Area Forecast Discussion
Issued by NWS State College, PA

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506
FXUS61 KCTP 271928
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
328 PM EDT Tue Sep 27 2016

.SYNOPSIS...
Dry conditions will prevail tonight. A storm system digging
southward across the Great Lakes tonight will bring an elevated
risk for heavy to excessive rainfall and flooding Wednesday night
into Thursday over south central PA. Unsettled weather will
continue into the weekend with periods of showers.

&&

.NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
Dry conditions prevail one more night before several days of
unsettled and quite soggy weather beginning late Wednesday.
Weak high pressure remains overhead tonight keeping skies mainly
clear for most with patchy river valley fog. No sig changes in
the near term as main wx focus is on potential heavy rain event
midweek.

In the pre dawn hours...winds will begin to shift to the southeast
signalling the beginning of deteriorating weather over the
commonwealth. Narrow ribbon of sub-0.5" PW will be sandwiched from
the west and east by deeper layer moisture advecting into the
region. Mins will range from the lower 40s north to the lower 50s
southeast.

&&

.SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
For Wednesday...southeast flow will bring increase in cloudiness
from southeast to northwest...with a chance of showers arriving
late Wednesday. The risk for heavy rain ramps up after 00z
Thursday. Highs will range from the upper 60s northwest to the
lower 70s southeast.

&&

.LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/...
*Increased risk of heavy to excessive rainfall and flooding
 Wednesday night into Thursday. Primary threat area is south
 central/east central Pennsylvania.

Latest guidance is narrowing in on the heavy rain potential for
Wednesday night and Thursday, with growing confidence of at least
a small shift southward for heaviest rain potential. Still looking
at high likelihood for a widespread 2 to 4 inch rainfall (with
the potential for higher amounts across the south). What is
certain is that a favorable cluster of parameters come together
and remain focused on central PA for a 24 to 36 hour period from
Wednesday evening through early Friday...bringing the greatest
heavy rain potential we`ve seen in several months to the region.

A pronounced convergence axis is progged to develop to the east
of an upper low that parks over the Ohio Valley, tapping into
deep moisture/high PW air off the Atlantic via anomalous east-
southeast low-level winds. The models also indicate some marginal
instability (a few hundred J/KG of CAPE) along the moisture
convergence axis, suggesting some embedded convective elements are
possible within the rain axis. The combination of strong forcing
aloft, impressive low level moisture convergence, magnitude of
850mb wind anomalies, terrain enhancement and slow overall
movement suggest an axis of heavy to excessive rainfall with
potential flooding is likely to evolve over south-central PA
Wednesday night into Thursday. The main question of course is
exactly where with model QPFs continue to differ somewhat at each
6hr interval. Therefore, generally followed the latest WPC QPF
which slightly shifts a broad area of 2-4" a bit further south
than previous runs (across mainly the south- central and
southeastern counties with localized amounts of 4-6" possible.

WPC continues the SLGT and MDT excessive rain risk for Wednesday
and Thursday. This is highlighted in the HWO and possible heavy
rain will be mentioned in grids/text products. Idea of a flood
watch was tossed around a bit, but event still 30+ hours out and
evolving. Most likely area would be southern tier/lower susq
unless later runs continue to shift heavy rain axis southward. The
other wildcard is the recent dryness, but given the favorable
synoptic set-up and magnitude of the anomalies the risk is
warranted.

The upper low is fcst to wobble back to the west/NW toward the OH
Valley on Friday before slowly moving to the ENE across the Lower
Great Lakes through the weekend. This will keep the pattern
unsettled with periods of showers - but we are not expecting the
heavy rain risk to persist beyond Thursday. Temps still holding
close to seasonal normals as we close out September and head into
October.

&&

.AVIATION /19Z TUESDAY THROUGH SUNDAY/...
Now that morning fog has burned off...VFR conds will prevail
through tonight.

Wednesday should be the last good VFR day before more showery and
cloudy conditions develop later in the week into the weekend with
widespread reduced conditions likely.

Outlook...

Wed...Showers/cig reductions possible late.

Thu-Sat...Sct showers/cig reductions possible, esp w mountains.

Sun...Reductions possible north with shra/cigs...fair elsewhere.

&&

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

&&

$$
Synopsis...DeVoir
Near Term...DeVoir
Short Term...DeVoir
Long Term...RXR/Steinbugl
Aviation...DeVoir/Gartner



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