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Area Forecast Discussion (AFD)

Issued by State College, PA (CTP)

FXUS61 KCTP 211452

Area Forecast Discussion
National Weather Service State College PA
1052 AM EDT Sun Jul 21 2019

The dangerous heat wave will last one more day today before
cooler and less humid air spells relief from the sweltering
conditions this week. As the heat wave breaks down, the risk of
flash flooding and severe thunderstorms will increase into
Monday. An extended period of dry weather is forecast Tuesday
through the remainder of the upcoming week. 


After a very warm night with near-record mins (assuming they
hold prior to midnight) and full sunshine, temperatures this 
morning are off and running in the low 90s in the Harrisburg 
area at 10AM. HX values are also exceeding 100F at CXY and LNS.
Heat warning/advisory continues through the evening with highs
in the 90s and max HX 100-115. 

Hires models are generally in agreement in developing isolated 
to scattered convection along/south of slow moving to nearly 
stationary frontal zone over central PA this afternoon. 
Anomalously high PWATs 1.5-2.0 inches and 2000-3000 J/kg of ML 
CAPE support risk of heavy downpours with isolated flash 
flooding possible. There is also the potential for locally 
strong/damaging wind gusts in the strongest pulse/multi-cell 
storms. Weak deep layer shear and poor mid level lapse rates 
will limit organized severe risk. The most likely area should be
to the south of US-6 based on blend of latest CAMs and HREF 
mean. Intensity of storms and overall coverage should fade into 

Low temps will be in the low 60s tonight to the north of the 
quasi stationary boundary draped near the I-80 corridor. Another
warm night across south central PA with lows in the 70-75 range. 


Monday looks to be an active day. The frontal system will slow 
during its passage and with a deepening upper trough overhead, 
at least one wave is expected to form on the front, 
potentially helping to focus locally heavy rain. 

At this hour I don't have enough confidence to say exactly 
where the best rains will fall, but early indications suggest my
SERN zones where PWATs around 2 inches (2-3 sigma) will pool 
along and ahead of the wavy front. In general, when warm season 
tropical moisture combines with upper forcing more in line with 
a cool season system, we usually see problems develop. We are
leaving it for the day shift to evaluate and possibly issue a 
Flood Watch to start preparation for the potential heavy rain 


The 00Z deterministic NAM and ECMWF are slower in making the
last of the wavy front and the rain exit my SERN zones, with
rain lingering much of Monday night. The GFS and Canadian are 
faster and bring the drier air in sooner.

What is more certain is that by the end of Tuesday we will see
markedly less humid air moving into the forecast area. This 
will be the beginning of a period of pleasant mid summer days 
with temperatures running several degrees cooler than normal, 
and with lower dewpoints and humidity. 

Models agree with keeping the anomalous eastern upper trough the
dominant feature through the end of the week when some
indications of rising heights returning to the NE are depicted.


12Z TAFS sent.

The few showers and storms that formed around 4 AM are mainly
gone now.

Hard to believe it is 80 degrees outside the office at 7 AM.
We did not get the outflow from any storms last night, as was
the case the night before. Other nights we cooled down to around
70 degrees.

Anyway, expect at least some showers later today, as a cold
front moves toward the area. Left thunder out for now. Used VCSH
for the most part.

Trend for Monday is cooler and wetter.


Mon...Widespread showers and tsra impacts possible. 

Tue-Thu...No sig wx expected.


Excessive Heat Warning until 8 PM EDT this evening for PAZ028-
Heat Advisory until 8 PM EDT this evening for PAZ019-025>027-


NEAR TERM...Steinbugl
LONG TERM...La Corte