-- Discussion containing changed information from previous version are highlighted. --
718
FXUS61 KCTP 192115
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
515 PM EDT Wed Jun 19 2024

.SYNOPSIS...
The center of a subtropical upper level high over Pennsylvania
will sink southward over the next few days. A weak cold front
will approach and stall out over the region Friday and
Saturday. A more significant cold front is likely to push
through Sunday night or early Monday.

&&

.NEAR TERM /UNTIL 6 AM THURSDAY MORNING/...
230 PM EDT Update:

Little change to the going forecast for the remainder of today
and tonight. We`re in standby mode for now, awaiting the
development of SHRA/Sctd TSRA across the NW Mtns of PA.

This portion of the CWA will also be within an area of weak
mesoscale UVVEL linked to the thermally direct cell of a weak
mid/upr level shortwave lifting NE across the Lower Glakes and
along the axis of highest PWAT air of around 2.00 inches.

Will have to keep a close eye on the potential for some strong
to potentially SVR and training cells on an axis from near KGKJ
to KJHW and KOLE. Locations along this axis have 1 hour Flash
Flood Guidance values reduced to between 1.25 and 1.5 inches
from Tuesday evening`s TSRA and will be susceptible to flash
flooding from any training cells.

High clouds for the remainder of today will be thickest over
the west. They will make little difference in max temps today as
the 8H temps will be higher in the NW than SE. The deepest
mixing (up to 5 kft AGL) should also be found across the NW Mtns
while mixed layer depths across the SE half of the CWA will be
around 4 kft AGL.

This will lead to max temps in the u80s on the ridge tops and
l90s in the valleys - very close to what we experienced on Tues.

Pulse storms with a slow NE drift will likely be the mode today
with a very small loop look to the hodograph, rather than a
longer/smooth line or curve. Similar to Tuesday, CAPE could max
out >2000J/kg in the nrn tier, and that could make a wet
microburst out of an isolated tall cell or two.

Another muggy night is in store with convection diminishing
during the mid to late evening. We`ll leave a small PoP across
the north after midnight as the ring of fire flattens out and
any transitory cells could travel along the lakeshore and into
wrn NY. Mins of 65-70F will be a little lower than recent days
because some subsidence under the big upper high could inch
dewpoints down into the lower 60s in spots overnight. The higher
dewpoints will be stubborn over the nrn tier due to earlier
rainfall.

&&

.SHORT TERM /6 AM THURSDAY MORNING THROUGH THURSDAY NIGHT/...
The Mid level high that was located from Eastern PA to the
Southern New England Coast today will retrograde slowly to the
west on Thursday, becoming centered across Central PA and Ohio.

500 MB flow becomes much more zonal and quite diffluent which
will combine with height falls and the approach of a sfc
frontal boundary to allow the daily pop-up/pulse storms to grow
a little taller via better storm relative inflow and deep layer
directional shear to have a higher potential for making wind
damage. Increase POPs by 10-20 percent and into the Likely
Category across the Northern Tier counties for Thursday.

SPC Day2 puts areas N of I-80 into the MRGL risk and far
NE (Tioga/Sullivan Cos) into the SLGT risk category. Capping
should keep things less active across srn PA on Thurs.
Persistence will be the best forecast for maxes, and the
dewpoints seem to recover just fine from any lowered numbers in
the morning, getting back into the u60s almost everywhere and
around 70F in the N for the middle of the day.

Again, we`ll hold onto the heat advy for both Thurs and Friday.
Friday does look a little more muggy and the heat index values
will be a couple of degrees higher than Thursday (and Wed).

&&

.LONG TERM /FRIDAY THROUGH WEDNESDAY/...
Heights lower just a little more on Friday, and a weakness in
the sfc pressure field develops over the nrn tier. How far S
the weak/diffuse front will make it on Fri will help determine
the eventual coverage of storms. Will go with a 40-50pct
coverage over the nrn half of the CWA in the aftn and evening
Fri. Again, one or two of these storms could produce a damaging
wind gust. There should be a little bit of movement (W-E) to
the storms on Thursday and a little bit more on Friday. But, the
shear is still pretty weak.

Hot early summertime temperatures will continue into this
weekend as an upper level high retrogrades west over the Ohio
and Tennessee Valleys. At the surface, the Bermuda high looks to
remain strong and anchored across the western Atlantic through
early this weekend. This will help to maintain H85 temperatures
of 20C+, resulting in heat indices topping 100F through Sunday,
particularly across south central PA into the Lower Susquehanna
Valley. Have maintained the Heat Advisory through Saturday as
Sunday still raises some questions as to exactly when the front
and associated rain will arrive.

Afternoon/evening showers and storms may become more numerous
Friday and Saturday, especially across northern PA, as heights
lower with the westward movement and weakening of the upper
ridge in combination with an approaching upper trough and
downstream energy moving along the central Canadian/CONUS
border. Areas farther south look to see showers/storms later
Sunday into Monday as the aforementioned trough dives south and
brings a cool front into the Commonwealth. This will bring a
period of drier/cooler northwest flow Monday/Tuesday, before
backing zonal flow and rising heights bring a return to more hot
and humid conditions by mid next week.

&&

.AVIATION /21Z WEDNESDAY THROUGH MONDAY/...
-- Changed Discussion --
VFR conds were found areawide at 21z, beneath a broken shield
of high clouds and a few cu at 4-5 kft.

An ensemble of short-range models continues to suggest that a
few SHRA/TSRA could develop over NW PA this evening. However,
any activity should quickly diminish with the loss of daytime
heating.

Otherwise, it will be a warm and somewhat muggy night. If any
rain falls around BFD, we could see some fog formation there
and have hinted at this in the TAF. Elsewhere, we could briefly
see some light fog/haze towards daybreak.

Thursday will feature predominantly VFR conds once again.
Similar to today, there could be a few late-day SHRA/TSRA across
mainly northern PA.

Outlook...

Fri-Sun...Predominantly VFR, with continued heat and iso
aftn/eve SHRA/TSRA. Density altitude concerns possible.

Mon...Predominantly VFR, with brief reductions possible in
scattered SHRA/TSRA as a cold front crosses the area.
-- End Changed Discussion --

&&

.CLIMATE...
A period of warmer than average temperatures is expected across
much of central Pennsylvania through the end of the week, with
record high temperatures at multiple cities being challenged
every day through Saturday.

Two records were either set or tied for sites on Tuesday:
* A record high temperature of 92 degrees was set at Altoona
  yesterday, which tied the old record of 92 degrees set back
  in 1994.
* A record high temperature of 90 degrees was set at Bradford
  yesterday. This broke the e old record of 87 degrees set back
  in 1993.

The record high temperatures for some sites across
central PA that could be challenged are outlined below:

 Jun 19 Jun 20 Jun 21

Harrisburg      98/1994     98/1931     98/1923
Williamsport    96/1929     101/1923    97/1933
State College   94/1931     94/1953     94/1988

Highest Max Temperatures for Central Pennsylvania Observing
sites for June (date of occurrence is in parentheses:

State College 96 in 1952 (26th) and 1933 (29th)
Harrisburg 100 in 1966 (27th), 1952 (26th), and 1934 (29th)
Williamsport 104 in 1925 (5th)
Altoona 97 in 1971 (28th) and 1953 (21st)
Bradford 93 in 2022 (22nd)

Longest run of consecutive days with max temp >=90 in June:

State College 6 (24-29th 1966)
Harrisburg 10* (19-28th 1943)
Williamsport 9 (18-26th 1923)
Altoona 8 (13-20th 1994)
Bradford 0 (has not occurred since POR started at site in 1957)

*Harrisburg had a run of 11 days with temperatures greater than
 90 degrees that started on 26th and ended on July 6th in 1901.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM EDT Saturday for PAZ004>006-010>012-
017>019-024>028-033>037-041-042-045-046-049>053-056>059-
063>066.

&&

$$

SYNOPSIS...Lambert/Dangelo
NEAR TERM...Lambert/Dangelo
SHORT TERM...Lambert/Dangelo
LONG TERM...Guseman/Dangelo
AVIATION...Lambert/Evanego
CLIMATE...Evans/Martin