447
FXUS61 KCTP 222325
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
725 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/...
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 rain reaching the ground off to our
west was near a KCLE to KZZV line in Ecent Ohio with weak echos
further east representing some light virga.

23Z temps across the CWA were mainly in a 5 degree range in the
mid to upper 60s, though locations near and to the west of KBFD
and over the Lower Susq Valley were hanging onto L70s where
late day sunshine was seen. These latter, warmer locations will
likely flip this evening/greatly tighten the temp spread under
better radiational for several hours before thicker, layered
clouds advect east over the region.

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 get into the western zones around or even a
little before sunrise.

&&

.SHORT TERM /MONDAY NIGHT/...
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. 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.

&&

.LONG TERM /TUESDAY THROUGH SUNDAY/...
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, have stuck close to NBM guidance which generally
paints SChc PoPs (~15-25%) across the western PA with some
enhancement of precipitation along the Laurels.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
MVFR ceilings continue at AOO/UNV/IPT this afternoon with VFR
prevailing at all other airfields. The bulk of model guidance
continues to suggest gradual improvement over the next two hours
with IPT expected to prevail by 19Z, with AOO/UNV trending VFR
closer to the 20Z timeframe. Given model output, coupled with
satellite observations, there is moderate (50-60%) confidence in
VFR conditions prevailing by 20Z, with high confidence (> 80%)
by 22Z. Once these conditions prevail, expect these conditions
to continue area wide through 03Z Mon w/ high (~70%) confidence.

Lower ceilings will gradually make way into the western
terminals after 04Z Monday with GLAMP model guidance suggesting
JST/AOO 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 UNV/IPT slightly
earlier; however, lower confidence on this solution so have
outlined scattered low-level clouds to outline this potential
after 03Z Monday.

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...Dangelo
LONG TERM...NPB
AVIATION...NPB