Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 201428

Area Forecast Discussion
National Weather Service Baltimore MD/Washington DC
1028 AM EDT Thu Apr 20 2017

High pressure off the coast will move further east today. A
cold front will drop into the Mid Atlantic Friday afternoon.
Weak high pressure builds over the area Friday night before low
pressure impacts the area Saturday night and Sunday.


The early morning showers have dissipated. Cloud cover has also
been rapidly eroding as subsidence briefly overspreads the
area...with mostly sunny skies across much of the area. Mostly
sunny skies should allow for a quick warm up...with afternoon
highs near 80 across much of the region.

Scattered thunderstorms are expected to develop over the higher
terrain early this afternoon and then gradually push east
through the afternoon into the evening. CAPE/shear parameter
space supports general thunderstorms (e.g., MLCAPE ~600-900
J/kg...0-6 km EBS ~20-30 kts)...with perhaps a very brief and
localized pulse-type thunderstorm. The higher shear totals (~30
kts by late afternoon) are mainly focused to our north across
PA...but could slip into the northern tier of counties near the
Mason-Dixon line by early evening. While still low probability, this
area is the most likely to experience a stronger thunderstorm.

Any thunderstorms should weaken shortly after sunset as instability
wanes with the loss of daytime heating.


Boundary appears to be hanging around right into the weekend.
This will lead to extensive cloud cover from Thursday night
right through Saturday. The better chance for rain looks to be
during Saturday as low pressure forms over the TN Valley and
progresses east during the late afternoon.

Highs look to reach the low 80s over much of the area Friday,
but with the extensive cloud cover/rain Saturday highs will only
reach climb to the low/mid 60s.


Unsettled weather looking likely for much of the weekend and into
next week.

An area of low pressure will move from the lower Mississippi River
Valley Saturday to the Mid-Atlantic/Southeast coastline by Sunday,
tracking eastward along a baroclinic zone. There remains some
discrepancies between model suites as to how far north/south the
zone becomes established and therefore the low track and area of
steadiest/heaviest rainfall, but the setup should be favorable for a
period of overrunning rain Saturday into Sunday.

Energy will likely be slow to depart the east coast in the
Monday/Tuesday timeframe, and with low pressure possibly lingering
offshore as high pressure builds into New England, this may promote
onshore flow, cool temperatures, lingering clouds and chances for

By Wednesday, should see more of a southerly flow develop as a low
pressure system moves through the Great Lakes. This should bring
warmer temperatures, as well as the possibility for additional
showers, especially west.

Temperatures will be on the cooler side in the extended period, with
highs likely holding in the 50s for much of the region Sunday with
rain/clouds. Some improvement Monday/Tuesday, with highs generally
in the 60s. Warmest day looks to Wednesday where 70s likely. Lows
during this time period mainly 40s to around 50F.



Conditions continue to improve this morning as skies clear and
rain pushes east of the terminals. VFR expected at all sites
through much of the day today. Scattered thunderstorms are
possible this afternoon into the early evening...with brief
flight restrictions and gusty winds possible.

Low clouds and periods of rain Saturday night and Sunday will likely
bring widespread sub-VFR reductions to both ceilings and
visibilities. Low clouds and areas of light showers may persist into
early next week.


While gradient winds are expected to remain below SCA values
today/tonight, scattered thunderstorms are possible this
afternoon into early evening (especially over the Upper/Middle Tidal
Potomac). Brief/localized gusty winds are possible in the
strongest thunderstorms...with a chance for a few Special
Marine Warnings. Rain is expected for the waters Saturday.

Generally sub-SCA conditions expected Saturday night and Sunday. SCA
conditions possible Monday with increasing northeasterly flow.


Water levels continue to decline...with tidal anomalies
generally a half to three quarters feet above normal. Current
forecast keeps all sites below action stage over the next
several tidal cycles.




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