Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 280904
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
504 AM EDT Wed Jul 28 2021

.SYNOPSIS...
Isolated strong storms are possible this afternoon across the
southwest portion of central PA and may produce locally
damaging wind gusts. The risk of severe storms has increased on
Thursday with a wind damage and isolated tornado threat possible.
A northwest breeze will bring cooler and less humid air into
the area on Friday which will set the stage for a beautiful
start to the weekend.

&&

.NEAR TERM /THROUGH TONIGHT/...
A weak cold front sinking south from the NY/PA border early this
morning is progged to stall out over southern PA near the MD
line this afternoon. Strong heating of relatively moist/high PW
airmass will result isolated t-storm development during the
afternoon/evening in vicinity of the NW-SE oriented frontal
zone. The 00Z HREF shows an axis of mean SBCAPE >1k j/kg over
the southwest 1/3 of the CWA where SPC has maintained a MRGL
risk; however weak/nebulous large scale forcing and <30kts of
deep shear suggests the overall threat of severe storms is low.
Steep low level lapse rates and high PW values support an
isolated strong to locally damaging wind gusts through early
evening. Brief heavy downpours are also possible in a few
locations across the southern tier of CPA with localized spot
amounts of 2+" possible based on the HREF max 6hr QPF.

Model RH profiles support partly to mostly sunny skies with
maxTs in the upper 70s in the northern mtns to upper 80s in the
south-central valleys. Highs will be about 4 to 8 degrees cooler
than yesterday across the northeast 1/2 of the CWA.

Hires models hint at showers lingering and even expanding a bit
in coverage over east-central PA late tonight into early
Thursday morning. This guidance also shows MCS tracking
southeast from MN/WI across lower MI and reaching Lake Erie by
12Z Thu. The NBM seemed to be handling this evolution reasonably
well utilizing a blend of the HRRR and other hires models. POPs
trend higher across the NW and eastern zones toward daybreak.
Fcst minTs tonight are very similar to this morning ranging from
around 60F in the northern tier to ~70F in the southeast metros.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
Better model agreement on the evolution of upstream convection
combined with new model data has resulted in an increased risk
of severe storms on Thursday; SPC has introduced a SLGT risk SWO
focused over south-central PA highlighted by a 5% tornado prob.

A belt of seasonably strong northwesterly mid-level flow will
be in place from the Upper Midwest through the Mid-Atlantic and
Northeast states on Thursday - in association with broad
cyclonic flow aloft beneath anomalous mean upper level trough
in eastern Canada. A sfc low is fcst to track northeast through
the lower Great Lakes and drag a cold front through the OH
Valley and Central Appalachians. T-storms are likely to develop
in moist/unstable airmass and move southeastward during the
afternoon and early-mid evening.

Concerning the severe potential, 0-6km shear values in the
40-50kt range would be supportive for supercells with an
isolated tornado threat (hence the 5% tornado probs from SPC)
although potential will depend on storm mode. If a linear mode
ends up being dominant, then the wind-damage threat could be the
greater of the two threats. We will begin messaging these risks
with a briefing later this morning.

PWs > 1.5 inches also favors potential for locally heavy
downpours. While the progressive nature of storms will likely be
a limiting factor, much of the area has observed well above
normal precip over the past few weeks making soils more
susceptible to flooding. WPC has maintained a MRGL (low) risk of
rapid-onset flooding for much of the area.

Improving conditions are expected in the wake of the cold FROPA
Thursday night with much drier air poised to arrive on Friday
via refreshing NW breeze. Look for a nice end to the week with
partly sunny skies and low humidity. The dry/low PW airmass will
set the stage for a rather cool Friday night/AM Saturday with
minTs ranging from the mid 40s over the NW Alleghenies to the
mid 50s across the southeast valleys.

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
The long term forecast features generally nice weather thanks
to an upper level trough over the northeast much of the time.

Saturday will feature pleasant temperatures with highs in the
70s to lower 80s and lows in the 50s for most. Surface high
pressure should result in dry weather and keep rain away to our
south with a stationary front over southern VA and KY.

A weakening cold front may bring clouds and a few showers or a
thunderstorm on Sunday, especially to northern PA. Drier and
slightly cooler air will again move in behind that front setting
up a comfortable and generally dry stretch of days early next
week.

&&

.AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...
Overnight fog likely at BFD and along the PA Northern Tier.
Elsewhere VFR will prevail through the night.

Showers and storms will be possible Wednesday afternoon mainly
along Southern PA. MVFR restrictions will be possible in TSRA.

Outlook...

Thu...Isolated/scattered PM TSRA impacts possible.

Fri & Sat...No sig wx expected.

Sun...Chance of a shower or thunderstorm.

&&

.CLIMATE...
It`s been a wet July in some parts of central PA. Through July
27th, the monthly precipitation at Harrisburg is 7.99 inches
which currently ranks 2021 as the 9th wettest July on record.

&&

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

&&

$$
SYNOPSIS...Steinbugl
NEAR TERM...Steinbugl
SHORT TERM...Steinbugl
LONG TERM...Ross/Steinbugl/Travis
AVIATION...Wagner
CLIMATE...Steinbugl


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