Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 231906
AFDLWX

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
306 PM EDT Tue May 23 2017

.SYNOPSIS...
Low pressure will move northeast off the DelMarVa tonight.
Upper-level low pressure will move toward the area Wednesday
through Thursday, and then depart on Friday. High pressure will
move across the area Friday night into Saturday, with a warm
front approaching on Sunday.

&&

.NEAR TERM /THROUGH TONIGHT/...
Latest surface analysis indicates that high pressure remains
over the northern Mid-Atlantic this afternoon. A stationary
front is draped south of the area. Dry air associated with the
high pressure is limiting northward progress and intensity of
rain with the first wave along the front. Expect areas of light
rain for the remainder of the afternoon.

A second trough is located near the Smokey Mountains, which
will help induce cyclogenesis along the coast this evening.
Some uncertainty remains with the evolution of rainfall tonight,
as well as a couple changes from the previous forecast. The
main difference amongst models is how much the dry air wins
out...with a wide range of QPF in the DC/Baltimore corridor.
Even if rain does eventually overspread the entire area as
suggested by several models, this corridor will also be farthest
removed from forcing from both the coastal low and a shortwave
trough over the Appalachians. This trough has resulted in a
substantial bump up in POPs over the mountains tonight...and the
overall exit of the two systems is a little slower than
previously suggested. While some areas may see a period of
moderate rainfall, guidance does not suggest there will be
enough to have flooding concerns.

With saturated low levels, fog development is still possible.
Lows will be in the 50s.

&&

.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
Most of the daytime hours of Wednesday will be dry as the
coastal low moves away and the wedged high pressure continues to
exert influence. A few showers or drizzle may develop in the
Allegheny Highlands. It will be cloudy across the entire area with
temperatures in the upper 60s to lower 70s.

Meanwhile, dynamic low pressure system will be winding up over
the Ohio Valley. Strong low level jet/moisture transport will
overspread the area Wednesday night, resulting in a widespread
moderate to perhaps locally heavy rainfall. With strong theta-e
advection aloft, there may be a few embedded rumbles of thunder
as well. If any areas in the mountains overperform on rainfall
tonight, it could set the stage for minor flooding concerns, but
will have to let the preceding events play out first. Otherwise
the system is progressive as the dry slot works northward
Thursday morning, with amounts from this wave ranging from 0.5
to 1.5 inches.

Jet energy will be moving off to the east on Thursday as upper
level low moves overhead and takes on a negative tilt. Surface
low looks to develop over northern parts of the area in response
to the forcing. The amount of clearing and destabilization is
in question, and if so, how much would overlap with the
departing shear. So while showers will be probable by afternoon,
the amount and intensity of thunderstorms is uncertain. Rain
chances will diminish Thursday night as low/occluded front move
to the east.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
Closed upper level low will continue to move NE
away from New England on Friday. Energy behind the upper trough may
enhance some showers over our area. Conditions dry out into Friday
night and Saturday as high pressure briefly builds over.

Shortwave trough and an approaching warm front will bring rounds of
showers Saturday evening and into Monday, as the front stalls near
or over our CWA. Cold front approaches from the west Monday into
Tuesday keeping the unsettled weather conditions over our area.

High temperatures will be near normal mainly in the 70s and low
80s... some 60s at higher elevations.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
MVFR cigs have been spreading back to the northwest as wave of
low pressure moves along the coast. Rainfall through this
afternoon will be light in nature. Considerable uncertainty
still exists about this evening into tonight. While MOS guidance
still suggest IFR cigs develop, actual model RH fields show dry
air advecting in from the northeast, causing cigs to lift.
Intensity of rainfall also remains in question, especially
across Baltimore/Washington. Have trended TAFs more
optimistic, but did not go VFR until after dawn Wednesday.

Break in low clouds and precip during the day Wednesday,
although remaining OVC. Next wave of rain will arrive Wednesday
night and be moderate to possibly heavy at times. IFR or lower
conditions seem likely. LLWS may be an issue with southeasterly
winds at surface and southerly aloft. A break in rain is
possible Thursday morning in addition to rising cigs, but
additional showers and perhaps thunderstorms will occur Thursday
afternoon. Precipitation exits Thursday evening but cigs may be
slow to lift.

Periods of sub-VFR conditions possible on Friday,
becoming VFR Friday night into Saturday. Then sub-VFR conditions
return Saturday evening and into Sunday with rounds of showers
moving through our region. Gusts as high as 22 kt possible on Friday.

&&

.MARINE...
Low clouds will result in poor momentum transfer though the midweek.
Coastal low pressure will move past the Delmarva tonight. Am
maintaining Small Craft Advisories for the mid Bay and lower
Potomac through 6 AM. Southeast winds increase Wednesday night
in response to low pressure in the Ohio Valley. Have hoisted a
Small Craft Advisory for larger waters of the Bay and lower
Potomac where mixing should be better, although moderate rain
may eventually limit this potential. There may be a lull on
Thursday morning, but winds may increase again later in the day
as low pressure passes by to the north.

Scattered showers expected on Friday with breezy conditions,
gusting as high as 22 kt, which would require a small craft
advisory. Conditions dry out Friday night into Saturday before
other rounds of rain move through into Sunday. Winds will be
below the SCA threshold Saturday and Sunday.

&&

.TIDES/COASTAL FLOODING...
Water levels are less than a half foot above astronomical normals
this afternoon. However...coastal low pressure will strengthen the
onshore flow a little this evening...so anomalies will increase a
bit during this time. The flow should be light enough for water
levels to remain below minor flooding thresholds for most
areas...but it will be close for sensitive areas such as Straits
Point in St Marys County and Annapolis in Anne Arundel County.
Anomalies will have to be monitored through this evening.

The low will move away from the area late tonight into
Wednesday...but deep low pressure will cross the area Thursday, with
southeast winds picking up Wednesday night. The early Thursday
morning tide cycle will need to be monitored, as the preponderance
of evidence suggests that minor flooding is likely. Minor flooding
is possible during the high tide cycles Thursday afternoon through
Thursday night...but confidence is lower since the wind is expected
to turn west to southwest.

&&

.LWX WATCHES/WARNINGS/ADVISORIES...
DC...None.
MD...None.
VA...None.
WV...None.
MARINE...Small Craft Advisory from 6 PM Wednesday to 6 AM EDT Thursday
     for ANZ531>534-537-539>541-543.
     Small Craft Advisory until 6 AM EDT Wednesday for ANZ533-534-
     537-543.

&&

$$
SYNOPSIS...ADS
NEAR TERM...ADS
SHORT TERM...ADS
LONG TERM...IMR
AVIATION...ADS/IMR
MARINE...ADS/IMR
TIDES/COASTAL FLOODING...BJL


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