-- Discussion containing changed information from previous version are highlighted. --
916
FXUS61 KCTP 230259
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
1059 PM EDT Sun Sep 22 2024

.SYNOPSIS...
Welcome to Autumn! An amplifying upper level trough and slow
moving frontal boundary headed our way from the midwestern
states will bring several periods of rainy conditions across
Central Pennsylvania beginning Monday morning and lasting
through at least Wednesday.

&&

.NEAR TERM /THROUGH MONDAY/...
-- Changed Discussion --
A slightly anomalous SSE low-level flow of moist air was
supporting a broad area of upslope stratus over much of the
Central mtn and Susq West Branch region between routes 219 and
Route 11/15. skies were mostly clear over much of the Laurel
Highlands and portion of the CWA east of the Susq Mainstem.

The leading edge of any light rain reaching the ground off to
our west was near KERI to KCAK and KZZV line. Weak radar returns
further east near the I-79 corridor represented some light
virga as a fairly deep 6-8 KFT thick layer still resided over
far Western PA.

02Z Monday temps across the CWA were mainly in a 4 degree range
between 61-65F with a few cooler readings in the upper 50s over
and to the east of the Mid Susq Mainstem and milder temps in
the upper 60s across Warren county.

Temps should again stay much warmer than normal tonight. Most
places will be at least 10F above normal. We`ll hold off for now
mentioning drizzle for the overnight, but there could be a
patch or two on the windward slopes of the ridges.

Similar to the last system, the approaching sfc frontal system
runs into the CAD in place over Central PA and stalls. Moisture
aloft and forcing keeps on flowing overhead, though and
rain/showers should spread across much of the Western half
of our CWA around or even a little before sunrise.

The plume of moisture overhead continues eastward and hangs over
the CWA most of the day on Monday. Heavy rain signal is not
there, but there might be a thunderstorm or two, more probable
in the SW than E. Temps on Monday won`t go more than 10F above
the morning numbers. Some places may not get much more than a 5F
deg diurnal swing.

Highs Monday will be mainly in the low to mid 60s in most
places, except for around 70F across the Lower Susq Valley.
-- End Changed Discussion --

&&

.SHORT TERM /MONDAY NIGHT/...
-- Changed Discussion --
The 5H ridge axis doesn`t go east of our longitude, so the
showers might not get to the far eastern reaches of the CWA
early Mon night before fizzling. Another minor surge of WAA and
moisture aloft arrives in the Laurels before sunrise Tuesday.
So, the chcs for rain increase once again. PWATs get around 1.5"
Mon aftn before dipping then rising again later at night. The
storm motion doesn`t seem slow enough to make a widespread risk
of flooding. Basin average QPF total is only 0.5-0.75" at
highest. But, any storms can drop that much in a short time with
PWATs in that range. Still, a big factor against any flooding
is the very dry antecedent conditions, esp in the Laurels.
-- End Changed Discussion --

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
-- Changed Discussion --
Model guidance outlines another surface low-pressure system
tracking northeastward across western portions of the Upper Ohio
Valley and into southern Ontario Tuesday into Wednesday, with
slightly more moisture and lift available as the warm front
aligns itself north-to-south across west-central PA. Given
slightly better lift with this low-pressure system coupled with
enhanced moisture, have retained likely PoPs in the Tuesday to
Wednesday timeframe. Given PWATs in the 1.25-1.75" range on
Wednesday, locally heavy rainfall will be possible with
relatively slower storm motion also possible. This threat has
been outlined with marginal (level 1/4) risks in the D3 (area
wide) and D4 (SE 1/2) Excessive Rainfall Outlooks from the
Weather Prediction Center. Highest impacts are expected to be
across areas that have recently received heavy rainfall, such as
urban areas of the Lower Susquehanna Valley, along with hydro
sensitive areas across the northern tier.

Model uncertainty in previous packages still remain late
Wednesday/early Thursday and into the end of the long-term
period, so there is low confidence as we move past Thursday
morning on this forecast cycle. A (potentially) slower
progression of the departing low- pressure system retains
chances through Thursday, with slightly lower chances as we move
into Friday.

The remnants of a tropical cyclone could also bring about rain
chances this weekend, but uncertainty with respect to
interactions with a trough that could limit chances of
precipitation/increase chances as we get closer to an event.

At this time range, have maintained numbers close to NBM
guidance which continues to paints SChc PoPs (~15-25%) or below
across the western PA with some enhancement of precipitation
along the Laurels.
-- End Changed Discussion --

&&

.AVIATION /03Z MONDAY THROUGH FRIDAY/...
High clouds are beginning to stream in from the west as an upper
trough pushes off the great lakes. This coupled with a building
low stratus deck will signal the lowering of cigs generally
throughout the evening.

Lower ceilings will gradually make way into the western
terminals at the beginning of this TAF package with GLAMP model
guidance suggesting JST/AOO/UNV will be the first terminals to
trend towards MVFR restrictions based on ceilings. There remains
some signal for low-level clouds to begin making way into
IPT/BFD slightly earlier; however, lower confidence on this
solution so have outlined scattered low-level clouds to outline
this potential after 03Z Monday. Many sites will likely see IFR
cigs before sunrise.

Closer to sunrise, SHRA is expected to begin entering W PA and
have outlined a general west-to-east onset of VCSH/SHRA from
08Z-11Z Monday. Relatively drier air at the lower-levels could
slow down timing of SHRA at MDT/LNS so have progged these
conditions closer to the 14-15Z Monday timeframe with slightly
lower confidence. With SHRA onset, borderline MVFR/IFR ceilings
start to become a concern with the HREF/GLAMP guidance
suggesting all airfields outside of MDT/LNS experiencing periods
of IFR thresholds. If IFR thresholds manage to get into the
airfield, generally expect these conditions to hold through the
end of the TAF period.

Outlook...

Tue-Wed...Chc/Likely SHRA with some TSRA possible.

Thu...Lingering/scattered showers possible area wide.

Fri...Scattered SHRA SW PA; otherwise, trending drier.

&&

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

&&

$$

SYNOPSIS...Lambert/Dangelo
NEAR TERM...Lambert/Dangelo
SHORT TERM...Lambert/Dangelo
LONG TERM...Lambert/NPB
AVIATION...NPB/Bowen