Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
620 PM EDT Monday Sep 1 2014
warm and humid conditions will continue into Tuesday until a cold
frontal passage brings drier and slightly cooler air to central
Pennsylvania through middle week. Humidity will return toward the
end of the week ahead of the next frontal system that will bring
the chance of rain for the upcoming weekend.
Near term /until 6 am Tuesday morning/...
fairly tranquil late Summer evening to close Labor Day Holiday
weekend over central PA...after some earlier heavy downpours
tracked across some portions of the central and south central
mountains. These were associated with a shortwave trough which
tracked into the Catskills this evening. Farther south...another
shortwave trough tracking from the WV panhanlde to southern Maryland
touched off isolated coverage of showers and a few thunderstorms
from southeast PA through central New Jersey. A third upper trough is
approaching southwest PA from northern Kentucky and extreme southern
Ohio...initiating isolated to scattered shower activity across
southwest PA and extending into the west central mountains at this
hour. Main convective activity is far west of central
PA...occurring with the surface cold front extending from Southeast
Michigan into Northwest Ohio and eastern Indiana this evening.
Upstream storms should weaken Post sunset...with focus for
convection shifting westward towards the approaching cold front...
expected to cross central PA Tuesday afternoon and evening.
Areas of fog and low clouds will form once again late tonight with
winds remaining light and low level moisture abundant. It will be muggy
with lows 65-70f.
Short term /6 am Tuesday morning through Tuesday night/...
a well-defined cold front /with sharp moisture- precipitable water gradient/ will drop
across the region Tuesday afternoon and evening. A conditional
risk for strong to isolated severe storms will exist from late Tuesday
morning through the afternoon with the primary potnl thunderstorm hazard
being damaging winds.
Cloud cover from upstream convection likely ongoing at 12z along
with residual overnight low stratus/fog should act to limit
destabilization early tomorrow. Although 700-500mb lapse rates
will generally remain weak to poor...the models show a belt of
stronger westerly middle-level flow and deep layer shear. This should
help to support an increase in convective precipitation/intensity as
renewed development occurs along outflows/differential heating
zones by late morning into the afternoon...with some organization
into line segments/clusters. This should yield at least an isolated
damaging wind threat...warranting an upgrade by Storm Prediction Center to a categorical
/slgt/ risk designation. This has been reflected in an updated
severe weather potential statement.
Early clouds should break for a warm and humid day with forecast
highs expected to range from the middle 70s over the northwest to near 90
over the lower susq valley. The second half of the period especially
after midnight should feature a cooler/drying trend behind the
front with precipitable waters falling at or below normal by 12z Wednesday.
Long term /Wednesday through Monday/...
cold front exiting to the east Tuesday night along with northwest
flow around building high pressure area over Ohio Valley will
bring a transition to a noticably drier airmass for midweek.
Daytime highs will remain slightly above normal...but lower
dewpoints will make it feel more comfortable. Overnight lows Wednesday
night will dip to near 50f in the northern mountains to around 60f in
The high will move off the East Coast Thursday into Friday...allowing more
humid air to begin returning to the region. A surface low slides
from the northern plains across the Great Lakes and lifting into
S Canada on Friday...dragging a cold front along behind. As this
front approaches...an area of higher precipitable waters /1-2"/ surges into the
Ohio Valley...and then into PA Friday night into Sat. At same time a
broad upper trough sags southward. This will bring elevated
chances for showers/thunderstorms starting Friday night in the northwest and
spreading areawide on Sat.
Behind the front...airmass change will usher in much cooler and
drier air for late weekend into early next week. Highs will fall
back about 10f...ranging from the upper 60s to middle 70s. As large
high pressure area tracks across New England...morning lows should
dip well into the 40s across the northern mountains...and in the lower
to middle 50s across the south.
Aviation /22z Monday through Saturday/...
18z...cigs/visbys have improved to VFR or MVFR as expected.
However despite a general increase in shower activity there is
still low confidence in point terminal impacts. Evolution of
convective activity and collocation of best instability suggests
the southern tafs have the best chance to see scattered showers or a brief thunderstorms and rain
downpour. Have added some 1hr temperatures through 19z with showers moving
Other concern will be cloud trends overnight with moist airmass
likely promoting another period of low ceilings/fog into Tuesday morning. A
cold front moving eastward through the Midwest will also bring the chance for
some showers to reach the northwest 1/4 to 1/3 of the airspace by
early Tuesday morning. This front will be a focus for scattered-numerous
showers/thunderstorms Tuesday afternoon-evening as it moves southeastward across central PA. The
recent guidance suggests a slightly higher probability for local
severe storms and at least an isolated threat for damaging thunderstorm wind
Wed-Thu...patchy am fog...otherwise VFR.
Friday...mainly VFR. Chance of PM thunderstorms west.
Sat...VFR/MVFR with scattered showers/thunderstorms associated with cold frontal passage.