Area Forecast Discussion
Issued by NWS Sterling, VA-- Highlight Changed Discussion --
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FXUS61 KLWX 251329
Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
929 AM EDT Tue Apr 25 2017
Low pressure near the South Carolina North Carolina border will
move slowly northeastward along the North Carolina coast through
this evening, and finally off the Mid-Atlantic coast Wednesday.
A cold front will approach the region late Thursday and remain
nearby through the weekend. A stronger cold front will pass
through the region early next week.
.NEAR TERM /THROUGH TONIGHT/...
-- Changed Discussion --Bands of heavier rain will pivot northwestward across the Mid-
Atlantic region in response to an 850mb warm front this morning.
Additional rain of 0.25 to 0.5 is expected through the
afternoon. NE-E winds, clouds and rain will keep the Mid-
Atlantic region cool today with temperatures in the 50s to L60s
Low pressure will remain nearly stationary over North Carolina
today before moving up the Mid-Atlantic Coast tonight. High
pressure over southern Canada into New England continues to
wedge south and west into the Mid-Atlantic. A northeast flow
around these systems has caused cool conditions for this time of
year. The onshore flow has also resulted in plenty of moisture.
Warm and moist air around the low will continue to overrun the
cooler air in place...resulting in more clouds and rain today.
There has been lull in the rain early this morning...but a
soaking rain is likely to fill from southeast to northwest
between about 11z and 15z this morning. This is associated with
the convergence from the 850mb warm front that will propagate
into the area. Rain will continue through midday before tapering
off in coverage later this afternoon into this evening as the
dry slot rotating around the low builds into the area.
Therefore...the steadiest rain is expected from about mid-
morning through the middle afternoon hours. However...even when
it is not raining that will be plenty of moisture underneath the
subsidence inversion for low clouds and areas of drizzle. Leaned
cooler than most guidance due to the cloud cover and rain
The low become vertically stacked tonight as it moves up the
Mid-Atlantic Coast. Therefore...it is expected to weaken. The
weakening low along with the dry slot in place will continue to
cause rain to diminish in coverage...but plenty of moisture
underneath the inversion will result in more low clouds along
with areas of light rain and drizzle. The best chance for
measurable rainfall will be across northern and eastern portions
of the CWA where wrap around moisture may build in as the low
passes by just to our east. Areas of fog are possible overnight
as well...especially in the valleys west of the Blue Ridge
Mountains where there may be some breaks in the stratus deck
toward morning. Fog may be locally dense.
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.SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/...
The low will continue to weaken and move away from the area
Wednesday and upper-level ridging will build overhead. High
pressure over the Atlantic Ocean will begin to take over the
weather pattern...allowing for a southerly flow to develop. The
southerly flow along with increasing sunshine and riding aloft
will allow for noticeably warmer conditions compared to the
previous couple days. Max temps will range from the upper 60s in
northeastern Maryland where clouds may hang on a bit longer to
the upper 70s/near 80 degrees across the central Shenandoah
Valley where more sunshine is expected.
A southerly flow will cause mild conditions Wednesday night as
well. A cold front will approach from the west Thursday before
moving into the area and weakening overhead Thursday night. A
south to southwest flow will usher in unusually warm conditions
for this time of year Thursday with max temps well into the 80s
for most locations. A few showers and thunderstorms are possible
ahead of the front...mainly late Thursday in the Potomac
Highlands into Thursday night across the rest of the area.
Coverage should remain isolated to widely scattered since
forcing will be weak. Best chance for gusty thunderstorms will
be across the Potomac Highlands perhaps into portions of the
Shenandoah Valley Thursday afternoon and evening. More mild
conditions are expected Thursday night.
.LONG TERM /FRIDAY THROUGH MONDAY/...
Warm and humid airmass will reside over the Mid-Atlantic through
much of the long term as our area remains in the warm
sector...with temperatures remaining well above normal through
the period. Heat and humidity will combine to foster moderate-
to-strong instability Friday into the weekend...though focus for
convective development expected to be rather nebulous...with
terrain circulations likely main source for any diurnally-driven
thunderstorm activity. Isolated strong storms are possible
during the afternoons/evenings. Stalled front looks to remain
far enough north initially to impact the area...though it could
sag toward the area late in the weekend. It remains unclear if
storms will remain over the higher terrain...or if they will be
able push east toward the metro areas.
Potential for more widespread thunderstorm activity (some of which
could be strong) arrives Monday as strong cold front approaches
.AVIATION /13Z TUESDAY THROUGH SATURDAY/...
-- Changed Discussion --Cigs will continue to lower to IFR and even LIFR levels this
morning. Rain will become more widespread by mid-morning and it
will continue through most of the day before diminishing in
coverage late this afternoon and evening. Low clouds will hang
around. IFR conditions are likely...but there may be breaks of
MVFR cigs at times late this morning and afternoon.
Low pressure will pass by to the east tonight. Low clouds will
hang around underneath the subsidence inversion. IFR conditions
are likely. Cigs may improve at KCHO...but areas of fog may
develop due to light winds and saturated soil. Low clouds and
fog should give way to VFR conditions Wednesday...but the exact
timing of that occurring is uncertain at this time. Most likely
scenario is for MVFR conditions to develop in the morning and
become VFR by the afternoon.
Light winds and mainly clear skies may allow for areas of fog to
develop overnight Wednesday into Thursday morning. A southerly
flow will develop Thursday and an approaching cold front may
trigger a shower/thunderstorm Thursday night. Coverage will be
isolated to widely scattered.
VFR expected to prevail Friday into the weekend...though,
isolated to widely scattered thunderstorms are possible
(especially during the afternoon/evening).
-- End Changed Discussion --
Low pressure over North Carolina this morning will weaken as it
moves up the Mid-Atlantic Coast later this afternoon through
tonight. A strong pressure gradient for today will cause gusty
northeast winds. A Small Craft Advisory is in effect for the
waters. A Gale Warning is in effect this morning for the
Maryland Chesapeake Bay south of Drum Point and the lower Tidal
Potomac River south of Cobb Island. The strongest winds will be
between 11z and 15z across these areas with gusts up to 35
Winds will gradually diminish later today and tonight as the
pressure gradient weakens. Will continue with the Small Craft
Advisory for the Bay and lower Tidal Potomac River tonight...but
that may be able to be cancelled early based on the latest
guidance. A southerly flow will develop Wednesday and the
southerly flow will strengthen ahead of a cold front Thursday
into Thursday night. A Small Craft Advisory may be needed for
portions of the waters Thursday into Thursday night.
Light winds expected Friday into the weekend...though, isolated to
widely scattered thunderstorms possible (especially during the
Easterly flow around low pressure to the south will continue to
promote persistent onshore flow over the waters through
today...which will keep tidal anamolies elevated. Minor tidal
flooding is expected at Straits Point/St. Mary`s county and a
coastal flood advisory has been extended for this afternoons
high tide cycle. There is still some uncertainty in flood
potential at other sensitive sites for the next high tide
cycle...though it currently appears they will remain below minor
flood stage. The flow should gradually turn northerly tonight
into Wednesday as the low moves away from the area. Water levels
should begin to decrease during this time...though minor
coastal flooding will remain possible through tonights tide
cycle at the most sensitive sites.
-- Changed Discussion --DC...None.
MD...Coastal Flood Advisory until 5 PM EDT this afternoon for
MARINE...Small Craft Advisory until 4 AM EDT Wednesday for ANZ530>533-
Small Craft Advisory until 4 AM EDT Wednesday for ANZ534-537-
Gale Warning until 11 AM EDT this morning for ANZ534-537-543.
Small Craft Advisory until 6 PM EDT this evening for ANZ535-
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