608
FXUS61 KCTP 210527
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
127 AM EDT Sat Sep 21 2024

.SYNOPSIS...
A ridge of high pressure through our region will make for
another pleasant afternoon and evening. An approaching warm
front on Saturday will bring scattered showers and
thunderstorms, a few of which may be severe. Drier weather
returns for Sunday, before an approaching trough brings the
next chance of rain from late Sunday night through midweek.

&&

.NEAR TERM /UNTIL 8 AM THIS MORNING/...
Tonight will be a relatively warm night as the beginning of
fall approaches with lows ranging from the mid 50s across the
northern mountains to the low 60s in the Lower Susquehanna.
Clouds will generally be on the increase overnight; however,
patchy valley fog is still possible mainly across the
Susquehanna Valley into the early morning hours on Saturday.
Surface winds tonight out of the south southeast will bring
increased strato cu into the eastern PA overnight while an
approaching shortwave to the west will allow for building
stratus over our western zones.

&&

.SHORT TERM /8 AM THIS MORNING THROUGH MONDAY NIGHT/...
A warm front approaching from the west will struggle to push
east into the cooler, more stable easterly flow over east
central PA through the day on Sat. Nevertheless, the approach of
the left exit region of a relatively strong upper level jet on
Saturday will combine with moderately high sfc dewpoints in the
low to mid 60s (with corresponding PWAT of around 1-1.25 inches)
to spark scattered showers/TSRA first through the Alleghenies
and northwest mountains Sat morning, then expanding eastward
into the afternoon. Given the progged 1000+ J/kg of SBCAPE and
NWrly 0-6 km shear near 40 kts, there is a MRGL risk for severe
weather, mainly in the form of localized damaging straight line
wind gusts and hail during the late afternoon/early evening
hours across west central PA. Although tornadoes are not
mentioned in the SPC outlook, given substantial veering of the
albeit weak winds in the lowest 1 km and LCLs less than 1000 m,
a brief, weak tornado can not be ruled out where there is
sufficient instability (Laurels and northwest mountains).

In general, forecast QPF is less than 0.50 inch, but localized
higher amounts are possible in thunderstorms, and amounts near
0 certainly possible for places that get missed given the
scattered nature of the convection. No flooding issues expected
given how dry it has been over the past month.

The timing of cloud cover in the morning hours Saturday will
play a significant role in the max temps and amount/eastward
extent of CAPE to fuel the convection in the afternoon and
evening. Early cloud cover (especially if the clouds move in
right around sunrise) will greatly flatten the temp curve and
subsequent intensity of convection that develops.

Model guidance indicates the shortwave and weak surface low
pass east of the region Sat night, then a cold air damming
scenario begins to develop between a stalled warm front along
the OH/PA border and high pressure over the Canadian Maritimes.
Forecast soundings indicate an upsloping southeast flow will
result in persistent stratus and perhaps a bit of fog or drizzle
over the Central Mtns early Sunday morning, with partly sunny
skies possible over Eastern PA. The cloud cover and flow off of
the Atlantic should result in markedly cooler afternoon temps
Sunday than on Saturday, with highs likely stuck in the 60s over
the Central Mtns.

A weak shortwave and associated plume of higher pwats
overrunning the warm front could produce scattered showers late
Sunday night into Monday.

&&

.LONG TERM /TUESDAY THROUGH FRIDAY/...
Upper trough energy and wavy frontal systems will be kicked
from the Plains/Midwest into the Great Lakes/Ohio Valley and
northern Mid Atlantic next week. An associated threat of
rain/showers will accompany these systems with some potential
for heavier rainfall within an amplifying mid/upper level
upstream flow pattern during the second half of next week.

Given model variability/growing uncertainty by later next week,
we did not identify any actionable targets of opportunity and
therefore weighted forecast basis heavily toward the National
Blend of Models (NBM). These data indicated a modest cooling
trend from the 9/21 weekend warmth trending closer to climo
(near or slightly below average) for daytime highs in the
60-70F range. Meanwhile, overnight low temps continue to run
above the historical average for late September in the 50-60F
range.

&&

.AVIATION /06Z SATURDAY THROUGH WEDNESDAY/...
Looking at just some mid level clouds overnight for the
most part. Some lower CIGS and fog possible across the
east for a short time, especially IPT.

A few showers and storms possible today, mainly across
the west, where the combination of instability and forcing
will be better. Left storms out for now, later shifts can
time them better, once something starts to form.

Earlier discussion below.

Meanwhile, a warm front transitioning to an occluded front
approaching from the west will trigger scattered SHRA and TSRA
across mainly west central PA, pushing eastward into the
evening.

As the frontal system moves east tonight, areas of fog and -DZ
will be possible across central and western PA into Sun morning.

Some guidance also hints at MVFR cigs sticking around for much
of the day on Sunday for portions of central PA in upslope SE
flow.

Outlook...

Mon-Wed...Scattered showers.

&&

.CLIMATE...
The first day of autumn 2024 (Fall Equinox) begins at 8:44 am
EDT on Sunday September 22nd.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

SYNOPSIS...Colbert/Bowen
NEAR TERM...Colbert/Bowen
SHORT TERM...Lambert/Colbert/Bowen
LONG TERM...Steinbugl
AVIATION...Martin/Gartner
CLIMATE...Steinbugl/NPB