Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
1112 am EDT Wednesday Aug 20 2014
an upper low over the west Great Lakes will drift slowly southeast across upstate
New York and new eng by late this week. A large surface high over the Canadian
Maritimes will ridge southward into PA during the upcoming weekend
and early next week.
Near term /until 8 PM this evening/...
a broad zone of weak-moderate cyclonic flow aloft will persist
from the upper Midwest/Ohio Valley to the Appalachians...to the
southeast of a weakening deep-layer 500 mb low over the Great Lakes. A
moist south-southwesterly flow beneath a diffluent exit region of middle/upper jet
rotating around the Great Lakes upper low will contribute to weak
instability and general increase in showers/thunderstorms through the afternoon.
At the surface...a frontal boundary will push slowly eastward from the
upper Ohio Valley along with a ribbon of high precipitable waters /1.50-1.75 inches/.
The combination of sufficient lg scale lift...weak
destabilization and deep moisture/high precipitable waters /60-65+ surface dewpoints will
support enhanced risk for heavy-excessive rafl with localized
runoff/flash flood issues possible - especially in training SW-NE oriented bands.
The hires model guidance generally shows this threat with 2"+ pt
amts over a short duration. A few stronger storms may also produce
brief wind gusts mainly below severe limits. Have updated the severe weather potential statement to
highlight short duration heavy rain/flash flood threat and Storm Prediction Center d1 see
text outlook/5% severe probs. Have also added mention of +ra into
the text forecast.
Early clouds should give way to some peaks of sun later today and
allow temperatures to climb to near seasonal norms...with highs from the
m70s over the high terrain of the alleghenies to the m80s across
the lower susq valley. Moist airmass will keep mins in the l-M 60s.
Short term /8 PM this evening through 6 PM Thursday/...
focus of dwindling rain showers/thunderstorms and rain will shift to the eastern counties
this evening along axis of weakening ll jet/plume of higher precipitable waters .
After a lull in the activity Thursday am...another round of convection
is expected Thursday afternoon as upper tracks north of the state.
Steep lps rates caused by cool pool aloft will promote widespread
afternoon convection...especially across the north mountains have bumped probability of precipitation upward
to the likely category across most of central PA based on latest
gefs/consall. Gefs 925 temperatures imply another day of near seasonal
temperatures with highs from the m70s to m80s.
Long term /Thursday night through Tuesday/...
latest medium range guidance pointing toward drier weather Friday into
early next week...as subtropical high is prognosticated to edge toward
the region from the Midwest. Daily ring of fire convection on NE
periphery of this ridge will threaten western PA during the
Friday-Monday time frame...while eastern PA is likely to remain dry.
Despite approach of upper ridge...no big warmup expected...as
large surface high over the Canadian Maritimes will circulate an
easterly flow into central PA. There may be issues with am low
clouds/fog due to the expected moistly southeasterly flow over the area.
Aviation /15z Wednesday through Sunday/...
MVFR ceilings over eastern sections should improve to low VFR by this afternoon.
Expect showers/thunderstorms to increase in coverage over the central and
eastern prtns of the airspace through the afternoon...with periods of IFR
visbys likely in +ra/+tsra.
Thu-Sun...morning fog chances each day. Overall VFR...but chance of
mainly afternoon rain showers/thunderstorms and rain and associated restrictions.
Sun...mainly VFR. Still chance of rain showers in the morning.