Area Forecast Discussion
Issued by NWS Sterling, VA
FXUS61 KLWX 191853
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
253 PM EDT Tue Jul 19 2016
A cold front will stall out overhead before passing through the
entire area tonight. High pressure will build overhead for
Wednesday and Thursday. High pressure over the Atlantic will
control the weather pattern for Friday through Sunday. A cold
front may impact the area early next week.
.NEAR TERM /THROUGH TONIGHT/...
Severe thunderstorms are currently present across the Central
Foothills this afternoon. The Mid-Atlantic region is between a
surface trough to the south and a dewpoint gradient near the MD/PA
border. This leaves the region in a humid airmass with dewpts in
the mid to upper 60s mainly across the southern half of the
outlook area. Instability is greater than 2k j/kg with 20-25 kt
0-6 km shear this afternoon and storms have been intensifying
rapidly. Large hail and damaging winds are the main threats with
these severe storms. Numerous showers and thunderstorms are
expected across the Central Foothills...Piedmont...far southern
MD and surrounding waters the rest of the afternoon.
Further north...dewpts in the 50s will advect into the region and
only isolated showers are possible. The advection of lower dewpts
from the north will continue this afternoon and evening as the
gradient or frontal boundary sags southward. A shortwave trough
will cross the region early evening and may initiate showers mainly
across the Chesapeake Bay region. Confidence is low at this time
as dewpts will be dropping. Clearing will occur from N to S
tonight and into Wednesday. Patchy fog is possible especially for
places receiving rain this afternoon.
.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
High pressure will remain over the region for Wednesday through
Wednesday night...bringing dry and seasonably warm conditions
along with lower humidity. The high will shift offshore Thursday
and a return flow will develop. Humidity will begin to increase
during this time. A popup shower or thunderstorm cannot be ruled
out over the Potomac Highlands into central Virginia during the
afternoon and evening hours...especially over the higher terrain.
Thursday night will turn out warm and humid as high pressure
settles off to the south and east.
.LONG TERM /FRIDAY THROUGH TUESDAY/...
The return and amplification of heat and humidity will be the main
story for the end of the week and into the weekend.
Much of the time period will feature northwest flow aloft with
warming temperatures and a gradual return in humidity. Core of
large upper ridge will be centered over the central US with the
Northeastern US/Mid Atlantic on the periphery with largely
west/northwest flow. With this type of pattern would expect a risk
of some showers/thunderstorms with daytime instability and
potential MCS`s moving along the periphery of the ridge.
Expecting widespread 90s for high temperatures Friday through
Monday. Humidity should be initially tempered in the westerly flow,
but as northwest flow weakens Sunday and possibly turns southwest
ahead of an approaching front, will see humidity values rise. Thus
the combination of heat/humidity will likely lead to heat index
values in excess of 100F, and heat headlines may be necessary. The
most oppressive days currently appear to be Saturday and Sunday.
As frontal system approaches Monday, will see the chances for
synoptically driven showers/thunderstorms increase.
.AVIATION /18Z TUESDAY THROUGH SUNDAY/...
TSRA/SHRA expected at cho this afternoon. Gusty winds and large
hail are possible in thunderstorm activity. Elsewhere...vcts
expected for the rest of the afternoon at IAD-DCA but coverage is
expected to be isolated. VFR expected tonight and continue through
Generally VFR expected from Friday and through the weekend. A few
showers/thunderstorms are possible which may bring a brief period of
sub-IFR conditions. Patchy fog also possible during overnights, but
coverage not likely to be widespread.
A cold front will slowly drop through the waters today into
tonight. A few showers and thunderstorms are possible mainly
across the Tidal Potomac and lower Chesapeake Bay. Thunderstorms
will be capable of producing locally gusty winds. A pressure surge
will develop behind the cold front late tonight into early
Wednesday. Winds may approach SCA criteria for a brief period
during this time. However...the pressure surge will be weak so
confidence was too low for an SCA headline at this point.
High pressure will remain over the waters Wednesday through
Wednesday night before moving offshore Thursday. A return
southerly flow will develop for Thursday into Thursday night.
Mostly sub-SCA conditions expected for the end of the week and
weekend, however brief intervals of gusts up to around 20 knots