Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 271459
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
Issued by National Weather Service BINGHAMTON NY
1059 AM EDT Thu Apr 27 2017

.SYNOPSIS...
A strong subtropical ridge over the western Atlantic will
dominate our weather through Monday. A front will weaken as it
moves into the ridge late today, triggering thunderstorms. Then
ridge will build westward over the weekend. We will be on the
warm, moist western side of the ridge most of the weekend. A
classic dirty warm up with warm moist air and potential diurnal
thunderstorms. Alas, a strong trough should bring cooler drier
weather to the region for the second part of next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
1030 am update...
GOES-16 red band visible imagery continues to show a stubborn
marine layer that was "stuck" in the ridge and valley region of
central PA up to the north central mountains and into east
central and northeast PA. In addition, the clouds were covering
a large portion of the lower Susquehanna valley. West of the
Allegheny front from the Northern Mountains to the Laurel
highlands, skies were pretty much clear under a cirrus layer.
The low-level flow will remain from the south to southeast
through the afternoon. This will keep this marine layer in
place. Insolation from the strong late April sun is expected to
break up this cloud layer but looking at the 1 minute satellite
data, this cloud layer looks very stable especially in central
PA. So we have backed way off on clearing in much of the county
warning area and lowered maximums in these areas as well. We may
have to adjust this even more with the 18z update. Rest of
forecast has not been changed as waiting for more 12Z guidance.
See previous discussion below.

Previous Discussion...
GOES-16 3.9-11.2um loop at 09Z showing stratus becoming more
extensive over eastern Pa, as boundary layer cools. It looks
like most of the region east of the Allegheny divide will begin
the day either cloudy or foggy. Model soundings/SREF prob charts
suggest patchy low clouds could linger until midday over the
eastern half of the state. Eventual clearing and very warm air
at 850mb should result in temps rising to near 80F this
afternoon.

Main forecast issue remains the potential of a few strong to
severe tstorms this evening associated with arrival of a
weakening cold front. All guidance timing convection into the
western counties between 21Z-00Z. Model soundings showing fairly
impressive shear, both 0-6km and 0-1km. However, only modest
low level moisture return ahead of front and thus marginal CAPE
values expected. SPC has placed roughly the western half of Pa
in a marginal risk this evening. Given the strong winds aloft
and borderline CAPE, expect isolated damaging wind gusts to be
the greatest threat, rather than large hail. Not sure how much
weight to place on 00Z NAEFS, which is indicating a high
probability of STP>1 across the north-central counties as front
comes through around 00Z, suggesting a brief tornado can`t be
ruled out.

&&

.SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/...
Diminishing frontal showers/storms should push through eastern
Pa after dark, as CAPE diminishes and bulk of large scale
forcing with parent shortwave lifts north of the region.

A dry and slightly cooler Friday is expected, as surface ridging
builds in behind cold front. 850 hPa temps remain above normal
across the Lower Susq Valley, where readings could again push
80F. Model RH profiles support a mostly sunny forecast for most
of the area Friday. Some increasing mid level cloudiness appears
likely across the northwest counties during the afternoon, as
WAA aloft spreads in ahead next shortwave working across the
Grt Lks.

&&

.LONG TERM /SATURDAY THROUGH WEDNESDAY/...
A weak shortwave passing across southern Ontario/Quebec is
progged to push a shallow cold front through Pa Saturday. Timing
of this front will favor diurnally-driven convection across the
southern counties, where pre-frontal heating could potentially
push temps into the mid 80s based on model 850mb temps near 15C.
A cold air damming scenario expected Sunday, as dying frontal
boundary slips south of the border and sfc high over New England
directs a cool southeast flow into central/eastern Pa. Have
nudged max temps several degrees below National blend guidance
Sunday. Much warmer/unstable airmass west of the mountains could
lead to PM tstorms across western Pa, potentially affecting our
western counties.

Max POPS during the extended period remain centered on
Monday/May 1st associated with passage of a potent cold front.
A severe weather threat may evolve downstream from the OH
Valley into the Mid Atlantic late Monday before temperatures
decline to near normal behind the cold front for next Tuesday
and Wednesday.

&&

.AVIATION /15Z THURSDAY THROUGH MONDAY/...
GOES-16 shows the low clouds than spread westward across PA
overnight. This feature is associated with a strong gradient in
the potential temperature field. This marine front is stuck sort
of north-south along the mountains. Mainly MVFR/IFR to east of
it and VFR to the west. This low cloud boundary should erode
west to east this AM restoring VFR to KUNV and KIPT before KMDT
and KLNS.

Winds a few thousand feet up are over 20kts. When we mix in the
15 to 18 UTC timeframe winds will get a bit gusty.

The frontal system to our west is in Ohio and KY. It is forecast
in most models to bring showers and thunderstorms to western
areas in the 20 to 22 UTC timeframe and later as one moves east.
Initially, there could be some strong storms with this front.
But they should weaken as they move into eastern areas mainly
after 0000 to 0200 UTC.

Any rain and low-level moisture will triggers some low clouds
and patchy fog. Most likely to the east, closer to the front and
higher moisture values. Some pretty moist air still stuck in SE
PA in the 0600 UTC timeframe Friday AM. Much drier to the west.
Mainly VFR behind the front in west MVFR/IFR in east especially
southeast.

Friday looks like shortly after sunrise the dry air should make
for a great flying day. VFR should dominate.

The chance of showers and rain increases Friday night into
Saturday.

Outlook...

Fri...No significant weather.

Sat-Sun...A few showers possible. Mainly dry with VFR
conditions for much of the time.

Mon...Breezy with showers and thunderstorms likely with FROPA.

&&

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

&&

$$
SYNOPSIS...
NEAR TERM...
SHORT TERM...
LONG TERM...
AVIATION...



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