Text Products for AFD Issued by CTP

Area Forecast Discussion
Issued by NWS State College, PA
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FXUS61 KCTP 182309
AFDCTP

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

.SYNOPSIS...
-Partly sunny over Central and Western PA and cloudy east of the
 Susquehanna Valley late this afternoon and evening.
-Becoming cloudy overnight.
-A few periods of rain showers precedes and accompanies 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.

Min temps will be in the 40s and only span a range of about 5
degrees F across the CWA, though the range will be slightly
greater if we account for some cool spots across the higher
terrain of Schuylkill and Sullivan Counties.

The moist east to southeast upslope flow will create some patchy
drizzle and fog later tonight and early Friday, especially to
the east of the Susq Mainstem.

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. Lows will range from around
40F on the ridgetops in Schuylkill & Sullivan County to 50F over
southwest Somerset County.

&&

.SHORT TERM /6 AM FRIDAY MORNING THROUGH FRIDAY NIGHT/...
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.

PWs peak around 1" ahead of the front over far south central PA
where some mdt to briefly heavy downpours/embedded t-storm
cannot be ruled out.

Mean MU CAPE via the HREF may push 500 J/KG across a small
portion of Scent PA and the Laurel Highlands during the mid to
late afternoon hours Friday, which could support a few
relatively low-topped TSRA. Elsewhere, very minimal CAPE values
will negate TSRA formation and lead to lighter QPF of generally
around 0.10 of an inch.

Conditions gradually improve Friday night and early Saturday
with sharply falling sfc dewpoints/PWATs.

&&

.LONG TERM /SATURDAY THROUGH THURSDAY/...
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 frost and freeze risk.

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 /22Z THURSDAY THROUGH TUESDAY/...
A north-south quasi-stationary front lies across the Susq Valley
early this evening, with mostly clear skies to the west of it
and low cigs/stratus covering most locations east of the
boundary. Models indicate this boundary will shift west
overnight, resulting in deteriorating flying conditions over
Central PA, as a wind shift to the east draws increasing low
level moisture into the area.

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.

&&

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

&&

$$

SYNOPSIS...Lambert
NEAR TERM...Lambert
SHORT TERM...Lambert
LONG TERM...Fitzgerald/Colbert/NPB
AVIATION...Fitzgerald
National Weather Service Regional Headquarters
