Napierville, Pennsylvania Weather Forecast Discussion

197
FXUS61 KCTP 280734
AFDCTP
Area Forecast Discussion
National Weather Service State College PA
334 AM EDT Sun Sep 28 2025
.SYNOPSIS...
* Fog developing, especially in the NW and S.
* Mostly sunny and very warm to close out the final Sunday of
  September; start of another extended dry spell.
* Mild/warm start to the week before turning much cooler with
  stretch of dry weather continuing into October.
&&
.NEAR TERM /UNTIL 6 PM THIS EVENING/...
330 AM update.
Fog has become a bit more widespread, so issued a dense fog
advisory for areas that have some dense fog and have the best
chance to not have clouds. The far south is stil showing some
wind and a few clouds. The far north has not fogged in as much,
a bit of a breeze at times in these spots.
Earlier discussion below.
Still some clouds to the east and south, but much of the area
is clear, with some fog. May issue a SPS here, if not an
advisory, will continue to look at trends. Most areas had little
if any rain on Saturday, except for Somerset County. At the
same time, the area not dry now and nights are longer.
Once any fog burns off this morning, the day will feature warm
temperatures and plenty of sunshine, a nice day for the last
weekend in September.
Earlier discussion below.
Hires data shows rain exiting southeast PA by 12Z Sunday as
high pressure starts to move in from the Great Lakes. Clearing
from west to east along with residual moisture and light/calm
wind will promote fog formation which may become locally dense
and significantly reduce the visibility during the predawn/post
sunrise period.
Max temps fcst in the 75-85F range are 10 to 20 degrees above
the historical average for late September.
&&
.SHORT TERM /6 PM THIS EVENING THROUGH TUESDAY NIGHT/...
High pressure settling over CPA Sunday night will favor more
radiational fog into early Monday morning.
Monday and Tuesday also will be quite warm by end of September
standards with fcst highs very close to/a few degrees cooler vs.
Sunday. High pressure continues to dominate the pattern, setting
up what should be an extended period of dry weather for the
week.
&&
.LONG TERM /WEDNESDAY THROUGH SATURDAY/...
00Z guidance support an extended period of dry wx into early
October, as the complex area of weather over the southeast not
as likely to back northwest toward the coast now.
The main issue will be the potential for patchy frost across
the north later in the week, especially Wednesday night. This
based on a backdoor cold front dropping southward from eastern
Canada, with shallow dry air working in from the northeast.
This is normally how we can get cold, as lakes will not modify
the airmass, as with the case with northerly or northwesterly
flow. Got a small area of frost in the northern tier for
Wednesday, Thursday, and Friday nights.
More information below.
October will start with 1030+mb Canadian high moves southeast
from Hudson/James Bay into New England. This airmass change will
drop daytime max temps by 5-10F day/day Wed & Thu and increase
the risk of late night/early AM frost particularly in the
northern tier where NBM min temps are fcst below 40 degrees.
&&
.AVIATION /07Z SUNDAY THROUGH THURSDAY/...
Precipitation has moved out of the central PA airspace, and now
fog will be the main concern over the next eight hours. There
is a strong signal in much of the guidance for fog and low
stratus over most of the region. The best chance for IFR or
lower conditions will be at BFD, JST, AOO, and IPT, but all TAF
sites will have the potential to see fog and/or low stratus
prior to sunrise. Confidence is increasing on multiple hours of
1/4 mile visibility and dense fog for sites across southwestern
Pennsylvania. Several sites are already reporting LIFR
conditions as of the beginning of the 06Z TAF package. Nighttime
microphysics on GOES satellite imagery shows rapidly developing
fog in the valleys, and this fog will likely become widespread
in the next few hours.
The fog and low clouds will dissipate during the morning as
high pressure begins to build into the area, giving way to VFR
condition with light northwesterly winds. High pressure is
expected to be in place over the region for the foreseeable
future, and little to no precipitation chances are present for
the next five days.
Outlook...
Mon-Tue...Low confidence in sub-VFR cigs/rain across the
southern/eastern portions of the airspace. Mainly VFR across the
central and northern two thirds of PA.
Wed-Fri...No sig wx.
&&
.CTP WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 9 AM EDT this morning for PAZ010>012-
017>019-024>028-033>036-041-045-046-049>052-056-063-064.
&&
$$
SYNOPSIS...Dangelo/Martin
NEAR TERM...Dangelo/Martin
SHORT TERM...Martin/Tyburski
LONG TERM...Martin/Tyburski
AVIATION...Bauco/Bowen

NWS CTP Office Area Forecast Discussion