Harrisburg, Pennsylvania 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

591
FXUS61 KCTP 140538
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
1238 AM EST Thu Nov 14 2024

.SYNOPSIS...
A weakening low pressure system will track south of
Pennsylvania late Thursday. Rain is likely throughout the day
Thursday with southwest PA seeing the highest possible rainfall
amounts near 0.5". Upper level ridging will build over the
region during the upcoming weekend.

&&

.NEAR TERM /UNTIL 7 AM THIS MORNING/...
High clouds continue to enter the region ahead of an
approaching low pressure system. Cloud cover will continue to
thicken overnight as moisture advection increases with the
arrival of a 40 knot low-level jet, but precipitation will
likely hold off until after sunrise with a very dry airmass in
place. Temperatures across eastern portions of the area are
already down into the 30s and will continue to fall into the
mid to upper 20s overnight with light winds expected.
Temperatures will stay slightly warmer overnight across the
west, where more cloud cover is anticipated.

&&

.SHORT TERM /7 AM THIS MORNING THROUGH 6 PM FRIDAY/...
Guidance continues to show a later arrival of rain on Thursday,
with the region likely remaining dry through most of the
morning. The 12Z HREF seems to capture this best and the current
forecast follows its timing. Rain likely moves into Somerset and
Cambria counties during the late morning/early afternoon and
spreads eastward through the afternoon.

The surface low will weaken significantly as it approaches
central PA and the parent shortwave will be passing well to our
south. This along with low PWATs suggest that rainfall will be
light and mainly confined to the western half of the region.
The highest rainfall totals are expected over the far southwest
corner of the area where up to half an inch may fall, while
locations farther to the north and east will likely see around a
tenth of an inch. Cold air damming/easterly flow associated
with high pressure over Eastern Quebec should result in max
temps Thursday a bit below NBM guidance. Expect the higher
elevations of the Alleghenies to struggle to reach 40F.

Any lingering light rain over Southern PA should taper off by
late Friday morning, as a secondary surface low develops off of
the east coast and large scale subsidence arrives behind the
exiting shortwave. However, a trailing inverted surface trough
and lingering low level moisture will likely result in stubborn
stratocu, especially over the Laurel Highlands due to an
upsloping northwest flow.

&&

.LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/...
11 PM Update keeps the tweak downward with the dewpoints on
Friday (mainly E) and Saturday, pulling Min RH down to 30pct in
the Susq Valley where comparitively lesser amounts of rain will
fall Thursday/night vs the western counties. Gusts over 20MPH
could mix down there on the downslope flow. Cold front looks
like it may arrive slightly earlier (Sun Night), and the dryness
will probably dry things up as they get into Central PA from the
Upper Lakes. Still enough support for 30-40 PoP in the NW. Big
trough/closed low forming over the central CONUS warrants PoPs
for Tues-Wed at this point. However, the system may slow as it
spins there, perhaps just far enough away to keep most of the
precip from getting too far into PA.

Prev...
Upper level ridging will provide mainly dry weather through the
weekend. A fairly tight pressure gradient across the eastern
half of the region between a developing surface low off of the
east coast and a high moving in from the west will lead to gusty
winds on Saturday, with gusts of 20 to 30 mph possible. High
temperatures will range from the low 50s across the northern and
western mountains to the low 60s in the southeast. Fire weather
concerns could also return for Saturday given dry northerly
surface flow under high pressure. Medium range guidance is
supportive of dewpoints lower than what NBM originally suggests.
Min RHs in eastern PA are currently forecast to be in the low
to mid 30s.

The trend of mild weather will continue through the long term
forecast period. Surface high pressure moves overhead on
Sunday, which will support continued tranquil conditions and
lighter winds. The decreased winds should limit wildfire spread
potential on Sunday. Increasing cloudiness is expected later
Sunday as a shortwave approaches the region.

The next chance for rainfall will come on Monday as the
shortwave passes to our north. Moisture is limited with this
system and any rainfall will be light. Ridging builds back in
for Tuesday as temperatures remain +10 to +15F compared to
average. By the middle of next week, most guidance shows
another system approaching from the west, but there is still
considerable spread in the ensembles as to how this system
evolves.

&&

.AVIATION /06Z THURSDAY THROUGH MONDAY/...
Not a lot of change for the 06Z TAF package. High clouds over
the region as of late evening. Current timing of the raIn
looks to be on track.

Earlier discussion below.

Wind is gusting over the western highlands (JST/BFD). Added
mentions of gusts back in, but also left the LLWS in the
forecast for the overnight there since it is very
notable/strong LLWS.

Prev...
Wind is pretty light and we are stratifying, so gusts overnight
will be negligible. A SErly low level jet will develop shortly
over the region, with the highest speeds based very low
(FL020-025) with the tight gradient starting at the top of the
boundary layer (FL010-012) thru the night and only rising
slightly thru the daytime on Thurs. The LLWS will impact JST in
the next 2-3 hrs, then BFD before midnight and AOO and UNV
shortly thereafter. JST is expected to have wind just above the
boundary layer of 40-45KT and BFD close to 40KT. These
conditions will last thru most if not all of the TAF valid
period (thru 00Z/15th). Have mentioned LLWS at IPT, as well. The
sfc wind will get stronger at the western terminals, but the SE
(MDT/LNS) will stay under 08KT all 24hrs. Thankfully, the LLJet
should not be as strong there, barely touching 20KTs. Have left
out mentions there for the time being.

The SE flow will bring in low clouds (MVFR at first) around or
just before dawn at JST and AOO before spreading farther north
and east during the day. MVFR ceilings are most probable between
12Z and 18Z, but there is a 40% chance of IFR ceilings at JST
during that time. IFR is then expected as rain moves in from the
W. The rain should reach JST around 16-17Z and keep things IFR
thru at least 00Z/15th. The rain will struggle to get into IPT,
but should occur at MDT and LNS very late in the day. Expect
further deterioration areawide with IFR conditions also becoming
likely (>=70%) at AOO, UNV, and BFD.

The storm system producing the rain will slide to our S Thursday
night and leave everyone dry on Friday. But, the low cigs will
last all day in the wrn terminals as NW flow kicks in. A patchy
shower or two could even linger VCTY JST on Fri and Fri night
as the NW flow picks up LH/LE moisture and pushes it uphill into
into the srn Laurels (mainly S of 2G9/Somerset). IPT/MDT/LNS
should improve to VFR Fri.

Outlook...

Sat...IFR cigs rising to MVFR cigs JST/BFD. MVFR cigs AOO/UNV.
VFR elsewhere.

Sun...IFR JST early. Otherwise, VFR.

Sun night-Mon...CFROPA. MVFR cigs/vis in -SHRA, mainly NW half.

&&

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

&&

$$

SYNOPSIS...Bowen
NEAR TERM...Bauco/Bowen
SHORT TERM...Bauco/Bowen
LONG TERM...Dangelo/Banghoff/Bauco
AVIATION...Dangelo/Martin