instability lies, could see a pop-
up shower or thunderstorm develop over the higher terrain. Late in
the day and into the first half of tonight, any ongoing convection
over PA/OH will tend to move southeastward and into the area
(mainly DC metro northward) on NNW flow aloft and as MLCAPE
increases to around 1500 J/KG. It should be noted that if the
activity near Lake Erie this morning can become more organized,
its arrival would be more towards mid-late afternoon. 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.

Otherwise...partly to mostly sunny and hot. Highs should max out in
the low to mid 90s. With dew points in the mid/upper 60s, heat
indices peak around 100F, highest east of the Blue Ridge Mountains.

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. Isolated to scattered
showers/thunderstorms around both this evening and Saturday, but
coverage/confidence not high enough to warrant mention at this
time. 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 Sunday.

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. A few 20
knot gusts possible in the early morning hours across the central
Chesapeake and lower tidal Potomac. 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.
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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