Scientific Forecaster Discussion
fxus61 kctp 140329
Area forecast discussion
National Weather Service State College PA
1029 PM EST Wed Dec 13 2017
an Alberta clipper will track across southern Pennsylvania
overnight. Behind this system, a ridge of high pressure will
slide across the region Thursday and Thursday night with cold
but mainly dry conditions.
Another clipper will move north of the region overnight Friday
and Saturday morning with limited moisture.
High pressure will build across the area early Sunday with
a brief moderating trend.
A series of cold fronts will move across the area late Sunday
into Wednesday. Windy and colder temperatures will follow for
Tuesday into Wednesday.
Near term /until 6 am Thursday morning/...
an Alberta clipper, located over Northwest Ohio at 03z, will
streak east-southeast through southern PA overnight and off to our east by
early Thursday. An impressive mid level jet streak is progged to
dive through the Ohio Valley and just south of the Maryland border
overnight, placing the region in the favorable left front
quadrant of the jet. The low level response will be a
strengthening southerly flow and a period of strong warm
advection and isentropic lift centered over central PA.
Light snow has overspread nearly the entire forecast area at
03z and expect strongest warm air advection at nose of low level jet to move
from the Laurel Highlands at 03z to the susq valley around
05z-06z. Many areas will likely see snow diminish after 06z, as
low level jet shifts east. However, 00z hrefv2 suggests a moist
upslope flow into the The Laurels/central mtns should keep the
accumulating snow going in that area until almost dawn.
As expected with a clipper, model Omega/temp time-sections
indicate fairly high snow/water ratios. Close to 20:1 expected
across northern PA, where Max lift if progged to occur within
the best dendritic growth zone. Across the southern counties,
indications are for slightly lower ratios with some riming
likely based on profiles of temp and lift.
Model blended qpf, including contributions from latest hrrr and
00z runs of the hrefv2/NAM, support storm total snowfall
ranging from less than an inch from Chambersburg and York
southward, to around 6 inches across Cambria and southern
Clearfield counties. Considered upgrading to an warning for
Cambria, didn't see much to be gained by upgrading one County
for what would be a borderline, low end warning.
Strong wind gusts across southwest Ohio have caught my eye this
evening. Although not anticipating the 47kts observed at Kiln,
do expect winds to become gusty late tonight across portions of
Somerset County, as this area briefly breaks into the warm
sector south of sfc low. BUFKIT soundings support gusts to
around 25kts down there by around 09z.
Short term /6 am Thursday morning through 6 PM Thursday/...
the snow should have ended by dawn over nearly the entire
region, as fast-moving clipper passes off the New Jersey coast. High
pressure will build in during the day leaving little more than
some scattered snow showers over northwestern sections and possibly the
Highs ranging from the upper teens across the northern tier to
around freezing in the far southeast will average about 10-15 deg
colder than normal.
Long term /Thursday night through Wednesday/...
a few snow showers for Thursday evening, then clearing and
quite cold conditions for overnight under the center of high
pressure. Min temps will vary from the single digits over the northwest
mtns, to the teens throughout the susq valley and southern
A mid level wind Max will drop southeast Friday night and
early Saturday. Some snow showers expected, mainly across the
west. I did up probability of precipitation some.
For the first part of the weekend, did lower temperatures some
Still looking mainly dry later Saturday into the first part of
A complex pattern will prevail after Sunday, with some energy
from the southern branch of the westerlies becoming more active.
Moisture from the Gulf of Mexico will be limited, with a ridge
Expect colder and more in the way of wind by later Tuesday into
Overall, minor changes to the package.
Aviation /03z Thursday through Monday/...
* cloud/ceiling info will be missing at kbfd until new part can
be installed *
Most terminals are VFR as of early afternoon, the exceptions
being the usual higher elevation culprits from jst north into
northern mountains where there remains some residual lake
effect snow showers.
The near term guidance shows the snow showers over the north
dwindling over the next few hours, just in time for a larger
area of snow to begin overspreading the western part of the
flying area between about 22-00z. The snow will reach eastern
areas before 02z.
Expect the brief improvement this afternoon to deteriorate once
again with widespread IFR being the predominant condition
overnight. MDT/lns being on the southern edge of the expected
snow shield could waver between MVFR and VFR.
The snow will taper off quickly in the pre-dawn hours with rapid
improvement setting in as the storm moves east of the area. The
higher elevation terminals could remain MVFR for a good part of
Thursday, but elsewhere airports should become VFR by mid
morning and continue throughout the day.
Thu-Sat...scattered snow showers and local restrictions
Winter Weather Advisory until 8 am EST Thursday for paz004>006-