420
FXUS61 KCTP 141114
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
714 AM EDT Sun Jul 14 2024

.SYNOPSIS...
Another prolonged and significant heat wave is expected today
through Wednesday with max heat index values reaching between
100 and 110 degrees F over parts of south-central PA and the
Susquehanna Valley.

Isolated late afternoon or early evening thunderstorms are
expected to develop from the heat and humidity, however,
most places are expected to stay dry through tonight.

More organized, and potentially strong to severe thunderstorms
are expected each afternoon and evening, Monday through
Wednesday.

&&

.NEAR TERM /UNTIL 6 PM THIS EVENING/...
630 AM Update: a few minor modifications to the mid morning
period of the forecast to capture some highly isolated, but
borderline glaciated shower that formed over northern Centre
county and tracked ENE and nearly parallel with and along I-80
between snowshoe and elimsport in far southern Lycoming County.

The left exit region of a nwrly 300 mb jetlet carving its way
into a fairly sharp mid/upper level trough axis across central
PA was the impetus for this compact area of weak convection
from just south of KIPT to KUNV.

Rainfall amounts and duration will be very light and brief.

Previous...

Anomalously warm and muggy conditions with mostly clear skies
are in store through this morning clearing skies are expected
overnight with MinTs running in the lower 70s across the LSQ
with some potential for lower 60s across the Laurels and N PA.

There remains a likelihood of some radiational/valley fog early
today, especially in areas that received rainfall late Sat
afternoon/evening with model soundings outlining some low-
level moisture overnight.

Lows at sunrise will vary from near 60F throughout the perennial
cold spots across Northern PA and Somerset County in the
southwest.

A weak, stationary frontal boundary extending from the Poconos
to the Lower Susq Valley early today, separated low 70 dewpoint
air from mid and upper 60 dewpoints across the Scent Mtns and
Middle Susq Valley.

Surface ridge with dry mid-level air across central PA will
allow for a very warm to hot day today, with heat index values
across the LSQ pushing towards and upwards of 100F and a Heat
Advisory (see NPWCTP) has been issued.

The biggest uncertainty with regards to heat index values will
be PWAT values and SFC dewpoints, the former of which will be
lowest this afternoon (0.9 to 1.0 inch) in a narrow band from
the Ncent Mtns to the Laurels, while the far SE and NW corners
of the CWA will see PWATs AOA 1.4 inches or nearly +1 sigma.

If the drier air is able to mix down from the top of the
boundary layer (5000-6000 KFT AGL today), heat index values
will have some potential to under-perform this afternoon.

Highs this afternoon will vary from the mid and upper 80s across
the higher terrain of the North and West, to the low and mid 90s
over Scent PA and the Lower Susq Valley.

PWAT values will increase late today/this evening ahead of weak
shortwave which will provide another opportunity for scattered
convection.

At this time, HREF guidance and its individual members
indicates the best moisture and lift for convection will be
focused near and just to the east of the stalled out boundary
over the Lower Susq Region beginning during the early to mid
afternoon, with the second area later this afternoon into early
tonight ahead of an approaching/albeit rather flat mid-upper
level shortwave where diffluence aloft and deep
layer/unidirectional westerly shear will be somewhat better.
POPS in the SE will peak around 20 percent this afternoon, and
30-40 percent across the NW Mtns this evening.

The Allegheny Front and Central Ridge and Valley Region should
see the lowest POPS of 13 percent or less during the daylight
hours.

A slightly better westerly breeze of 5-10 kts with some gusts
into the low-mid teens will boost temps by 2-3 deg F today, but
the breeze may help make it feel the same or slightly cooler
than Saturday via evaporation of perspiration off the skin.

&&

.SHORT TERM /6 PM THIS EVENING THROUGH MONDAY NIGHT/...
Cloud cover across N/W PA from weakening convection will
provide fairly uniform MinTs tonight into Monday morning,
ranging from the mid- to- upper 60s to the lower 70s across the
LSQ.

Confidence in excessive heat risk is higher on Monday, and has
lead to the issuance of a Heat Advisory (see NPWCTP) for the
south-central Alleghenies and Middle Susquehanna Valley on
Monday. The interaction of a modest shortwave trough, coupled
with a very warm and humid airmass is currently expected to
trigger thunderstorms Monday afternoon into the evening hours
with a slightly higher severe threat than over the weekend with
shear being be the biggest limiting factor.

Collaborated with LWX to maintain the Excessive Heat Watch
(through a few more model cycles) for Monday afternoon into
Tuesday for the lower Susq given some model uncertainty in SFC
dewpoints and precip amounts/impacts from afternoon convection.

SPC has placed all of the CWA within a MRGL risk of Severe TSRA
Monday as stronger mid and upper diffluence and the approach/presence
of a better defined left exit region of the jet will lead to
enhanced larger scale lift to help break any weak cap. The Susq
Valley and points east, along with sou swrn zones may see the
best chance of SHRA/TSRA though on Monday.

Precipitation becomes more limited to NW PA overnight Monday
and into Tuesday morning. The more important sensible weather
overnight into Tuesday will be the lack of reprieve from warm
temperatures with MinTs ranging in the upper 60s (N PA) to the
mid-to-lower 70s (LSQ) with muggy conditions.

&&

.LONG TERM /TUESDAY THROUGH SATURDAY/...
A hot and humid airmass will be across the area on Tuesday.
Held onto current high temperatures for Tuesday, given that
the thickness is quite high. Some concern for any days up
to Wednesday could see showers and storms, given that weak
shortwaves track just to our north, around a rather large
500 mb upper level low for mid July. Main chance for storms
would be from about route 6 northward.

For later Wednesday, a cold front moves across central
Pennsylvania. Expect widespread showers and storms across
the area, as the cold front drops southeast across the region.

Wavy front just to the south and east of the Keystone state
on Thursday could still result in a shower or storm, mainly
across the southeast.

After this, high pressure builds into the area, resulting in
less humid weather, with mainly dry conditions.

Still some hints that slightly higher dewpoint air could work
back into the area by late in the weekend, resulting in a small
chance of showers.

More information below on the heat for Tuesday.

Heat pattern ramps up and peaks early next week 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 100-110F. High confidence in
excessive heat risk lead to the issuance of a heat advisory for
the south central Alleghenies and middle Susquehanna Valley on
Tue and xheat watch for the lower Susq where max heat index
could reach 105-110F.

Heat index values will likely push 100F or advy criteria over
the Lower Susq Valley Wednesday afternoon.

&&

.AVIATION /11Z SUNDAY THROUGH THURSDAY/...
Valley fog was observed across much of northern PA as the sun
came up this morning (and a very low cloud deck at IPT). There
is also a bit of light fog/haze across portions of southern PA
(including AOO and LNS).

Any fog will quickly burn off this morning, leaving
predominantly VFR conds today. A few pop-up SHRA/TSRA are
possible this later today, focused on NW PA and the Lower Susq
Valley. Kept the mention of VCSH in the TAFs for BFD, MDT and
LNS.

Outlook...

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

Wed...More widespread SHRA/TSRA, with brief restrictions likely.

Thu...Lingering -SHRA diminishing.

&&

.CLIMATE...
RECORD TEMPERATURE - A record high temperature of 86 degrees
was set today at Bradford, PA. This ties the old record which
was set back in 1979.

The duration of warm nights is noteworthy -- Harrisburg has not
dropped below 70 degrees since July 3rd. Here are the longest
stretches with temperatures at or above 70 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
     .................................................

The minimum temp dipped to 68F yesterday morning (7/12) and
therefore ends the streak at 8 consecutive days. Tonight will
start another run of lows >=70F which should last until next
Thursday night (7/18 or morning of 7/19).

&&

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

&&

$$

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