Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 210930
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
530 AM EDT Mon Aug 21 2017

.SYNOPSIS...
High pressure moving off the Mid Atlantic coast will bring a
warm and increasingly humid southerly flow to the region into
Tuesday. A strong cold front will move through the region late
Tuesday and Tuesday night. Cooler and drier conditions will
return for the second half of the week.

&&

.NEAR TERM /THROUGH TONIGHT/...
Radar shows scattered showers and thunderstorms over Lake Erie
moving into NWRN PA and SWRN NY. The activity is on the leading
edge of some modest instability associated with a weak mid
level shortwave. The HRRR weakens this activity by shortly after
sunrise as it moves away from the instability.

High pressure will slide off the coast and a warm and more humid
airmass will return to the region. The shortwave will drift
across the area this afternoon providing weak large scale
ascent. Models agree in developing a fair amount of CAPE and
eastern areas are also expected to see some moderate deep layer
shear develop as well. SPC has outlooked our eastern zones with
a Marginal Risk which looks entirely reasonable. Convection
will tend to decrease quickly after sunset with a warm and muggy
night in store ahead of an approaching cold front.

The amount of cloudiness today could hamper eclipse viewing as
clouds and showers begin to pop up this afternoon. As it stands
now, it looks like my SWRN zones will enjoy the least amount of
cloudcover.

&&

.SHORT TERM /TUESDAY/...
The big weather maker still looks to be a potent late summer
cold front that will pass through the region Tuesday evening
into early Wednesday.

Consensus model timing shows the front just entering NWRN PA by
8PM Tuesday/00Z Wednesday. Models develop just some modest CAPE
east of the front over much of central and western PA. The
exception is over eastern PA in what appears to be a lee side
surface trough feature. The ECMWF/GFS and the ensembles all
develop a finger of moderate to strong instability over SERN
areas along the coastal plain. Deep level shear is also progged
to increase and exceed 35kt.

Interesting to note that the GEFS shows very little unstable
air from the cold front into central PA and the SREF which
paints more CAPE overall, has a distinct minimum of instability
over our area during the day into the evening. This suggests
there could be a fair amount of pre-frontal cloudiness hindering
heating. The whole region is painted under a Slight Risk which
seems based more on expected impressive wind fields than model
generated instability.

Highs are still expected to rise into the 80s, to around 90
over southern areas before the clouds increase ahead of the
front. A very humid airmass will make it feel warmer.

&&

.LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/...
After our surge of heat and humidity, the strong front will
bring a noticeably cooler and drier airmass to the region
starting midweek and lasting through the end of the week into
the weekend. Interesting to note that the deterministic models
show 850 temps as cold as 2-3C just north of the Canadian
border behind the cold front, a reminder the change in seasons
is not all that far away. Chilliest readings look to be the
mornings of Fri/Sat/Sun with high centered overhead, dipping
lows into the 40s over the Northern Mountains (and deeper
valleys of the Central Mountains) and solidly into the 50s
elsewhere.

&&

.AVIATION /10Z MONDAY THROUGH FRIDAY/...
Minor adjustments to 09Z TAFS.

Less weight on fog now, given clouds moving in.

Some storms near Lake Erie now. Mid level wind fields
may support some stong storms today.

Yesterday I was thinking today would be dry. However,
strong warm advection may result in some showers and
storms late.

Earlier discussion below.

03Z dewpoint depressions indicating patchy fog development will
begin between 04Z-06Z. However, increasing clouds late tonight
will complicate the forecast and could limit fog formation.
Climatology suggests there is very little chance of fog at KJST
and KMDT. However, 03Z observations combined with latest
downscaled NAM output suggest a period of IFR visibility is
possible elsewhere overnight.

Any early fog should burn off between 13Z-14Z, then the focus
will shift toward the possibility of scattered thunderstorms
Monday afternoon, as an upper level disturbance passes through.
Despite predominantly VFR conditions, a brief reduction is
possible from an afternoon or evening thunderstorm. A blend of
the latest convection-allowing models suggest the best chance
will be across the eastern airfields, including KIPT,KMDT and
KLNS, between 18Z-22Z.

.OUTLOOK...

Tue...Patchy AM fog poss. Late day/evening tsra impacts poss.

Wed...Early AM low cigs poss BFD/JST.

Thu-Fri...No sig wx expected.

&&

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

&&

$$
SYNOPSIS...La Corte
NEAR TERM...La Corte
SHORT TERM...La Corte
LONG TERM...La Corte
AVIATION...Martin



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