897
FXUS61 KCTP 130733
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
333 AM EDT Sat Jul 13 2024

.SYNOPSIS...
A stalled front along the East Coast will bring a beneficial
rain to Southeast Pennsylvania tonight into early Saturday. A
large Bermuda High will then result in building heat this
weekend into early next week. A cold front is likely to push
through late Wednesday or early Thursday of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
The main feature of interest tonight will be the stalled frontal
boundary along the East Coast. A few light showers are falling
in the vicinity of this front across York and Lancaster counties as
of 02Z. However, upstream radar imagery shows some heavier
showers/tsra over Northern VA associated with a northward
drifting mid level vort max. Model guidance indicates the bulk
of the large scale forcing with this feature will pass just east
of our forecast area. However, some heavier showers appear
possible early Sat morning across Lancaster County. Ensemble
plumes indicate most likely rainfall over Southeast Lancaster
County of around a half inch early Saturday, but convection-
allowing models indicate there is at least some potential of
localized 1-2 inch amounts.

For most of Central PA, dry mid level air and a
cooling/stabilizing boundary layer should result in dry weather
tonight. However, can`t completely rule out an isolated evening
shower along the spine of the Alleghenies, where building cu
and low level convergence is noted at 23Z. Mostly clear skies
and a calm wind should once again lead to late night valley fog
across the central and western part of the forecast area, as
the latest SREF and NAMNest suggest. Also, models indicate a
light SSE flow a couple KFT AGL and associated moisture
advection could result in broken stratus forming late tonight
east of the Allegheny Divide. See no reason to deviate
significantly from NBM min temps, which range from around 60F in
the coolest valleys of the NW Mtns, to around 70F over the
mainly cloudy Lower Susq Valley.

&&

.SHORT TERM /SUNDAY THROUGH TUESDAY NIGHT/...
Showers are likely to exit the southeast corner of the forecast
area around 12Z Sat, as the mid level vort max and plume of
highest pwats shift east. Rising heights and dry mid levels
argue for dry weather over the vast majority of Central PA Saturday,
as the 12Z HREF suggests. However, passage of a weak surface
trough and lack of a significant cap in model soundings opens
the possibility of an isolated PM shower/tsra (<10pct POP).

Model RH profiles support a mostly sunny Saturday after any low
clouds/fog mix out. Mixing down 850mb temps of around 18C
translates to expected highs ranging from the mid 80s over the
higher elevations of the Alleghenies, to the low 90s in the
valleys of Southern PA. Fair and mild weather looks likely Saturday
night, as surface high pressure builds over the state.

Surface ridging and dry mid level air over Central PA should
result in fair and hot weather for most of the area Sunday.
However, surging low level moisture ahead of a weak shortwave
over the Grt Lks could potentially spawn convection over the NW
Mtns toward evening. 850mb temps near 20C support highs in the
upper 80s to mid 90s Sunday. However, mixing of drier air aloft
should result in falling dewpoints during the afternoon, likely
keeping max heat indices just below advisory criteria of 100F
over the Lower Susq.

&&

.LONG TERM /WEDNESDAY THROUGH FRIDAY/...
A large high pressure system near Bermuda will pump increasingly
hot and humid air into PA on a southwest flow early next week.
The passage of a weak shortwave Monday should result in
scattered PM convection. Medium range guidance indicates upper
level ridging over PA Tuesday, which will likely suppress
convection over most of the region and progged 850mb temps
rising to around 22C supports highs peaking between 90-100F
Tuesday. The latest GEFS and EPS both track a cold front into
the region Wed PM, likely accompanied by a round of
showers/tsra, some of which could be severe. Confidence is
increasing for heat indices above 100F early next week,
especially Tuesday over the Lower Susq Valley where values
>105F are possible.

High pressure building in from the Grt Lks will likely bring a
return to fair and seasonable weather late next week, with much
lower humidity. However, there is some model spread regarding
how far south the dying cold front gets, so can`t completely
rule out convection lingering over the Southern tier counties,
especially on Thursday.

&&

.AVIATION /08Z SATURDAY THROUGH WEDNESDAY/...
Predominantly VFR conds were found across central PA at 06z,
outside of LNS which is seeing a brief downpour temporarily drop
conds to IFR.

Low clouds (MVFR to possibly IFR) have formed over the Lower
Susq Valley, impacting LNS and MDT. The question is...how far
to the northwest will the low clouds be able to build in the
pre-dawn hours? Nighttime microphysics satellite imagery shows
the low clouds making a slow but steady march northwestward. We
will have to see if they can make it into AOO, UNV and IPT.

Farther to the northwest, we will see patchy valley fog
development. BFD has already begun to see it`s vsby bounce
around as the fog begins to form.

Any fog should quickly dissipate after sunrise. The low clouds
over the Lower Susq Valley should gradually lift as well, with
conds becoming VFR areawide by afternoon.

Outlook...

Sun...A late-day SHRA/TSRA is possible. Otherwise,
predominantly VFR.

Mon-Wed...Scattered SHRA/TSRA possible areawide.

&&

.CLIMATE...
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-??      2024
     .................................................

Low temps are forecast to be >=70 for at least the next 5-6
days which should put this streak in the top 3 warmest runs on
record at Harrisburg.

&&

.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 Sunday 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...Steinbugl
NEAR TERM...Fitzgerald/Steinbugl
SHORT TERM...Fitzgerald/Steinbugl
LONG TERM...Fitzgerald
AVIATION...Evanego
CLIMATE...Steinbugl/Banghoff