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Area Forecast Discussion (AFD)

Issued by State College, PA (CTP)

FXUS61 KCTP 211954

Area Forecast Discussion
National Weather Service State College PA
354 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.


Scattered to broken mainly thin high cloud remnants from 
overnight and early morning midwestern convective clusters are 
streaming across parts of western and central PA this afternoon.
A brisk north to northwest flow is making temperatures in the 
50s and 60s feel much cooler, but at least it's fair and 
pleasantly cool. Seeing gusts in the teens and lower 20s thanks 
to rather efficient 3-5k AGL mixing height. 

Expect to see better decoupling tonight as sfc high builds into
the region. Winds will become light after 00z, and given still 
dry air over much of the CWA, expect similar, if not cooler min 
temps early Wednesday morning. We saw local mid 30 readings over
Warren and McKean Counties this morning, so will mention patchy
frost there as most areas should remain in the upper 30s for 
lows. Official growing season began in these areas today. Min 
temps at sunrise Wed will generally vary from the upper 30s in 
the perennial cold spots of the north, to around 50F in the 
larger metro areas of the southeast, but we could again see
highly localized mid 30s in the coldest spots.


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.




SHORT TERM...Lambert
LONG TERM...La Corte