Scientific Forecaster Discussion
Area forecast discussion
National Weather Service State College PA
159 am EDT Tuesday Sep 16 2014
a weak area of low pressure will move east across the Ohio River
valley tonight before sliding off the Delaware coast during the
midday hours Tuesday. This low...will combine with a disturbance
aloft to bring numerous showers to northern Penn overnight...with
showers of the more scattered variety over southern Penn. The
rest of this week will be mainly dry as high pressure brings
plenty of sunshine and and slightly below normal temperatures to
Near term /until 8 am this morning/...
thickening/layered clouds will continue to spread east across the
Susquehanna and Juniata river valleys early this morning.
Weak...1016 mb surface low was centered in vicinity of kunv at 05z with a cold
front trailing southwestward through the Ohio River valley...and an
inverted trough extending northward into western New York. Precipitation in the
form of light to moderate rain showers has been linked closely to
the thermally direct transverse circulation of a 100 knots...300 mb
jet that stretched from the St Lawrence Valley...to Lake Erie.
The best vertical motion associated with this /right entrance/
region of the jet will slide across the northern half of Penn through the
middle morning hours.
A large area of light to MDT showers /covering much of northwestern Penn
and northestern Ohio/ will shift east through the overnight hours and
target mainly the northern one-third to one-half of the state.
21z sref...and 18z/00z operational NAM and GFS indicate basin
average quantitative precipitation forecast amounts of 2-3 tenths across the northern tier counties of
Penn. Ec ensembles also imply quite a sharp gradient in probability of precipitation
further south across the County Warning Area - between 1-80 and I-70/76. Less than
one tenth of an inch is expected to the SW of a line from kfig to
kunv and kmdt.
Low temperatures continue to look in pretty good shape with mins ranging
from 45-50f across the northern and western mountains...to the lower and middle 50s
Short term /8 am this morning through 6 PM Wednesday/...
cold front will be moving east across the area quite fast this
morning. Any light showers after 12z today should be confined
across the region just to the east of the susq mainstem. Skies
will become clear to partly cloudy this afternoon...though some
briefly broken...flat strato cumulus are possible...especially across the
northern mountains and Laurel Highlands
Winds will stay light and variable ahead of the cfront...while
northwest winds increase to 6-8 kts with some gusts into the l-M teens
range this afternoon across the central and western mountains
Main concern for late in the 2nd period will be potential for
frost and localized freezing temperatures across the northern mountains 00z
mav numbers are at the colder end of all guidance...and have bfd
below 32. Clear skies and light winds...along with high heights
and low dewpoints/precipitable water values support some potential for frost et cetera.
Still a ways out...later shifts can monitor and issue flags as
Long term /Wednesday night through Monday/...
by Wednesday...surface high pressure over Ohio Valley with drier cooler
northwest flow in lower levels. There may be some clouds lingering
over higher terrain under subsidence inversion early...but
central PA should see good deal of sunshine overall. Highs will be
about 5f below normal.
Trough aloft sharpens over Quebec Wednesday night/Thu...pushing a weak
frontal boundary south toward PA on Thursday. This front will continue
to weaken as it edges southward...but a weak shortwave will slide
overhead from the Midwest. This will bring an increase in clouds
cover along with just a chance for a sprinkle. Northern PA will
get a reinforcing shot of cooler air...keeping highs in the upper
50s to lower 60s.
High pressure then builds across the Great Lakes into a 1030mb
high over New England by Friday...bringing a period of quiet weather
and unseasonably cool temperatures. Morning lows in the 30s are
likely Thursday night if skies clear out as anticipated. Highs on Friday
will be mainly in the 60s as northwest flow continues.
Return flow begins weakly Friday night...taking a little of the bite
out of overnight lows /though still chilly with readings mainly in
the 40s/. But this will begin a warming trend for the weekend.
Temperatures Sat into sun will be 5-8f warmer than Friday...with highs
approaching 80f in the southeast. Lows will be noticeably milder as well
as low level moisture increases in strengthening SW flow.
Heading into Sunday...deepening low centered well to our north
as a trough axis rotates through the Great Lakes will bring plenty
of clouds and a chance for showers. Models still showing large
differences on evolution of this system...with GFS more
progressive and European model (ecmwf) actually bringing a second wave around the
backside of the trough and developing a stubborn closed upper low
over the region. For now will split the difference and hold on to
mention of showers into Monday along with another shot of cooler air
and see how this plays out over next couple of days.
Aviation /06z Tuesday through Saturday/...
showers continue to progress through northwest PA. Rain showers are
heavy at times...producing IFR ceilings and visibilities at bfd. These
conditions will continue through 09z. Reductions across the rest
of the state are possible...though outside of bfd they should
MVFR. The showers will continue their eastward trek...and some
shower formation will be possible in the lower susq before
sunrise. The winds should shift to the northwest after 13z...and
after that conditions should gradually become VFR. At the very
latest by middle to late morning as the cold front moves east of the
area. Gusty northwesterly flow remains probable...though maximum
gusts shouldn't exceed 20mph. Any gusty winds should subside
around sunset...with milder...VFR conditions settling in this
afternoon into tonight.
Some potential for fog Tuesday night...as high pressure
builds over the area.
Mainly dry conditions the rest of the week...as high pressure
Wed-Sat...mainly VFR conditions.