Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
1144 PM EDT Friday Sep 19 2014
generally fair weather will be interrupted overnight and Saturday
morning as Atlantic moisture pools over the central PA mountains.
A frontal system will bring a chance of showers to the region on
Sunday...before another large area of high pressure and a blocking
upper-level ridge over central Canada brings more fair and cool
weather for most of next week.
Near term /until 6 am Saturday morning/...
the remainder of the overnight will continue to see low clouds
invading north under the south-southeast maritime flow. Expect these clouds to
lower and become more widespread overnight as the long fetch off
the Atlantic ascends the mountains of central PA. This should
create a widespread overcast with areas of drizzle/fog which will
last through the sunrise hours.
Short term /6 am Saturday morning through 6 PM Saturday/...
the winds should continue to veer on Saturday allowing the low
clouds to break up by late morning and early afternoon. After a
cloudy start...temperatures will rebound nicely in the afternoon with
maxes rising into the 70s for most places - especially in the valleys of
the southeast where there should be less cloud cover. Will hold the temperatures
around 70f along The Spine of the mountains where the clouds will
take longest to break.
Long term /Saturday night through Friday/...
all of the guidance shows a frontal system moving northwest to
southeast across the state overnight Saturday into Sunday evening.
This is most likely the only rain event that should affect the
region of the next 7-12 days as a blocking 500 hpa ridge with over
2 sigma height anomalies should provide a prolonged period of dry
At this time the models and ensembles all imply an increase in
shower activity after midnight northwestern Pennsylvania and
toward sunrise in central areas early morning. The gefs and sref
both imply the 12 hour period for the best rainfall will be
between about 8 am and 8 PM Sunday. The rainfall lingers in the
ensemble pdf until between 8 and 2 am Sunday night into Monday in
the east. The rapidly lowering precipitable water field implies all rain gone in
east after midnight Sunday.
At this time the higher probability of precipitation and higher qpfs should be in the
northern areas. Few members of any efs show much more than 0.25
inches with perhaps 40 percent showing 0.50 inches in the northern
areas. Most qpfs imply a 0.10 to 0.20 inch rainfall event. Kind of
anemic. Did mention thunderstorms as there is a warm surge and
some hints of modest cape in the higher resolution models and efs
perhaps 600 to 800jkg-1. Lower in the gefs. A few thunderstorms
could boost qpfs up a bit if they appear.
High pressure builds in Monday behind the front and sort of Parks
itself over New York and New England with +2 to +3 sigma above
normal mslp anomalies by Wednesday-Thursday. High confidence in
this forecast as the models all show a blocking 500 hpa
anticyclone to the northwest as a massive 500 hpa positive height
anomaly of +2 to +3 sigma develops over central Canada by Thursday
and into Friday.
Our models and efs do well forecasting anticyclones. So I am
high on the fair weather forecast next week.
Only fun issues are how much rain Sunday and how sunny will it get
behind the front Monday. Did not attempt to draw the diurnal fog
and low clouds likely to affect the region once the big high takes
hold. This will be a nearly ideal diurnal fog pattern Wed-Sunday.
Aviation /04z Saturday through Wednesday/...
late evening 11-3.9 micron satellite imagery showing strato-cumulus banked
against the east slopes of the Appalachians...the result of moist
southeasterly flow off of the Atlantic and orographic lift over central PA.
Ceilings beginning to fall with kunv reporting a 2700ft ceiling as of 03z. Modest
radiational cooling of blyr will cause ceilings to continue lowering
overnight. A gradual wind shift from southeast to south overnight will
favor the north mountains for the best orographic lift and highest chance
of IFR ceilings. Further south...confidence in IFR conds late tonight
is not very high. Conditional climatology and latest sref output
suggest MVFR ceilings more likely late tonight across central and
southern PA...despite the more pessimistic MOS guidance. Will also
have to watch for possible IFR visibilities at klns in fog late tonight.
Dewpoint depression down to 3f at 03z and this area remains mclear
beneath surface ridge axis...where wind are very light.
Early low clouds should mix out by late am. Model data then
supports a high confidence of VFR conds during the afternoon and
evening...as high pressure ridge remains over the area.
sun...am fog possible eastern PA. Slight chance of PM thunderstorms and rain impacts West Mountains
Monday...am low ceilings poss West Mountains
Tue-Wed...no sig weather expected.
near term...la corte