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Area Forecast Discussion (AFD)

Issued by State College, PA (CTP)

FXUS61 KCTP 191959

Area Forecast Discussion
National Weather Service State College PA
359 PM EDT Mon Aug 19 2019

It will remain very warm and humid through mid week and a weak 
trough of low pressure will keep the chance of mainly afternoon 
and evening showers and thunderstorms in the forecast. Cooler 
and much less humid air will arrive for Thursday through 
Saturday with dry conditions expected.


Satellite shows a bright day in progress with a quiet radar as
of 2PM. The near zonal flow from the upper Plains into PA will 
continue aloft while weak high dominates on the surface today 
and tonight.

I further trimmed POPs down to just the very small chance of a
stray shower or storm over the far eastern zones in what the 
RAP is showing as some marginally unstable air. With warm air 
building in aloft, have to believe this will act as an effective
cap to most convection.

Heat indices are already approaching 100 over York and
Lancaster, and will top out near or slightly above 100 over most
of the SE for a few hours this afternoon. Either way the 
heat/humidity will be noticeable over much of the region
reminding us summer is not over quite yet, despite the
shortening days. 

Another muggy night is in store for most of the CWA with highs
near 60 north, to lower 70s over the SE.


Approaching mid/upper level short wave trough and increasing 
south to swrly LLVL flow along and to the south of a weak warm 
frontal boundary will help ramp up convective parameters Tuesday
afternoon, with scattered afternoon showers and storms expected
in all parts of the state. 

High temps Tuesday will be a tad lower lower than today given
the expected additional sky cover. Highs will range from the 
Lower 80s across the NW Mountains and Laurel Highlands to 
lower 90s over the SE.


A slightly higher probability for TSRA Wednesday afternoon and 
evening as a more vigorous upper shortwave and associated sfc 
CFROPA pushes southeast through the CWA late Wednesday and Wed 

Upper low crossing eastern Canada will anchor a trough that
traverses the Great Lakes/Northeast US by the end of the period.
The sensible wx forecast is tied to the movement and eventual 
position of the leading cold front which is expected to cross 
the area on Thursday and stall out to the south of the Mason 
Dixon line. This scenario should eventually allow cooler and 
drier (less humid) air to spread into the region behind the 
front and bring a nice stretch of weather from late week through
the weekend. 

The cold front will be a focus for thunderstorms as it pushes 
southeast on Thursday. A severe storm risk upgrade is possible
across a portion of the area (most likely southeast PA). A 
trailing wave of low pressure riding along the stalled frontal 
boundary could bring another round of rain to the southern tier 
of central PA on Friday. This outcome is supported by the latest
operational ECMWF but considered a low probability outcome at 
this time given lack of support from other guidance. 

Still expect Thursday to be the transition day with frontal
passage fueling storms followed by push of cooler and drier 
air Thursday night. The FROPA will also spell some temporary 
relief from the recent hot and muggy conditions. High pressure 
and lower humidity should translate into pleasant and warm days
and comfortably cool nights with valley fog Fri-Sun.


Expect widespread VFR to continue into the evening.
A mainly clear night with high humidity should lead to fog and 
low clouds causing restrictions at most airports during the few 
hours either side of sunrise Tuesday.

Showers and thunderstorms will develop and become most numerous
Tuesday afternoon and evening, causing local brief restrictions.


Wed...Rounds of PM showers/thunderstorms. Late night and early 
morning fog/low clouds.

Thu...Trending VFR northwest; chance of t-storms southeast. 
FROPA Thursday night.

Fri-Sat...No Sig Wx.


Harrisburg tied the previous record high of 97 degrees 

Record highs for August 19:

Harrisburg 97 in 1966
Williamsport 94 in 1899
Altoona 91 in 1983
Bradford 86 in 1978
State College 96 in 1899


Heat Advisory until 7 PM EDT this evening for PAZ065-066.


NEAR TERM...La Corte
LONG TERM...Gartner/Steinbugl