Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 221906

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

High pressure will remain offshore tonight, as a weak trough
remains over the Mid-Atlantic. A weak front will arrive from the
north on Saturday. 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.


Synoptic features are rather subtle over the Mid-Atlantic with
high pressure offshore and a weak front well to the north. Large
upper ridge over the Plains will be a predominant feature through
the weekend. An early morning MCS from Lake Erie finally decayed
over SW PA, although new convection is developing on its forward
flank. Think the best chance for some showers and storms this
afternoon will be over the western mountains. The dense remnant
cirrus has helped limit the cumulus field (in addition to the
temperatures). However, as the subtle boundary drifts south into
the warm and humid airmass late this afternoon and evening,
there`s a possibility there could be a few storms farther east as
well. Gusty winds are possible with high DCAPE values.

Any activity should diminish after midnight if not
sooner. Patchy ground fog in the typical rural spots will be
possible late tonight depending on opacity of cirrus. Lows will
fail to drop below 70 except in portions of the Shenandoah Valley
and mountains.


The weak front will be arriving from the north on Saturday. For
the most part it will only be a moisture boundary, with dew points
of 5-8 deg lower to the north. At the same time, a shortwave
trough will be crossing New England. The question for Saturday
afternoon and evening will be if these features will provide
enough lift to what could be a capped, hot atmosphere with poor
mid-level lapse rates. Computer models aren`t overly zealous with
QPF, but get the feeling we could at least see some scattered
storms, especially east of the Blue Ridge. If so, they will be in
an environment with sufficient CAPE and deep layer shear to become
locally severe. These trends will need to be monitored.

Heat will also be a concern for Saturday. Cirrus could again be a
concern, but looks like a better chance for low elevations to
reach the mid 90s, if not upper 90s. With elevated dew points
ahead of the boundary, heat indices will exceed 100, especially
from DC south. Am not sure we will reach Heat Advisory criteria of
105, so will pass these concerns along to subsequent shifts.

Any convection will diminish by late Saturday evening, leaving a
dry and mild overnight. The dew point boundary will never
completely clear the area, likely leaving a gradient across the
area for Sunday. To the north, despite continued heat again at
least in the mid 90s, humidity values will be lower, and heat
indices may not top 100. To the south, it will be a little
muggier, and can`t rule out an isolated storm. However, the ridge
will build east, likely capping most convection. Although models
are generally dry for Sunday night, there will be theta-e
advection aloft, so will have to watch the potential of
showers/storms arriving from the west.


Monday may very well rival Saturday as the most oppressive day over
the next week as 850 mb temperatures remain in the 22-24 C range but
dew points also climb back into the lower 70s F at the surface. High
temperatures in the middle to upper 90s F (possibly near 100 F if
full sunshine is realized) will result in heat index values in the
105-110 F range.

In addition to the heat, an upper-level disturbance and its
accompanying surface cold front will approach the area late in the
day. Ample instability is expected given the high heat and humidity,
so it seems quite plausible that the added energy with the
approaching front could set off a round of strong thunderstorms
Monday afternoon and evening.

The high pressure area behind the front is not particularly strong
which this time of year usually means the front will just weaken and
dissipate over the region. This will result in only a slight
degradation from Monday`s heat and humidity. It also means there
will be a lingering chance of showers and thunderstorms each
afternoon and evening through at least the middle of next week.

No significant relief from the heat and humidity is forecast for at
least the next week.


A few storms are developing to the west this afternoon, but
coverage is somewhat iffy. Have used VCTS at MRB and IAD, although
can`t totally rule out a storm at any terminal. Winds have been
somewhat hard to peg with a weak trough over the area, but they
should generally be from the SW or W. Some patchy ground fog may
form again late tonight, but far from certain, so have just hinted
with a 5 SM at CHO/MRB.

Scattered storms are possible again Saturday afternoon and
evening, with potential slightly higher (but far from certain) in
the DC/Baltimore area. Thunderstorm chances lower, if nonexistent
for Sunday. Light W/NW flow will become southerly late Sunday.

Sub-VFR/gusty winds possible in any showers or
thunderstorms that move across the area ahead of a cold front Monday
afternoon and evening. Winds will be southwesterly around 10 knots
outside of any convection through Monday evening, then should become
more westerly Tuesday.


SW flow 10-15 kt through this afternoon. Increase is still pegged
this evening over the lower Maryland bay and adjacent Potomac. A
Small Craft Advisory remains in effect for this area for this
evening and overnight. Am not certain at this time if the stronger
winds will be any more widespread than this. Lighter (and somewhat
variable) winds are expected Saturday through Sunday, with
southerly flow perhaps increasing Sunday night. The best chance
for scattered thunderstorms will be Saturday afternoon and

A weakening cold front will approach the waters by late
Monday. Southerly channeling ahead of the front may result in Small
Craft Advisory level wind gusts on portions of the waters Monday
afternoon and night. In addition, any thunderstorms that develop
Monday afternoon and evening would likely produce gusty winds.


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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