-- Discussion containing changed information from previous version are highlighted. --
501
FXUS61 KCTP 182208
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
608 PM EDT Tue Jun 18 2024

.SYNOPSIS...
*Isolated strong/severe T-storms this afternoon and evening
*Long duration and potentially dangerous heat wave this week
*Near-record high temperatures 90+ degrees into the weekend

&&

.NEAR TERM /THROUGH WEDNESDAY/...
Most of central PA sites have reached 90F as of 18Z/2pm this
afternoon with local heat indices around 100F. A heat advisory
remains in effect.

MCD#1318 from SPC details the environmental conditions and
convective regime over central PA this afternoon/evening:

A very moist low level airmass (sfc dewpoints in the upper 60s
to lower 70s) continues to advect northward and pivot around a
deep layer anticyclone currently positioned over the Mid
Atlantic region. Robust heating of this moist airmass is
resulting in convective temperatures (i.e. 88-90 F) being
reached over most of central PA, with thunderstorm initiation
already underway triggering off the higher terrain.
Overspreading this airmass are 6+ C/km mid level lapse rates,
which are supporting narrow, but tall CAPE profiles, with 2000+
J/kg MLCAPE already becoming widespread. The current synoptic
regime around the anticyclone is fostering weak vertical
shear/flow profiles, so pulse cellular is the expected mode of
convection, though multicells are also possible. Wet downbursts
with the stronger storms with elevated 50+ dBZ cores should
support strong wind gusts, some of which may cause localized
damage. However, severe (50+ kt) winds are not expected to be
particularly widespread, so a severe t-storm watch appears
unlikely at this time (20% probability).

"Ridge-running" storms will also be capable of producing
locally heavy rainfall with relatively slow cell motions and
pwats up to 1.75 inches. Lake breeze boundary downwind of Lake
Erie may help enhance low level convergence and forcing. The
12Z HREF guidance suggests rainfall rates should easily reach 1
to 2 inches/hour with the stronger storms, with the potential
for a lot of this to fall in as little as 30 minutes given the
high CAPE environment. Some localized storm totals of 2+ inches
will be possible, and this may result in some isolated instances
of flooding.

Bulk of t-storms should fade into the late evening hours due to
the loss of heating. Another warm and muggy night with patchy
fog and low temps in the 65-70F range or generally +10-20F
above mid June climo.

&&

.SHORT TERM /WEDNESDAY NIGHT THROUGH FRIDAY/...
Heat wave continues into late week with a heat advisory in
effect until 8pm Friday. Max temps and HX values peak from
Thursday into the weekend.

Isolated to widely scattered thunderstorms remain possible
on Wednesday coincident with the diurnal heating cycle. Robust
moisture and high dewpoints will also support potential for
locally heavy rainfall.

Surface front sinks south across the US/CN border on Thursday
and will encounter a very warm/moist environment. Favorable
timing with peak heating suggests scattered storms are likely
with a MRGL risk SWO over the northern tier of central PA.

&&

.LONG TERM /FRIDAY NIGHT THROUGH TUESDAY/...
The heat and humidity will continue into Saturday, although we
could begin to see some slight relief working into northeastern
PA in the form of a backdoor cold front. It will be another hot
day for much of the area though, and could easily see
continuing to push the advy out into the weekend for much of the
CWA. The proximity of the cold front will bring a slight
increase in PoPs, esp across north-central and northeastern PA.

Sunday into Monday should see the (slightly) cooler temperatures
expand southward as a cold front crosses the area. This front
will be accompanied slightly increased PoPs areawide Sunday
into Monday.

Any subtle cool down early next week looks like it will be
short-lived, with heat building once again by midweek.

&&

.AVIATION /22Z TUESDAY THROUGH SUNDAY/...
Hazy, hot and humid conds will be the rule across central PA
for the next few days. Spotty storms will continue to pop up
this afternoon with daytime heating, with the best chc of
getting wet being over the higher elevations of the Allegheny
Plateau.

Brief reductions and gusty winds are possible in any storms,
but this activity should be widely scattered. Added VCTS to
JST, which already has some thunder nearby, with VCSH added to
the remaining TAF sites outside of the Lower Susq Valley.
Despite the spotty storms, most sites will be VFR for the
majority of the day.

Another warm and muggy night is in store, with any lingering
showers/storms quickly diminishing as we lose the heat of the
day. Patchy fog is once again possible in areas that see
downpours today, but not confident enough to put it in any
specific TAF sites yet.

Outlook...

Wed-Sun...Predominantly VFR, with continued heat and iso TSRA.
Density altitude concerns possible.

&&

.CLIMATE...
-- Changed Discussion --
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 mutiple cities being challenged
every day through Saturday.

Two records were either set or tied for sites today:
* A record high temperature of 92 degrees was set at Altoona
  today that ties the old record of 92 degrees set back in 1994.
* A record high temperature of 90 degrees was set at Bradford
  today that breaks the 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 18 Jun 19 Jun 20 Jun 21

Harrisburg      97/1957     98/1994     98/1931     98/1923
Williamsport    97/2018     96/1929     101/1923    97/1933
State College   94/1923     94/1931     94/1953     94/1988

Highest Max Temperatures for Central Pennsylvania Observing
sites for June (date of occurrance 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.
-- End Changed Discussion --

&&

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

&&

$$

SYNOPSIS...Steinbugl
NEAR TERM...Steinbugl
SHORT TERM...Steinbugl
LONG TERM...Evanego/NPB
AVIATION...Evanego
CLIMATE...Steinbugl/Evans