Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 171420

Area Forecast Discussion
National Weather Service State College PA
1020 AM EDT Thu Aug 17 2017

Warm and increasingly humid air will surge into our region
from the southwest this afternoon and tonight as a warm front
draped across the Ohio River Valley lifts across the state.
Expect showers and thunderstorms, some with locally heavy rain
and frequent lightning to become more frequent tonight into
early Friday morning. An approaching cold front will spark more
thunderstorms during the day Friday, especially across the
eastern half of the state where some may become strong to
severe. Cooler and drier should improve our weekend weather. A
ridge builds in early next week and we should warm up before
another northern stream cold front triggers showers again and
ushers in cooler drier air around mid-week.


The sky cover is the main tweak in this update, with much more
sun than anticipated. One patch of lower clouds sliding over
Adams Co should begin to get eaten up by the strong
sunshine/heating/mixing. The increase in high clouds is on the
horizon - literally. Increased temps through the day just a
hair as well due to the overall sunnier outlook. We could see
something pop off the higher hills in the SE/SC before anything
goes in the west since the SE is much more muggy. However, low
POPs for low coverage.

GOES-16 10.3-3.9um fog product shows dendritic valley fog taking
shape across NRN PA along with the Susq Valley. The fog will
become locally dense for several hours during the mid morning
as a result of a high pressure ridge right over Central PA
providing mainly clear skies, light wind, and quite cool temps
in the lower to mid 50s across the nrn mtns (where air/stream
temp/dwpt spreads are 20 to 25 deg F.

The fog will lift into a thin bkn-ovc stratus deck between
13-15Z before dissolving and partly to mostly sunny skies follow
for the midday and afternoon hours as a warm front lift north
into the region and some mid and high clouds from upstream TSRA
spill to the ESE.

Will maintain 05-07Z National Blend of Models POPS for late
today and this evening which are in the CHC to low Likely range
over the approx western half of the CWA. If anything, these
pops may be slightly overdone across the Central Mtns and Susq

Temps will be warmer than Wednesday with more
humidity...especially late in the day as the warm front
approaches from the south. Highs this afternoon will be mainly
in the 80s, though the northern and southwestern mountains could
hold in the U70s.


The chance of rain and thunderstorms increases this evening
across the SW half of the CWA, and overnight elsewhere as a
deep cyclone moves into the western Great Lakes. Several
distinct LLJ features will stream north across the CWA during
this period ahead of an approaching sfc cold front, and will
interact with the Warm Front and approx 2 inch PWATs to support
locally very heavy rain in training TSRA. Some of the TSRA will
contain frequent lightning and possibly some brief strong wind

Cape actually climbs slightly across southern and central PA
late today and through the first half of tonight. This increase
in instability to these levels (of several hundred to just over
1000 j/kg during the overnight hours only occurs about once a
year, if that here in Central and Ern PA, and is notorious for
waking many from a sound sleep during the late night/predawn
hours with frequent vivid lightning and booming thunder. with
PWAT of 2 inches (or more across the SW part of the CWA) leading
to extremely heavy hourly and three hourly rainfl rates across
the Central and SW part of our CWA, will have to closely monitor
for training TSRA. Hourly FFG is still around 1 inch or less
across parts of the Lower Susq Valley, while much of the rest of
the region has hourly values of 2 inches or more. Therefore,
unless we can foresee a specific target area comprised of both
time and duration of potentially repeating TSRA (via later
version of the HRRR or other CAMS/mesoscale models), then we
probably won`t be posting a FFA with a tremendous amount of lead
time in excess of 6-8 hours.

Due to high PW and widespread/thick clouds, it will stay warm
overnight with min temps only in the low to mid 60s across nrn
and wrn PA, and L70s in the southeast.

Sfc cold front pushes East to the Susq Valley or Central Mtns
between 20-22Z (The prime hours to fire up some locally strong
to severe TSRA.

A few discrete TSRA (with some transient supercells) are likely
during this period. 0-1KM EHIs climb to between 1-2m2/s2 across
parts of central and scentral PA between 02-06Z Friday.
Although updrafts may not be rooted right at the sfc, the
presence of the aforementioned LLJ S-SSW wind maxes, very high
pwat (Low LCLs), and the close presence of the warm front (which
will add a source for LLVL shear and helicity), will create a
threat for rotating updrafts/supercells and even a small chance
for a few weak tornadoes. Locally heavy rain, and frequent
lightning maybe the biggest threats.


The GEFS and CMCEFS suggest our PW values drop during the
midday hours Friday (west) and afternoon/early evening over the
east - back to normal as the cfront is forecast to push to our
east. This means improving weather from morning to afternoon.
Lowering POPS over time.

We should remain relatively dry as the PW values stay near
normal until perhaps Tuesday. This said there are hints of a
weak wave and scattered showers perhaps Sunday.

Tuesday could be rather warm as there is a surge of above
normal 850 hPa temperatures and PW values above normal. Not
surprisingly the GEFS shows higher POPS Tuesday afternoon into
Wednesday with the wave and front.

We should enter the second half of the weak in a cooler and
drier air mass as currently forecast. And thus in the grids.

Bottom line in this period the NCEP guidance suggests low
probability of significant QPF.


Fog is gone and most of the day will be dry. Dewpoints fcst to
come up late today into Friday. This will lead to some storms
with heavy rain and gusty winds - mainly later tonight and
during the daylight hours on Friday. Impacts to flight
operations will be from TSRA - and should be brief. However, fog
could be a problem Sat AM with rain Fri then clearing overnight.

Improving conditions for the weekend, as the cold front
moves east of the area and dry air moves in under high pressure.


Sat-Mon...No sig wx.




NEAR TERM...Dangelo/Lambert
SHORT TERM...Grumm/Lambert
LONG TERM...Grumm/Lambert
AVIATION...Dangelo/Martin is the U.S. government's official web portal to all federal, state and local government web resources and services.