172
FXUS61 KCTP 171855
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
255 PM EDT Tue Jun 17 2025

.SYNOPSIS...
* Scattered to numerous instances of flash flooding possible
  this afternoon into early tonight across southwestern and
  south central PA
* Strong to potentially severe thunderstorms possible Thursday
  afternoon
* First significant heat wave to the season expected Sunday into
  early week with Major Heat Risk forecast (level 3 of 4)

&&

.NEAR TERM /THROUGH TONIGHT/...
Convective activity has begun to overspread southwest PA with
some clearing noted on the latest satellite imagery. Sufficient,
but not significant, low-level shear could support a brief
tornado across south central PA. Any spin ups would be short-
lived.

The far more notable concern today is the potential for
numerous flash floods with significant events possible. An
extremely moist near- record PW environment for mid June will
support a moderate risk excessive rainfall outlook (level 3 out
of 4 ERO) which [since 2009] has only occurred 5-6 days a year
or less than 2% of days annually.

Very moist southwesterly return flow from the Gulf - pinched
between Bermuda-type ridge off the southeast coast and digging
trough tracking east from the mid Mississippi Valley - will
interact with multiple mid level shortwave impulses and a
wavering sfc front to produce enhanced isentropic ascent
supportive of heavy convective rainfall this afternoon into
early tonight.

The high-res CAMs and Warn on Forecast System output are in
good agreement in targeting parts of southwest/southcentral PA
with increasing coverage of heavy t-storm downpours during peak
afternoon/evening heating. The heaviest rainfall corridor is expected
to be south of I-80, west of US-15, and north of I-76. Although
storm motions are expected to be somewhat progressive to the
northeast based on 20-30kt mean wind, the magnitude of deep
layer moisture available to tap will be close to/if not at
record levels for mid June with PWs approaching 2 inches. The
near-record PW and deep warm cloud depths of 13000-14000 ft will
support hourly rainfall that has a chance (20-40%) of exceeding
2"/hr, with sub-hourly rain rates of 3-4"/hr possible. Despite
a lack of organization, repeating rounds of thunderstorms with
these intense rain rates could produce stripes of rainfall
exceeding 3" (50-60%) and locally 5" (20-30%) with the greatest
potential for the higher accumulations suggested by both HREF
and REFS guidance occurring over CPA. With FFG extremely
compromised and reduced to 0.75" or less/1hr in some areas, WPC
has upgraded to a MDT risk for excessive rainfall. Given the
strong +RA signal and background antecedent conditions, a flood
watch was an easy decision to highlight and message what could
be a potential higher-end flooding day in CPA.

A majority of the rainfall is progged to fall within the
Juniata River basin. River flooding is possible in that area
overnight into Wednesday morning with a quick, several-foot rise
in river levels as rain moves through. After somewhat of a lull
in rain coverage/intensity after 03Z or 11pm, high-res ensemble
guidance favors an uptick on shower activity through early
Wednesday morning across the western Alleghenies which appears
tied to a trailing shortwave impulse.

&&

.SHORT TERM /WEDNESDAY THROUGH FRIDAY NIGHT/...
Another midlevel shortwave trough is fcst to move through the
central Appalachians by 00Z Thu along the northwest periphery of
the upper level ridge off the Carolina coast.

Rather strong heating of a moist environment will result in
moderate to strong buoyancy Wednesday afternoon particularly
across southeast PA. Scattered diurnal storm development will be
possible with guidance generally depicting a belt of 35-45 kt
midlevel flow across the region. This modest flow enhancement
could result in some storm organization, with large PW and
steepened low-level lapse rates resulting in a threat for at
least isolated damaging wind. The D2 MRGL risk SWO clips the
eastern portion of the lower Susquehanna Valley.

By Thursday, a seasonably deep 500mb trough will move across
the Great Lakes into the Northeast CONUS. With rich moisture
already in place, it won`t take much heating to drive
thunderstorm development Thursday afternoon ahead of and along
an approaching cold front. Timing of the cold front still varies
somewhat which adds an element of uncertainty to the severe
storm potential. That being said, SPC transitioned the D4 15%
severe prob to a D3 SLGT risk SWO which essentially covers the
southeastern 1/2 of CWA.

Higher confidence outcome is the front clears the area Thursday
night which will bring drier/less humid conditions to close out
the week an officially kick of astronomical summer (see climate
section).

&&

.LONG TERM /SATURDAY THROUGH MONDAY/...
As upper ridging builds in, a capping inversion will limit any
shower/thunderstorm chances to sub 20 pct for Sat-Mon. By
Sunday, highs in the low to mid 90s are becoming more likely as a
very strong ridge of high pressure with 500mb heights
approaching 600dm builds overhead. NAEFS guidance depicts a ~72
hour period of anomalous 500mb heights, 700mb temperatures, and
850/925mb specific humidity values (>99th percentile for this
time of year). If such a forecast materializes, we could be
looking at a several day stretch of extreme heat and humidity
across the region. Monday looks to be the hottest day of the
stretch, with highs currently forecast in the 90s for most of
the area. Dewpoints in the mid 70s would push heat index values
over 100F for most locations east of I-99 and south of I-80,
with values approaching 105+ in the Lower Susquehanna Valley.
Although this forecast is still several days out and
uncertainty remains, the fact that we have been in a stretch of
relatively cool conditions means the quick switch into
significant heat and humidity could exacerbate the risk for
heat-related impacts.

In addition to warm afternoon temperatures and heat index
values, overnight lows will provide little to no relief from the
heat. With the expected stretch of hot and humid conditions, those
with heightened susceptibility to heat, such as young children,
older adults, people with chronic medical conditions, and
pregnant women, should prepare to limit time outdoors, wear
light clothing, and drink plenty of water later this weekend
and early next week.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
12Z TAFS sent.

Not much to add, conditions at 7 AM about the same as the last
few days. Showers limited on coverage the last few hours.

Earlier discussion below.

06Z TAFS sent.

Main issue today into early Wednesday will be low CIGS.
Visibilities will low at times, mainly at night and in and
near showers and storms.

Chance for thunder will be a bit limited, unless low clouds
can break, and the airmass heated by the strong mid June
sun this aft.

No real change in the coming days.

Outlook...

Wed...VFR/MVFR with showers and thunderstorms possible, as warm
front pulls north of area.

Thu...Predominantly VFR, although strong PM t-storms are
possible with a cold frontal passage.

Fri-Sat...Predominantly VFR, with a slight chance of a rain
shower north.

&&

.CLIMATE...
Astronomical Summer (solstice) officially arrives on Friday,
June 20, at 10:42 p.m.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Flood Watch until 10 PM EDT this evening for PAZ010>012-
017>019-024>028-033>036-045-049-050-056-063.

&&

$$

SYNOPSIS...Steinbugl
NEAR TERM...Steinbugl/Banghoff
SHORT TERM...Steinbugl/Banghoff
LONG TERM...Steinbugl/Colbert
AVIATION...Martin
CLIMATE...Steinbugl