-- Discussion containing changed information from previous version are highlighted. --
000
FXUS61 KCTP 190254
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
1054 PM EDT Thu Apr 18 2024

.SYNOPSIS...
-A few periods of rain showers precede and accompany a weak
 cold front that will cross the region Friday afternoon and
 evening.
-Cooler conditions for the weekend with a gusty west breeze on
 Saturday.
-Frost/freeze risk for Sunday & Monday mornings.

&&

.NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
North-south oriented occluded front is stuck over the
Susquehanna Valley and will likely retreat westward by about 100
NM overnight as vertical mixing wanes west of it and boundary
layer decoupling/and the llvl east/west P-gradient enhances the
near sfc easterly flow. The persistent stratus deck based AOB
1000 ft agl will show a steady WWD push and bank itself up
against the Alleghenies (possibly as far west as the RT 219
corridor) late tonight through the mid morning hours Friday.

Lows will range from around 40F on the ridgetops in Schuylkill
& Sullivan County to 50F over southwest Somerset County. Showers
associated with an approaching cold front could reach the
extreme western part of the CWA by daybreak Friday, but most
likely hold off until after 12Z.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY/...
A few bands of light to briefly moderate rain showers will
precede and accompany a weak cold front as it drifts east across
the CWA Friday afternoon and evening.

Persistent southeast flow should keep clouds firmly in place
during the day on Friday. As such, expecting the threat for
thunderstorms to remain low across nearly the entire area. The
latest HREF paints a narrow corridor of 500J/kg getting into the
Laurel Highlands coincident with the cold front sweeping
through, which is probably the only spot where thunder is
possible.

PWATs ahead of the front will approach 1" across the southern
tier, but the lack of instability supports keeping QPF amounts
relatively low. QPF exceeding 0.25" will be confined to the
Laurel Highlands with <0.10" expected south and east of the
I-99/I-80 corridor. Highs will range from the mid 50s to low
60s.

As high pressure builds in on Friday night, conditions will
gradually improve with sharply falling sfc dewpoints/PWATs.
Current high-resolution guidance indicates the cold front could
be slow to clear the Lower Susquehanna Valley Friday night, so
have kept PoPs in the forecast a bit longer than guidance.

Morning lows will drop into the upper 30s across the northwest
mountains as westerly winds prevent a farther fall. Farther
southeast, cloud cover will support lows closer to 50F. Cooler
and drier air will arrive in CPA behind the cold front to start
the weekend, directed by a west wind gusting 25-35 mph. A
shortwave trough crossing the eastern Great Lakes may touch of
an isolated shower over north central PA late Sat morning into
the afternoon. Sfc ridging will ensure dry weather Saturday
night as focus shifts to a risk for frost and freeze.

&&

.LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/...
The setup for frost Saturday night is uncertain, depending on
the timing and coverage of low clouds that may develop in a thin
layer of upslope moisture beneath the low level inversion. If
more clouds develop earlier in the night, Saturday night could
be more of an advective marginal cold freeze setup (vs. more
favorable radiational/decoupling frost pattern.) But for now,
models suggest there could be at least a few hours of mostly
clear skies and light winds before the clouds develop, during
which frost can form. Low temps are fcst in the 30-35F range
over the northwest half of the CWA which could impact counties
activating the growing season April 21 along the US220/I99/I80
corridor.

The bulk of model guidance supports dry conditions Sunday
through through Tuesday morning as high pressure builds into
the area. Upper-level troughing does support below-average
temperatures for the majority of the period.

The combination of light winds, dry air, and mostly clear skies
associated with the surface ridge is likely to result in more
widespread frost Sunday night into Monday morning. The greatest
risk of frost is over the northern mountains where the growing
season is not active, but areas in the growing season further
south could also be affected.

Temperatures are expected to warm Monday into Tuesday with the
surface high pressure shifting eastward off the east coast and
an approaching shortwave bringing the next chance of showers.
Ensemble plumes indicate the best chance for precipitation will
come Tuesday PM, with scattered showers lingering into
Wednesday. Another shot of cold air will follow for Wed night
into Thu morning, with temperatures taking another dip below
the freezing mark across the northern tier.

&&

.AVIATION /03Z FRIDAY THROUGH TUESDAY/...
-- Changed Discussion --
A moistening southeast flow will cause stratus over Eastern PA
to expand westward overnight. Latest model RH time sections
and ensemble prob charts support predominantly MVFR cigs across
the lower elevations of the Susq Valley tonight. Further west,
expect a fairly rapid transition from VFR to IFR cigs between
06Z-10Z, as the increasing moisture is forced to ascend the
higher terrain of the Appalachians. Upon examination of the
latest model soundings have removed earlier mention of LLWS for
the overnight. However, a brief period of LLWS does look
possible over the NW Mtns (KBFD) between 12Z-16Z associated with
increasing winds aloft ahead of a cold front approaching from
the Grt Lks.

Very little improvement is expected Friday, as a moist
southeast flow off of the Atlantic remains in place. Diurnal
heating is likely to result in a very modest increase in cigs,
with VFR possible across the Lower Susq Valley (MDT/LNS) by
afternoon but borderline IFR/MVFR cigs likely holding on along
the spine of the Appalachians. In addition to the low cigs, a
few showers will move into the region during the late AM and
afternoon hours in advance of the approaching front.

Outlook...

Sat...Pre-dawn low cigs/showers possible Eastern PA.

Sun-Mon...No sig wx expected.

Tue...Slight chance of evening showers/cig reductions NW Mtns.
-- End Changed Discussion --

&&

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

&&

$$

SYNOPSIS...Lambert/Banghoff
NEAR TERM...Lambert/Banghoff
SHORT TERM...Lambert/Banghoff
LONG TERM...Fitzgerald/Colbert/NPB
AVIATION...Fitzgerald