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Area Forecast Discussion (AFD)

Issued by State College, PA (CTP)

FXUS61 KCTP 220035

Area Forecast Discussion
National Weather Service State College PA
835 PM EDT Tue May 21 2019

A ridge of high pressure will build east across the Commonwealth 
tonight and slide off the Mid Atlantic Coast by Wednesday 
evening. Fair weather will prevail through Wednesday afternoon,
but much higher humidity will surge into the state from the 
Ohio Valley Wednesday night and Thursday, bringing an increasing
chance for showers and Thunderstorms. Pennsylvania will be on 
the northern periphery of a building subtropical ridge late this
week and over the upcoming weekend. This will mean occasional 
bouts of mainly afternoon and evening showers and thunderstorms.


Fair weather and light wind expected overnight, as surface ridge
builds over the region. Satellite loop shows a good deal of mid
and high level cloudiness poised to overspread Pa from the west
overnight. Therefore, believe earlier concern for patchy frost
over the NW Mtns is diminishing. Min temps at sunrise Wed should 
generally vary from the upper 30s in the perennial cold spots of
the north, to around 50F in the larger metro areas of the 


After a fair tranquil and cool start with high pressure
overhead, southeasterly llvl flow will increase during the day 
as the high pressure ridge drifts off the NJ Coast. Several 
layers of clouds will also form/advect east into region as the 
nose of much higher PWAT air (30-35mm or +2-3 sigma) spreads 
into Western PA late in the day. A small chc for showers exists 
across the northern tier and Alleghenies of west-central PA 
later in the afternoon and evening Wednesday, and kept low chc
POPs in the forecast for the NW Mountains as a result. Highs
will moderate into the upper 60s and 70s throughout with a 
noticeable uptick in humidity in the afternoon as dewpoints 
rise through the 60s.


NAEFS indicates Pennsylvania will be on the northern periphery 
of a building subtropical ridge later this week into the weekend.

The most significant weather during the extended looks like it
will be on Thursday when the ingredients for a severe storm
outbreak look most likely.

Models show an advancing warm front late Wednesday night into
Thursday morning. The deterministic models all show some remnant
convection sliding through PA associated with the warm front and at
least one weak shortwave aloft that will be racing over the top
of the eastern ridge.

Progged stability indices indicate the warm front will move
through the CWA Thursday morning, with moderate instability
developing over the region during the late morning into the 
afternoon hours. A robust shortwave is made to approach during 
the second half of the day pushing a potent mid level speed max 
down and through the area during the afternoon hours. Deep layer
flow will be out of the WNW with the models developing 
impressive helicity on the order of 2-3(m^2/sec^2). The most 
likely scenario will be for storms to develop over the lower 
lakes and spread ESE across the region during the afternoon in 
short lines and bowing segments along with the possibility of 
isolated supercells.

In the wake of the Thursday system a slightly cooler and drier 
day is in store for Friday. By Saturday the humidity will begin 
to return along with a chance of mainly diurnal showers/storms.
as a weak shortwave once again races over the top of the eastern
ridge. We look to get a break on Sunday as the 12Z models 
show a mainly dry day, but by the Monday-Tuesday timeframe more
showers become possible as we stay in fast flow aloft near the 
top of the subtropical ridge.


A ridge of high pressure will build east across the Commonwealth 
today and slide off the Mid Atlantic Coast Wednesday evening. 
VFR expected over all routes today and tonight. NW winds will 
persist at 10-15 mph with gusts around 20 mph before diminishing
by sunset. VFR conds prevail tonight and Wednesday, with winds
shifting to the southeast and increasing later Wednesday
morning afternoon.


Tue-Wed...Mainly VFR.

Thu...VFR into the afternoon, then strong thunderstorms possible
mid afternoon into Thu night.

Fri...Ceiling restrictions poss NW early, then mainly VFR.

Sat...scattered restrictions possible from potential showers 
and thunderstorms.




NEAR TERM...Fitzgerald
SHORT TERM...Lambert
LONG TERM...La Corte