Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
1033 am EDT Tue Sep 27 2016
mostly sunny and pleasant conditions are expected today. A storm
system digging southward across the Great Lakes today 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 /through tonight/...
benign conditions to last through tonight...with no sig changes in
the near term. Main wx focus is on potential heavy rain event
A well- defined dry slot/below normal precipitable water air overspreads the area
behind departing cold front and upper level low dropping south
from the northern Great Lakes. Look for a pleasant afternoon with
Max temperatures within a degree or two of late September climate
normals 65-75f. The model guidance is in good agreement in taking
the upper low to a position near the southern tip of Lake Michigan
by the end of the period/12z Wed. Not expecting any showers
tonight but the trend in rain probs will start to edge higher into
Short term /Wednesday through Thursday night/...
*increased risk of heavy to excessive rainfall and flooding
Wednesday night into Thursday. Primary threat area is south-
The aforementioned upper low will cut off from the mean flow and
drift south across the Ohio-Tennessee valleys Wednesday into Thursday. The
guidance has been coming into better agreement with this feature
over the past few days with the European model (ecmwf) only 100 miles west of the
consensus (near eastern ky) by 12z Friday at 500mb. The exact
evolution and location of the upper low will play a significant
factor in the magnitude and location of potential heavy rainfall
axis during the period. At this time, the favored location appears
to be over south-central PA.
The risk for heavy rain ramps up after 00z Thursday as the upper
low digs and takes on a slight negative tilt, resulting in
increasing upper level diffluence. A pronounced convergence axis
is progged to develop to the east of the low and tap into
deep moisture/high precipitable water 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
likely 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 differing slightly at each 6hr
interval. Therefore, used wpc quantitative precipitation forecast which shows a broad area of 2-4"
across the south-central and southeaster counties with localized
amounts of 4-6" possible.
That said, based on trends in the latest 27/00z guidance, wpc
will introduce a slight and MDT excessive rain risk for Wednesday
and Thursday. Included chance for +ra into wx grids during this
time and will highlight in severe weather potential statement. Flood watches may be considered
with later shifts. The other wildcard is the recent dryness, but
given the favorable synoptic set-up and magnitude of the
anomalies the risk is warranted.
Long term /Friday through Monday/...
the upper low is fcst to wobble back to the west/northwest toward the Ohio
Valley on Friday before slowly moving to the east-northeast 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
Aviation /15z Tuesday through Saturday/...
now that morning fog has burned off...VFR conds will prevail
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.
Wed...showers/cig reductions possible late.
Thu-Sat...sct showers/cig reductions possible, esp W mountains.