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Area Forecast Discussion (AFD)

Issued by State College, PA (CTP)

FXUS61 KCTP 171902

Area Forecast Discussion
National Weather Service State College PA
302 PM EDT Wed Jul 17 2019

----Dangerous heat and humidity possible this weekend---

We will be dealing with hazy, hot and very humid conditions 
until early next week when some relief is in sight. Dangerous 
heat is becoming increasingly likely this weekend, with heat 
indices on Saturday easily exceeding 100 over southern and 
southeastern areas.


A little something for everyone this afternoon and evening.

Radar has lit up as we hit the convective temp at about the same
time over most of the area. 

Flash flood watch NE as training or at least repeated storms 
are likely and have already set up. Near-tropical downpours are 
in store, and could affect lots of the area. But, it is the 
possibility training which is the most worrisome. The rest of 
the area will likely have a lower risk of flash flooding due to 
consistent storm motions to the east. 

SVR watch also on the table as shear is modest to strong over 
the NE and the heat and humidity are making big CAPE in the SE. 
Some strong storms are possible over all the area, though.
Already a few statements and a warning are out the door. Expect
most of the storms to be sub-severe, but a few will get tall
enough for severe wind gusts. Hail should not be severe size
today as it is so warm, and lots of melting will take place
before it would reach the ground.

Heat Advy in the SE will die off as the sun sets, but the HIs 
are already 100-105 in the SErn cities.


Deep layer moisture over 2.0" PW will hang on over my southeast
counties and could keep heavy downpours going into the early 
morning hours over Lancaster and York Counties, while slightly
drier air works in from the west elsewhere. We could have some
fog develop if some clearing happens, but will hold off on
mentions at this point. Mins will be a soupy m60s-m70s. The 
moisture will be slowest to leave the southeast and right along 
the MD border where isolated to scattered showers and 
thunderstorms cannot be ruled out into Thurs evening. Elsewhere,
Thursday will be a drier day with lower humidity but still 
quite warm throughout. Highs Thu range from the mid 80s north to
the lower 90s south. This keeps Heat Index very close to advy
numbers in the SE.


*Excessive heat risk Fri-Sat 
*Excessive Heat Watch issued for noon Friday through Sunday
 evening for eastern quarter of CWA

Model guidance continues to signal the potential for excessive
heat around the end of the week and into the weekend. Maximum
heat index values peak in the 100-105 range Friday, Saturday and
likely Sunday. In collab with neighboring offices, have issued 
an Excessive Heat Watch starting noon Friday for areas from
Harrisburg to Williamsport and east. Area in watch may need to 
be expanded later if confidence increases. Will also continue to
highlight heat risk via HWO, social media and DSS briefings in 
addition to the watch.

Very warm mid-level temperatures look to limit/cap t-storms 
Fri- Sun. That said, models hint at the potential for MCS 
activity propagating along wavy frontal zone on the northern 
periphery of summertime upper ridge/heat dome. 

Flow is fast and strong across the US and Canada border overall 
for mid July, but we are south of the main westerlies. The
airmass is warmer than what we have seen lately. 1991 and 1995
featured such warm air. In this case, the warmest air is
advected toward BUF late Friday. Warmest air for central PA is
Friday into Saturday. 850 mb temperatures fcst to be near plus
24 degrees at 850 mb late Friday across Lake Erie. Will be
interesting to see how fast water temperatures come up on Lake
Erie by next week. 

A cool down is expected by next Monday, as the mean upper level
trough is nearby.


Showers and thunderstorms developing across the region this
afternoon and will continue through 03Z. Timing any one cell
will be difficult so amendments will keep track of issues. Any
wind gusts of significance will occur in thunderstorms with
light winds otherwise.

Showers should taper off from northwest to southeast into
tonight. MVFR to IFR cigs are probable across the northern and 
western airspace with MVFR to low VFR most likely at central and
eastern terminals. 


Thu-Sun...Mainly VFR with isolated to widely scattered PM 
thunderstorms possible. Heat/humidity will make the density
altitude much higher - takeoffs/landings will take more 


Excessive Heat Watch from Friday afternoon through Sunday 
evening for PAZ028-036-041-042-046-049>053-056>059-063>066.
Flash Flood Watch until 1 AM EDT Thursday for PAZ037-041-042-
Heat Advisory until 7 PM EDT this evening for PAZ057-059-


NEAR TERM...Dangelo
SHORT TERM...Dangelo
LONG TERM...Gartner/Steinbugl