Area Forecast Discussion
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FXUS61 KLWX 271809
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
209 PM EDT Wed Jul 27 2016
A stationary front will remain over the area through Friday. An
area of low pressure will ride along this front across the area
Thursday night into Friday. The front will move offshore during
the weekend, then high pressure will build in by early next week.
.NEAR TERM /THROUGH THURSDAY/...
An unstable airmass has developed south of a frontal boundary that
bisects the area as of early Wednesday afternoon. Showers and
thunderstorms, some with gusty to locally damaging winds and heavy
rainfall, are likely in the central Shenandoah Valley given CAPE
of 2000+ J/kg, effective bulk shear of 35-40 knots and PWATs near
2 inches. Added forcing from the right-entrance region of an
upper-level jet streak may also aid in storm organization over
these areas. Further east from the Fredericksburg VA to southern
MD area, storm coverage may be more isolated to widely scattered.
Any showers and thunderstorms should dissipate quickly this
evening with the loss of daytime heating. High-res model guidance
indicates a shower or two could pop up near or just south of the
stalled from overnight, so left low POPs (20 percent) for the
southern third of the CWA.
The stalled front will lift slowly northward as a warm front
Thursday bringing a return to higher humidity and instability to
most of the rest of the CWA. A stout 500 mb shortwave will
approach in WSW flow aloft late in the day. This should result in
fairly widespread showers and thunderstorms by mid to late
afternoon. Given the increasing humidity, instability and shear,
isolated to scattered instances of severe thunderstorms seems
plausible especially for the southern half of the area.
.SHORT TERM /THURSDAY NIGHT THROUGH FRIDAY NIGHT/...
500 mb shortwave will induce a surface low that will move through
the region Thursday night. There is still uncertainty in the
timing/exact placement of not only the heaviest rainfall but also
the track of the low itself. The potential definitely exists for
localized flooding (see Hydrology section below).
The 12z NAM is the quickest moving the low east of the area...and
the 00z ECMWF is the slowest. Split the difference with
Superblend which has been performing best over the last several
weeks. This keeps likely POPs for showers and thunderstorms over
the area through Friday, before diminishing Friday night.
.LONG TERM /SATURDAY THROUGH TUESDAY/...
Unsettled weather continues into the weekend, as quasi-stationary
frontal boundary remains near the area and multiple impulses ride
along the flow aloft. Shower/thunderstorm coverage should begin to
become more isolated early next week, with front sagging south of
the area and weak high pressure building in. High temperatures
through the long term generally near normal (in the M/U80s).
.AVIATION /17Z WEDNESDAY THROUGH SUNDAY/...
Mainly VFR expected tonight into Thursday, though brief MVFR/IFR
is possible near CHO 19-23z with showers and thunderstorms this
afternoon. Gusty winds are also possible in these areas. Patchy
fog expected near MRB/CHO, with MVFR visibility possible 06-11z.
Showers and thunderstorms become more widespread areawide Thursday
evening with sub-VFR visibilities/ceilings becoming likely. Winds
will remain light/erratic in direction the next 24 to 48 hours
with a stalled front overhead.
A return to VFR is expected by Friday night as a cold front drifts
south of the area and winds become westerly at or below 10 knots.
Showers and thunderstorms over the weekend and into early next
week could lead to brief periods sub-VFR in the stronger activity.
Winds should remain light less than 15 knots through Friday night
with a weak pressure gradient over the waters thanks to a stalled
front overhead. Isolated showers and thunderstorms are possible
late this afternoon and evening. More widespread showers and
thunderstorms, some containing gusty winds, are expected late
Thursday afternoon into Friday. Winds will become westerly as a
front sags south of the waters Friday night but should remain
below Small Craft Advisory criteria.
Winds generally 10 kt or less outside of thunderstorms, but winds
and waves higher near thunderstorms are expected over the weekend.
An area of low pressure will cross the area Thursday night into
Friday. The right-entrance region of an upper-level jet combined
with frontogenesis around 850 mb, PWATs exceeding 2 inches, warm
cloud layers up to 15kft and storm motions largely parallel to a
stalled boundary all point to a potential for heavy rain. However,
the exact track of the low and subsequent placement/timing of
heaviest rain remains uncertain at this time.
DCA has had 3 consecutive days with low temperatures at or above
80 degrees. This is currently tied for the 3rd longest streak on
record (happened in 2012 and 2011). One more day would tie for
second (also happened in 2011). The record is 5 consecutive days
set in 2013.