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Area Forecast Discussion (AFD)

Issued by State College, PA (CTP)

                            
000
FXUS61 KCTP 171840
AFDCTP

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

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

The remnants of once Hurricane Barry will move through the area
today and early tonight producing thunderstorms and localized 
heavy downpours. Otherwise, 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.

&&

.NEAR TERM /UNTIL 2 AM THURSDAY MORNING/...
* Confidence in the possibility of flash flooding has increased
  for this evening.

Issued a flash flood watch for 18-05Z for the NE as the most
recent runs of the meso models continue to point to the higher
possibility of thunder lingering/repeating over our NErn
counties. Collaboration with BGM/PHI revealed their concerns as
well. This has increased our confidence level to the point 
where a watch is warranted for that area. We do try to keep the
FF watches to a shorter time frame in these situations where we
can get more specific on areal coverage as onset time nears
(much like severe/tor watches). While isolated/localized FFing 
is possible almost anywhere in the CWA this aftn/early tonight, 
the NE will be most at-risk.

* Heat index expected to get to 100 this afternoon over the 
  lower Susq valley.

Dewpoint in LNS 76F (HI of 89) and lower 70s elsewhere in the 
southeast already. The NW/mtns are still in the 60s for 
dewpoints. A minor tweak upward in the dewpoints has pushed
forecast HI for this aftn right to 100/101 in the bigger 
cities/valley towns of the SErn few counties. 

Otherwise, no changes to current thinking on heavy
rainfall/flash flooding. Most of these storms will be moving
with some speed. But the 2.0-2.5" PWATs and possibility of 
repeated rain (mainly N) later this aftn and early tonight could
lead to localized flash flooding, but not enough confidence in 
this occurring to put up a watch just yet. 

* Heavy downpours today due to moisture remnants of former TC 
  Barry.

Boundary layer convergence over eastern OH and western PA
maintaining scattered early morning showers and isolated storms 
over western third of PA. Isolated showers and storms reaching
into the Laurel Highlands, WC/C/NC Mountains as well. This as
deep layer moisture continues to increase ahead of the center of
Barry's tropical moisture remnants, which are focused over OH
this morning. 

The airmass will be very juicy with PWATs in the 1.75-2" range. 
Localized heavy downpours will occur with thunderstorms this
afternoon and evening, and some will produce 1-2"+ in a short
amounts of time. Held off on any Flash Flood Watches at this
time as per collab with southern neighbors at WFO LWX as 3-hr 
FFG values of 3.5 to 4.5" coupled with weak- mdt flow from 850 
mb to 500 mb reduces the areal threat of flash flooding, but 
will likely see ponding of water in poor drainage areas from 
efficient downpours this afternoon and evening, especially 
across southern and southeastern sections. WPC has entire area 
in SLGT risk of excessive rainfall, while CAMs seem to pinpoint 
my southeast as the most likely target for locally heavy 
rainfall.

It will be a steamy day with highs reaching the low 80s north
and the lower 90s south, with dewpoints in the low to mid 70s.
Heat Advisory criteria is reached south and east of the Lower
Susq River Valley.

&&

.SHORT TERM /2 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
Locally heavy rainfall threat persists into this evening
as the moisture remnants of former TC Barry swing through.
Pooled deep layer moisture over 2.0" PW over my southeast
counties may keep heavy downpours going into the early morning
hours of Thursday over Lancaster and York Counties, while drier
air works in from the west elsewhere. The moisture will be
slowest to leave the southeast on Thursday as well where
isolated to scattered showers and thunderstorms cannot be ruled
out. 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.

&&

.LONG TERM /FRIDAY THROUGH TUESDAY/...
*Excessive heat risk Fri-Sat 

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 and Saturday.
Heat headlines appear likely during this time and will continue
to highlight heat risk via HWO, social media and DSS briefings.

Very warm mid-level temperatures may act 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. 

New part of discussion below.

I did look over temperatures and dewpoints several times this
shift. Made a few minor adjustments to the temperatures and 
dewpoints. 

I did cut pops back some after early Thursday, and prior to
later Sunday, based on mew model data and a very warm airmass.

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 Tuesday, as the mean upper level
trough is nearby.

&&

.AVIATION /19Z WEDNESDAY THROUGH MONDAY/...
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. 

.Outlook...

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 
distance.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Flash Flood Watch until 1 AM EDT Thursday for PAZ037-041-042-
046-051>053-058.
Heat Advisory until 7 PM EDT this evening for PAZ057-059-
063>066.

&&

$$
SYNOPSIS...Dangelo
NEAR TERM...Dangelo
SHORT TERM...Dangelo
LONG TERM...DeVoir/Steinbugl
AVIATION...Tyburski