Area Forecast Discussion
Issued by NWS Sterling, VA
FXUS61 KLWX 281426
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1026 AM EDT Thu Jul 28 2016
A front will remain stationary along the Mason-Dixon line through
Friday morning. An area of low pressure will ride along this front
across the area tonight into Friday. The front will move offshore
during the weekend, then high pressure will build in by early next
.NEAR TERM /THROUGH TONIGHT/...
Surface features remain subtle this morning. Based on dewpoints,
it appears as though weak stationary boundary stalled out
somewhere near the Mason-Dixon line.
Initial area of showers, coincident with nose of theta-e ridge,
entering mountains west of the Shenandoah valley this morning
have been dissipating. This moisture likely going toward
moistening a dry layer around 700 mb. However, there has been
ample cloud cover associated with this feature. These clouds will
be incurring a delay in destabilization. That said, based on 12z
LWX RAOB, we aren`t exactly stable right now.
Anticipate insolation will further destablize atmosphere during
the midday hours. Based on forecast surface conditions (going
forecast and LAMP in good agreement, so used this in sounding
analysis), there should be almost 2000 j/kg MLCAPE by late day,
along with 30-35 kt effective shear. This should be more than
enough to support convection, which is already noted in upper Ohio
Valley. That area also has height falls and shortwave support to
provide more than adequate forcing. In short, storms should become
numerous by late day. At first, there may be a few discrete cells.
Eventually, the form will evolve into an more consolidate area of
showers and thunderstorms.
Given foreast parameters, localized wind damage will be a concern.
Reference MGRL/SLGT risk areas per SPC. After airmass saturates,
the threat will evolve as well. Since moisture rich air will be
converging upon a boundary, there is a threat of heavy rain across
northern half of forecast area...specifically, eastern West
Virginia, much of Maryland, and perhaps the northern Shenandoah
Valley. This area best characterized as north of I-66 (VA)/ US-50
(MD). However, antecedent conditions actually quite dry. Going
forecast, and current Flash Flood Watch, captures intersection of
ingredients and flood guidance well. There are no plans to adjust
Watch area at this time.
Timing is uncertain still, due to a disparity amongst guidance.
Certainly risk will be heightened for the second half of the
afternoon through the evening. It is less clear how far into the
overnight this will last.
.SHORT TERM /FRIDAY THROUGH SATURDAY NIGHT/...
Trough axis and low pressure will be shifting east of the area by
18Z Friday. Have focused additional rainfall in the morning hours,
although a few showers could linger into the afternoon. Airmass
should be a little more stable, but ultimately that will depend
upon how quickly the front can cross the area, how deep the
attendent low will be, and where it will be positioned. If things
progress slowly, there could be a few more decent thunderstorms in
the middle of the day.
Another shortwave trough is forecast to cross the area Saturday and
aid in additional shower and t-storm development with pockets of
heavy rainfall Sat afternoon and night.
.LONG TERM /SUNDAY THROUGH WEDNESDAY/...
Broad upper level troughing will gradually push across the area
during the first part of next week. A building ridge over the
Plains will begin to spread eastward on Wednesday. At the surface,
the quasi-stationary front which has been a persistent feature of
the local weather may get a shove south from low pressure moving
across the area on Sunday. Showers and storms will be likely
during this time. There is some question how far the south the
front gets, but high pressure over New England will be in a
favorable position to push drier air down the coast. There`s a
better chance of dry weather for Tuesday- Wednesday. Temperatures
will remain closer to normal through the first half of next week.
.AVIATION /14Z THURSDAY THROUGH MONDAY/...
Numerous to widespread showers and thunderstorms expected mid-late
afternoon, continuing through at least the evening. These storms
may contain strong gusty winds, with the threat slowly evolving
into a heavy rain threat. How long these storms last less certain.
Best case scenario has an ending time by/near midnight. Worse case
supports a continuation into the morning push Friday. There should
be MVFR flight restrictions throughout at a minimum due to cigs.
IFR likley near any storms.
A lull in activity expected Fri afternoon and night before more
rain moves into the area Sat-Sat night.
Showers and thunderstorms will likely be present across the area on
Sunday. A smaller risk of thunderstorms will linger into Monday.
Winds generally 10 kt or less today except higher in thunderstorms,
which will be developing late today into tonight. Marine Warnings
likely near thunderstorms, especially this evening.
Winds strengthen late tonight and especially Friday as deepening low
pressure departs the area. A Small Craft Advisory remains in
effect for the Chesapeake Bay and lower Potomac for Friday.
Low pressure will cross the waters Sunday, helping to push a
front south of the area. At this time, gradient winds are forecast
to stay less than 15 kt. However, thunderstorms will be likely
Sunday, with the chance lingering into Monday, posing a threat of
locally higher winds.
Record high minimums were set at DCA and IAD yesterday.
DCA tied the record of 81F set in 1930. IAD set a new record with
75F breaking the previous record of 73F in 2006.
MD...Flash Flood Watch from 5 PM EDT this afternoon through Friday
morning for MDZ006-011-014-506>508.
MARINE...Small Craft Advisory from 7 AM to 6 PM EDT Friday for