Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 300118

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
918 PM EDT SUN MAY 29 2016

Tropical Depression Bonnie will slowly weaken as it lingers along
the Carolina coast through at least Monday. High pressure will
build across the northeastern United States through the middle of
the week. A cold front will aproach the east coast at the end of
the week.


A plume of moisture has ejected north within theta-e ridge axis
positioned along the eastern North Carolina/Virginia coast onto
the Delmarva peninsula. Warm rain processes have resulted in some
generous rainfall rates, in keeping with an environment containing
precipitable water of 2 inches. The plume generally set up
along/just east of I-95. However, the last few frames from
mosaiced radar loops suggest that the widespread showers beginning
to break apart. That is especially true across southeastern

Recent HRRR/RAP runs initializing decently, and suggest that the
overnight hours will be substantially drier than the late
afternoon and early evening. Am still a bit concerned about
isentropic lift and moisture convergence to the north of Bonnie,
so will not go as dry east of the Blue Ridge...but will
taper/trend PoPs down from categorical to likely for the overnight
hours. Believe that deep moisture will be lacking in the mountains
as lower dewpoint advect east, and will trend down to a dry
forecast overnight in the Appalachians/Potomac Highlands.

Given the tropical air mass and moist ground/low levels, as rain
departs fog may develop considering the lack of air movement. Am
keeping the wording patchy at this point. Altered hourly
temperature forecasts to account for rain-cooled air, but the
effect on low temperature forecast minimal.

Since the area where a Flash Flood Watch was posted earlier is
still receiving rainfall with the highest rates, will leave that
as is for now. Will monitor trends for the rest of the evening,
and then determine its fate.


 Energy associated with Bonnie will split off from the main area
of low pressure and round the western fringe of the ridge, getting
caught up with the passing trough and move through the Mid-
Atlantic during the day. This will act as a trigger for afternoon
convection across the CWA Monday with plenty of moisture and
warmth leading to increased instability. Shear still on the weaker
side so thinking it will just be another day of pulsey
thunderstorms for areas west of the I-95 corridor. To the east
where the bulk of the upper level forcing looks to align will be
more persistent showers with embedded thunderstorms.

Monday night ridging from the west begins to build in as high
pressure returns at the surface. The precip will begin to taper
off from west to east with the building subsidence.

Many areas will see dry weather on Tuesday, though showers will be
possible central Virginia into southern Maryland depending on how
far north the remnants of Bonnie push. 12Z guidance is keeping
these remnants to the south on the Carolina coast through midweek
so not anticipating the precip to push much further north/west.


Low pressure over the North Carolina coast will slowly move
north- northeast along the Mid-Atlantic coast Wednesday into
Thursday. Moisture advection will allow for showers and
thunderstorms for both days... but it will mainly depend on
location of the low pressure system.

Slow moving cold front will approach from the west late on Thursday
into early Friday... and move across our CWA later on Friday. Front
will stall through Saturday to our east as upper trough over
the Great Lakes pushes east. These systems will allow for a
continued pattern of showers and thunderstorms through the end of
this period.

High temperatures will be in the upper 70s and low to mid 80s... and
low temperatures in the 50s and 60s.


As tropical showers overspread the terminals, ceilings and
visibilities did lower...just not consistently. Flight conditions
either MVFR or VFR at this time, but a few patches of IFR remain
lurking. Am covering it with TEMPO groups thru 3Z at DCA/BWI,
which are the two terminals at greatest risk of seeing a heavier
shower for the next couple of hours.

Radar trends and recent meso-model runs suggesting that showers
may be becoming more scattered overnight. If that does pan out,
then the humid air mass and little wind should combine to support
fog development. Either way, conditions seem favorable for
deteriorating flight conditions. The only terminals where
overnight IFR advertised are DCA and BWI, which is where rain has
been most persistent. AM keeping MVFR elsewhere. That is certainly
subject to change.

Although a few showers or thunderstorms remain possible on
Monday, the deeper moisture will depart. Therefore, the TAFs
suggest that conditions improve back to VFR on Monday.
Limited/brief restrictions possible, as does a return for
nocturnal fog Monday night.

VFR conditions expected for Tuesday through Friday, with some
showers and thunderstorms possible, bringing periods of sub-VFR


Generally, winds expected to remain below SCA values through
Tuesday. Periods of showers remain probable through Monday
morning, and then possible for the rest of Monday. Winds will be
variable through the period.

For the rest of the week, the gradient remains rather light. Winds
should mainly be at or below 10 kt.


MD...Flash Flood Watch until 2 AM EDT Monday for MDZ011-013-014-016.
VA...Flash Flood Watch until 2 AM EDT Monday for VAZ057.


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