Area Forecast Discussion
Issued by NWS Sterling, VA
FXUS61 KLWX 191342
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
942 AM 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/...
A surface trough extends from coastal New England to southern VA
this morning. Further north...a cold front or dewpt gradient is
across southern PA. Winds are northerly across the Mid- Atlantic
however lower dewpts have yet to advect into the region as they
are still north of Harrisburg PA. This is keeping it warm and
humid this morning. It won`t be until late this afternoon when we
start to see dewpts dropping into the 50s from N to S and will continue
into the evening.
Scattered mid-high level clouds will continue this morning with
temps rising into the upper 80s by afternoon. Due to the higher
dewpts today...marginal instability is expected with higher amts
across the southern half of the outlook area where dewpts are
higher including CHO-CJR-EZF-NHK. Showers and scattered
thunderstorms are expected to form around noon as convective temps
are reached. The coverage will likely be isolated across the MD/PA
border to scattered/numerous across Central VA... where the
dewpts are greatest. A shortwave trough will also dive into the
Mid- Atlantic mid afternoon and into the evening which may cause
additional thunderstorms to form. This disturbance and the lower
dewpts coming in from the north will compete. If the dewpts are
slower to advect in towards evening then thunderstorm coverage
will likely be greater across the region...if lower dewpts are
faster then the thunderstorms will be limited to the southern
portion of the area.
The cold front will stall out just to the south of the area
tonight and a northerly flow will allow for drier and more
comfortable conditions overnight.
.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 MONDAY/...
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 /14Z TUESDAY THROUGH SATURDAY/...
VFR condtions are expected most of the time today through
Wednesday night. A cold front will slowly drop south through the
area this afternoon into tonight. A few showers and thunderstorms
are possible this afternoon and evening. Coverage will be isolated
to widely scattered. The best chance for a thunderstorm will be
across KCHO...and any thunderstorms that do develop may contain
gusty winds or hail. High pressure will build overhead for
Wednesday through Wednesday night. The high will move off the
coast for Thursday and Thursday night...and a return southerly
flow will develop. However...VFR conditions will likely persist.
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 will develop along and
ahead of the boundary. Coverage of convection will be isolated to
scattered...but 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
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