317
FXUS61 KCTP 151148
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
748 AM EDT Mon Jul 15 2024

.SYNOPSIS...
High pressure will remain near Bermuda for the first part of
this week, resulting in a prolonged heat wave over the region.

A compact but potent upper level disturbance and strong winds
aloft will bring a round of strong to severe thunderstorms that
will be focused across Central PA and the Mid Susquehanna
Valley later this afternoon and evening.

A cold front will push southeast across Pennsylvania late
Wednesday with another round of strong to severe thunderstorms,
then high pressure will build in from the Great Lakes late in
the week.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
Fairly tranquil, but warm and humid conditions with variable
amounts of clouds and even a few showers were present across
central and northern PA early this morning.

A relatively flat mid/upper level shortwave and associated weak
sfc streamline convergence with mostly cloudy to overcast skies
were noted on regional sat loop and mesoanalysis approx 40 NM
either side of a line from KCBE to KUNV and KELM.

This feature aloft could spark a few showers or a TSRA across
the Susq Valley around and shortly after daybreak today, where
the latest RAP and 03Z SREF display higher instability, SREF
indicates 40-70 percent chcs for CAPE to exceed 1000 J/KG
through 13-14Z near and to the east of the Lower and Mid Susq
Mainstem. Virtually no chc of exceeding 1000 j/KG across the
Central and Western Mtns during this time, considering that a
slight PWAT min of around 1.3 to 1.4 inches was found across the
western half of the CWA with PWATs of 1.6 inches across far
Western PA and Sern PA.

Min temps at sunrise today will range from the mid 60s in the
coolest northern valleys to the mid 70s in the more urbanized
spots of the Lower Susq Valley.

Diurnal heating, combined with the approach of a more potent
shortwave/MCV (extending from Southern Lower Mich into Indiana
and NW Ohio at 0830Z) will be accompanied by a belt of strong
40 to 50 kt WSW to SWrly winds later this morning and this
afternoon. This prompted SPC to coordinate a SLGT risk SVR area
for this afternoon and this evening that extended from Central
and Western NY south into the Middle Susq Valley and Ncent PA.

The severe weather threat will be focused on strong to
potentially damaging straight line winds from mid level jet
inflow/mini bow echoes embedded within a bkn line of TSRA, not
to mention the possibility of even a few discrete supercells
given broad looping hodographs with helicity values of 250-300
m2/s2 and mean storm motion from 280-290 deg.

The more-certain wx problem will be the heat. An excessive Heat
Warning covers the Lower Susq Region for this afternoon and
Tuesday afternoon with little relief during the overnight hours
tonight.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
After the convection diminishes and/or moves east and out of
the area, weak ridging at the sfc and aloft builds across the
Commonwealth tonight.

Mins again will range from mid 60s in the northern valleys to
the mid 70s in the urbanized areas of the Lower Susq Valley.

Heat wave peaks Tuesday afternoon, as a large Bermuda high
pumps increasingly hot and very humid air into CPA. 850mb temps
rising to around 22C supports highs peaking between 90-100F with
max heat indices 100-105F southeast of the Alleghenies. High
confidence in excessive heat risk lead to the issuance of a heat
advisory for the south central mountains and Middle Susquehanna
Valley Mon & Tue and an excessive heat warning for the Lower
Susq Valley, where max heat index around 105F appear likely.

A round of at least semi-organized/possible severe convection
is possible Tuesday PM ahead of cold front pressing southeast
through the Great Lakes toward Lake Erie. However, a better
chance of more widespread showers/tsra is in the cards for Wed
PM with the passage of the front.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
All medium range guidance supports a return to fair and
seasonable weather with much lower humidity late in the week, as
high pressure builds southeast from the Grt Lks. Will maintain
just a slight chance of convection along the southern tier
Thursday to account for some uncertainty regarding how far the
dying cold front makes it south of the Mason Dixon Line.

The stalled front is likely to return northward next weekend,
resulting in the return of diurnal convection, mainly over the
southern counties.

&&

.AVIATION /12Z MONDAY THROUGH FRIDAY/...
At 12z, primarily VFR conds were found across central PA
beneath patches of mid-level cloudiness. With that being said,
a deck of borderline IFR/MVFR cigs is slipping southeastward
from wrn NY into the NW mtns, and will impact BFD through
13-14Z. KJST also briefly dipped to a MVFR stratocu CIG, but
that should also mix out within the next hour or 2.

LNS is also seeing some light fog (MVFR vsby) this mid morning.

Additionally, widely scattered showers are beginning to pop up
over the Susq Valley, and a few showers (and even a bit of
thunder) are approaching the Laurel Highlands and also aimed at
the KBFD area with the arrival time there around 1330Z.

Additional showers and storms are expected to pop up
with the heat of the day. There is some discrepancy between
short-range models as to the location and coverage of this
activity. However, it still appears that nrn PA has the best
chc of seeing thunder today, so have maintained VCTS in the TAFs
for BFD, IPT and UNV.

Expect more of the same for Tuesday, before a potent cold front
crosses the area in the midweek and ushers in a cooler and
drier airmass for late week.

Outlook...

Tue...Scattered, mainly aftn, SHRA/TSRA possible.

Wed...Numerous SHRA/TSRA, with some restrictions expected.

Thu...Lingering -SHRA diminishing.

Fri...No sig wx.

&&

.CLIMATE...
The duration of warm nights is noteworthy. Here are the longest
stretches with temperatures at or above 70F on record at
Harrisburg.

# OF DAYS   DATES      YEAR
17        7/19-8/04    2020
13        8/16-8/28    2021
13        7/13-7/25    2010
12        7/18-7/29    2022
12        7/17-7/28    1999
12        7/20-7/31    1940
11        8/04-8/14    1977
10        8/10-8/19    2016
10        7/14-7/23    2013
10        8/01-8/10    1968
10        7/21-7/30    1898
9         12 times, most recently 7/3-7/11/2020
8         7/04-7/12    2024
     .................................................

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Heat Advisory from noon today to 8 PM EDT Tuesday for PAZ019-
025>028-034-035-045-046-049>053-058.
Heat Advisory until noon EDT today for PAZ036-056-057-059-
063>066.
Excessive Heat Warning from noon today to 8 PM EDT Tuesday for
PAZ036-056-057-059-063>066.

&&

$$

SYNOPSIS...Lambert/Fitzgerald
NEAR TERM...Lambert/Fitzgerald
SHORT TERM...Lambert/Fitzgerald/Dangelo
LONG TERM...Fitzgerald/Bowen
AVIATION...Lambert/Evanego
CLIMATE...Evans