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Area Forecast Discussion (AFD)

Issued by State College, PA (CTP)

FXUS61 KCTP 180126

Area Forecast Discussion
National Weather Service State College PA
926 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 
especially southeastern areas.


Well, a little bit of everything going on this evening. First,
heat advisory for parts of region earlier today expired as
strong tstms slid across the lower Susq, leaving just the 
excessive heat watch ongoing for Fri-Sun. That severe weather 
threat has since moved off to the east, so severe thunderstorm 
watch #516 has ended as well. 

But showers and isolated tstms will linger into the early
morning. So, what to do with current flash flood watch over the
NE? Best shot at training of cells with torrential rainfall has
passed, though some spots did come in with over 2 inches of rain
for the day. Amounts going forward not looking like enough to
cause problems, but with area of rain ongoing in watch area with
high PWATs and a few heavier downpours persisting, will not 
rush cancelling the flood watch even though it will likely be 
ended early.

Mins will be a soupy m60s-m70s, though some slightly drier air
will be working in from the west. But lingering ground moisture
and high dewpoints will lead to some fog development with patchy
dense fog likely. Will have to see how the fog develops before
deciding on need for a dense fog advisory.


Patchy dense fog to start the day as 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.


00Z TAFS sent.

Strong to severe storms just east of the MDT and LNS areas.

Several more hours of showers and isolated storms to the west.

Main issue after that will be low CIGS and fog forming.

Expect improving conditions by early afternoon on Thursday.

Left showers out of the fcst for Thursday, expect warming 
aloft and slightly drier air to hold storms down.


Fri-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 

Mon...Still a chance of showers and storms.


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-


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