-- Discussion containing changed information from previous version are highlighted. --
537
FXUS61 KCTP 180518
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
118 AM EDT Tue Jun 18 2024

.SYNOPSIS...
*Long duration and potentially dangerous heat wave this week
*Near-record high temperatures 90+ degrees into the weekend

&&

.NEAR TERM /THROUGH TODAY/...
1030 pm update... Convection has weakened considerably, with
lightning activity now almost completely gone, and just a few
residual showers across the northern tier. We expect all
showers/storms to have dissipated before midnight.

We did speed up the fog timing a bit for many of our northern
zones, with locally heavy rain earlier this evening, and patchy
fog already developing.

It will be a muggy night, with lows generally ranging from the
mid 60s over the northern mountains, to the lower 70s in the
Lower Susquehanna Valley.

8 pm update... Broken convective band has intensified a bit in
the last 30 minutes or so, as activity encounters a synoptic
stationary front (convergent dew point boundary) over central
PA, plus localized outflow, within a sufficiently unstable
environment. The threat for locally severe wind gusts/hail
should be with us until 8:30-9 pm.

Thereafter, we still anticipate a weakening trend, as the
environment stabilizes diurnally, and also becomes generally
less favorable into eastern PA with a drier column in place.

6 pm update... A broken band of showers and thunderstorms, tied
to an upper-level disturbance over Lake Ontario, extends from
western and central NY, through western and north-central PA,
then back into OH at 6 pm. An isolated strong or perhaps
marginally severe storm could still impact areas north of I-80
and about as far east as the I-99 corridor through 8 pm or so.
Thereafter, waning instability and the development of weak
subsidence behind the aforementioned disturbance should cause
any residual storms to weaken and dissipate.

We still anticipate some patchy fog later tonight, owing to
increasing surface dew points, locally wet ground conditions,
and light surface flow.

Lows by daybreak should generally range from 65 to 70.

Previous discussion... Still anticipate a gradual uptick in
[isolated] convection over the NW Alleghenies this afternoon on
the southern periphery of remnant MCV moving eastward across
southern Ontario. SPC has maintained the MRGL risk SWO and
recently sent MCD #1302 which included a portion of Warren
County. The limited vertical shear generally <20kt and weak
forcing for ascent suggest storm organization will remain
limited and therefore a WW is not expected (20% probability).
Recent radar loop showed a couple of t-storms developing near
KBFD which is reporting a dewpoint of 70F. One of those storms
briefly pulsed up over northern Elk County with a 50dBZ core
reaching the -20C level, prompting a short duration SPS.

Isolated to scattered convection mainly over northwest PA
should fade with loss of heating after sunset. Mostly clear
skies, light wind and surging low level moisture will likely
result in patchy valley fog late tonight. A muggy night with low
temps in the 65-70F range or +10-15F above climo in most spots.

&&

.SHORT TERM /TONIGHT THROUGH THURSDAY NIGHT/...
The heat and humidity are progged to build through midweek, as
the center of the subtropical high lifts north over PA. Current
guidance indicates heat indices will reach 95 to around 100F
especially in the valley locations during the afternoons Tue-
Thu. We extended the heat advisory through 8pm Thursday which
aligns well with neighboring WFOs to the west, north, and east.

Model guidance continues to favor isolated to scattered diurnal
t-storm risk Tue-Thu downwind of Lake Erie and over the higher
terrain along the spine of the Alleghenies. Thursday looks like
the hottest day of the period with maxT 90-95F or +10-20F above
climo.

&&

.LONG TERM /FRIDAY THROUGH MONDAY/...
-- Changed Discussion --
Late evening update brings about little changes outside of
potentially some lower MaxTs (still well above climo for mid-
to-late June) with slightly higher dewpoints keeping heat index
values high into the weekend. Diurnally driven afternoon showers
and thunderstorms remain plausible across the northern tier,
with some uptick towards Chc PoPs across the Endless Mountains.

Prev...

Medium range guidance continues to show that a prolonged heat
wave will continue throughout at least early this weekend.

A 595-600 dm 500 mb ridge looks to begin breaking down by late
week, but temperatures will still easily reach the 90s across
most of the region each day with heat indices in the upper 90s
and low 100s.

Northern PA will stand the best chance of any afternoon/evening
showers/thunderstorms through the period closer to an
approaching upper trough and farther away from the core of the
upper ridge. Better rain chances and possibly a reprieve in the
heat could materialize by late weekend or early next week
depending on the evolution of the aforementioned trough to our
north/northwest.
-- End Changed Discussion --

&&

.AVIATION /05Z TUESDAY THROUGH SATURDAY/...
-- Changed Discussion --
For the 06Z TAF package, looking at some low clouds and dense
fog from time to time until around 12Z, mainly in locations
that had showers and storms on Monday. This being BFD and
perhaps IPT.

Otherwise looking at mainly a dry day with VFR conditions.

More information below.

03Z Update: Lower dewpoint depressions have allowed for some
BR/FG formation at BFD earlier than expected with IFR/LIFR
conds. These are expected to continue overnight, prevailing LIFR
with some bounces towards low-end IFR overnight. Outside of BFD,
the 00Z TAF package remains on track.

Prev...

Winds have already started to diminish this evening, allowing
for some patchy fog formation across areas where rain has fallen
with clearing skies as the evening progresses. Higher (70+%)
confidence in some restrictions at BFD has allowed for IFR
mentions, and could realistically see LIFR conds in the 09Z-11Z
Tuesday timeframe. There is still some uncertainty as a light
wind could limit these restrictions, so have opted to keep a
period of MVFR-to-IFR restrictions for the 00Z cycle. Airfields
along the Laurels (JST/AOO) could also see some restrictions but
a light wind is more likely to keep restrictions minimal
overnight.

VFR conds will prevail areawide after 12Z Tuesday, but cannot
rule out isolated to scattered storms early Tuesday afternoon
over similar areas as Monday. If storms do develop, generally
expect less widespread coverage and low confidence (< 20%) does
not warrant a mention in this TAF cycle.

Outlook...

Wed-Sat...Predominantly VFR, with building heat and iso TSRA.
Density altitude concerns possible.
-- End Changed Discussion --

&&

.CLIMATE...
A period of much warmer than average temperatures is expected
across much of central Pennsylvania next week, with record high
temperatures being challenged. The record high temperatures for
some sites across central PA 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 Temperature for June:

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 in POR since 1957)

*Harrisburg had a run of 11 days that started on 26th and ended
 on July 6th in 1901

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
-- Changed Discussion --
Heat Advisory from noon today to 8 PM EDT Thursday for
PAZ004>006-010>012-017>019-024>028-033>037-041-042-045-046-
049>053-056>059-063>066.
-- End Changed Discussion --

&&

$$

SYNOPSIS...Steinbugl
NEAR TERM...Jurewicz/Fitzgerald/Steinbugl
SHORT TERM...Fitzgerald/Steinbugl
LONG TERM...Guseman/NPB
AVIATION...Martin/NPB
CLIMATE...Steinbugl/NPB