Area Forecast Discussion
Issued by NWS Sterling, VA

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FXUS61 KLWX 210126

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

A cold front will drop into the Mid Atlantic Friday. Weak high
pressure builds over the area Friday night before low pressure
impacts the area Saturday night and Sunday.


Thunderstorms were able to feed off of steep low-level lapse
rates, decent CAPE (around 1000 j/kg), and 30-40 kt bulk shear
to organize into a line that crossed the forecast area late this
afternoon and evening. These storms entering a slightly more
hostile airmass (less CAPE and a bit of CIN), likely aided by
nightfall and the loss of daytime heating. Will be clearing the
forecast area by 10-11 pm. Database will follow suit.

The next area of thunderstorms are back in northwestern
Pennsylvania. This area pre-frontal as well. There should be a
weakening trend overnight, but the HRRR suggests echoes will
approach the northwestern corner of the forecast area after
midnight. Will have chance PoPs returning at that time. Most
likely just showers, but with a little lingering MUCAPE,
adequate shear and lapse rates...wouldn`t rule out an elevated


Cold front will push through the area Friday. Still some
uncertainty with regards to timing...which will have a large
impact on sensible weather across the area. Thunderstorms are
expected along/ahead of the front. A few of these storms could
become marginally instability increases compared to
today (e.g., MLCAPE ~1000-1400 J/kg). Latest SPC Day 2 Marginal
Severe risk was shifted further north into our area...which
makes sense considering the timing uncertainty of the frontal
passage and thus how much destabilization can occur. While
slightly better than today...strong wind field remains to the
north of the area...which results in only marginally favorable
shear profiles for storm organization.

Front will stall to our south late Friday into Saturday, leaving
us with extensive cloud cover as we remain in the cool sector.
Low pressure will approach the area from the SW during the day
Saturday...allowing for overrunning precipitation to develop
over the area during the afternoon and through the overnight.
Uncertainty remains in regards to how far north the greatest
QPF sets up...latest trends in the guidance continue to push the
highest totals south...keeping them over central VA. 1-2 inches
of rain are possible across central VA...with generally less
than an inch expected north of I-66.


Low pressure at the surface will likely sink south of the area
Sunday as the upper level low forcing it becomes cutoff from the
westerlies. The low may meander off the coast for a couple days with
high pressure building to the north. This would result in a cool and
dreary pattern into the first half of next week.

By the middle of next week, the low should move away and high
pressure rebuilds to the southeast of the area, resulting in
southwesterly flow and a marked increase in temperatures.


Thunderstorms over BWI/MTN at this time. Otherwise, all activity
has pushed east of the area. In metro Baltimore, activity will
push east by 10pm. No additional storms anticipated tonight. Fog
is another story. There is now sufficient low level moisture.
Will be adding MVFR to TAFs shortly, and will reassess to see if
IFR warranted. There is certainly a chance at that.

Thunderstorms with gusty winds are possible Friday...with flight
restrictions possible in any stronger thunderstorms...VFR
expected outside of thunderstorm activity.

Low clouds and periods of rain could impact the terminals
Saturday afternoon through Sunday morning...with sub-VFR

Sub-VFR likely in low clouds/onshore flow (generally
northeasterly around 10 knots) Sunday into Monday.


Weakening showers and thunderstorms reaching the Bay at this
hour. We believe that gusts will be under 30 kt, but are
monitoring in case additional Marine Warning will be warranted.

A cold front will cross the waters Friday. Additional
showers/storms...some with gusty winds...will again be a
consideration. All should monitor in case another round of
Marine Warnings are issued.

Rain is expected for the waters Saturday.

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

Northeasterly flow between high pressure to the north and
low pressure to the south may push gusts into Small Craft Advisory
territory early next week.




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