Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
139 PM EDT Friday Jul 31 2015
high pressure over the central U.S. Will nose into the region this
weekend. A persistent trough aloft will sit over the Great Lakes
and northeast at the same time. Weak fronts with little moisture
moving down on the northwest flow over the Great Lakes will try
to break down the high pressure.
Near term /until 6 PM this evening/...
high pressure ridge extending NE into the region from the Ohio
River valley will provide abundant sunshine for this afternoon
with surface dewpoints in the upper 50s across the mountains and l-M 60s
throughout the lower susq valley.
Late today and tonight...increasing moisture...reflected by a few
SW/NE oriented ribbons of higher Theta-E at 850 mb /and 1.25-1.5
inch precipitable waters / will push into the region ahead of an upper trough
and cfront dropping east-southeast from the upper glakes.
Surface based cape /via the more aggressive NAM/ will remain under
500 j/kg across the southeast half of the state...while ramping up to
over 1000 j/kg across far northwestern Penn by 00z Sat. 14z run of the 13km
rap...and a similar solution given by the 09z sref indicate that
enough instability to produce scattered rain showers/thunderstorms and rain wont arrive across
our far northwest zones until at least 02z Sat /enhanced by cooling middle
levels and increasing upper level diffluence/.
This agrees pretty well with our current gridded pop values and
timing. However...will be trimming low-end 10-15 percent probability of precipitation for
our northwest mountains later this afternoon.
Earlier Storm Prediction Center marginal dy 1 risk for northwest Penn has been trimmed back to
general thunderstorms and rain /which extends across the northwest half of our County Warning Area by 12z
Temperatures are on track to reach late afternoon highs that will be
several degree f above normal.
Short term /6 PM this evening through 6 PM Saturday/...
upstream convection will try to get into the northwest later this
evening and will probably succeed is making isolated to scattered
showers/storms for the northern third to half of the County Warning Area tonight. But
the loss of instability and the storms running into drier air and
a good cap will likely cause them to die out as they get to PA.
Nighttime lows 55-65f.
Dewpoints Don/T move much into Saturday...but better dynamics
aloft and slightly lowered heights will allow for a few more
storms to form on Sat afternoon/evening - but again confined to the northern
half of the area due to strong inversion in the southern half. 800 mb temperatures
a little cooler on Sat vs Friday...so maximum temperatures will likely be
slightly cooler as well - especially in the northwest.
Long term /Saturday night through Thursday/...
little change is anticipated in large scale flow pattern across
the Continental U.S. Through the extended period. A broad upper trough
extends southward from eastern Canada across the eastern U.S.
Temperatures should average near normal into early next week before
trending cooler later in the week. Several short waves will rotate
around the upper low in a fashion reminiscent of winter. As is the
case with most upper low vs. Short wave spokes...confidence is
low on timing of precipitation chances.
Aviation /18z Friday through Wednesday/...
surface high pressure ridging NE into the region will support VFR
conditions across most or all of the region through at least
early Saturday morning. Scattered thunderstorms and rain possible beginning late this
evening in vicinity of kbfd...but they will become more isolated further
south and east overnight as instability wanes.
Sat...mainly VFR...scattered showers/thunderstorms.
Mon-Tue...mainly VFR...scattered showers/thunderstorms.