Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 200552

Area Forecast Discussion
National Weather Service State College PA
152 AM EDT Tue Mar 20 2018

Big changes to our weather on the way as a storm system will
affect the area for the first day of Spring tomorrow. The storm
will primarily affect the southern half of the area. It will
initially bring a mixture of rain and snow but by Tuesday night
and Wednesday primarily snow will fall and significant
accumulations are possible across southern Pennsylvania. Cooler
than average temperatures will dominate the pattern through the
end of the week, with the next opportunity for precipitation
coming this weekend.


Near term in good shape with increasing clouds tonight. Latest
HRRR/NAM12 shows precip shield moving towards CWA in the pre
dawn hours. Latest HREF2 lags by a couple of hours, bringing
precip shield into Laurels and South Central mountains after
12z. Siding with the earlier HRRR/NAM12 at this time into the
Laurels. Temperatures should be cold enough for snow across the
south, but QPF should be light enough to limit much in the way
of accumulation initially.


Complicated storm system with snow and potentially some mixed
precipitation across southern half of CWA. Initially the
temperature profile and intensities of the precip suggest snow
will have trouble sticking in valleys and banding will determine
where accums are more efficient. Higher elevations will see
more accumulating snow. Temperature profiles cool dynamically
and this has become more pronounced in the 18z and early look at
00z Tue NAM12 run. This brings a better chance of accumulating
snow for valley locales as well. Snow will continue into
evening and Wednesday and Winter Storm Watch remains in effect
for southern zones for the long duration of potential snow fall.
Kept the Watch in place to allow mid shift opportunity to assess
latest numbers as we are approaching 12- and 24- hour Warning
criteria and trend from 00z runs and CAMs will provide critical

Besides the typical March complications with snow accumulations
(moderate to heavy snowfall rates needed to offset melting as
snow falls due to higher sun angles), the duration nature of
this system will make achieving Warning criteria difficult, with
only 30-40% probs of 6" or more in a 12 hour period in the Watch
area currently, and 30-40% probs of 8" or more in a 12 hour
period from Tuesday morning through Wednesday afternoon. Latest
guidance from the 12z Mon cycle has ramped up QPF/snow totals
over southern PA, but this may be in part due to an overzealous
NAM package. The current 90th percentile snowfall for Somerset
County is in the ridiculoso range (not to be mentioned here),
far beyond anything we are reasonably forecasting, so am hoping
that the addition of global model members into the pdf suite
will yield greater confidence and less spread probabilistically
for the overnight forecast tweaks.


By daybreak Wednesday the storm is expected to be reorganizing
off the Mid Atlantic before tracking off south of Nantucket
island. The upper trough/low will keep some light precip
lingering Tuesday night and Wednesday morning. An additional
coating of snow is possible as surface temperatures chill down
into the 20s in most areas Tuesday night.

By later Wednesday cool and drier air will begin filtering in.
Weak high pressure will keep us fair through Friday before the
next approach storm brings a chance of rain or snow Saturday
night and Sunday. This looks to be a clipper type way which will
move through the area quickly with high pressure following for
next Monday.


Main change to 06Z TAFS was to slow things down by an hour or

Earlier discussion below.

VFR will continue through the overnight and into early Tuesday
for most of Central PA. The area of concern overnight will be
the Laurel Highlands and south central mountains, where an
approaching low pressure area will push lowering ceilings in
from WV/MD after 09z followed by the onset of snow (mixed with
sleet at first) which will continue with IFR restrictions much
of the day. The ceiling restrictions look to spread into the
Lower Susq Valley by mid morning as low-level easterly jet
increases, and reach as far north as KDUJ- KUNV- KSEG by late
morning/midday. Precip reaches KMDT-KLNS after 13z, and should
fall as mostly snow, but sleet and rain could be mixed in at
times. Across the central mountains, snow will arrive midday
bringing further reductions.

Lower cigs and snow for southern PA continues Tuesday night into


Tue night and Wed...Snow and restrictions continue. Some
improvement late day.

Thu-Sat...No sig wx expected.


The Spring (Vernal) Equinox will be at 12:15 PM on Tuesday,
March 20. Sunrise in State College, PA will be 7:14 AM and
sunset at 7:23 PM, for a total day length of just over 12


Winter Storm Watch from this evening through Wednesday evening
for PAZ056>059-063.
Winter Storm Watch from 6 AM EDT this morning through
Wednesday evening for PAZ033>036-064>066.


NEAR TERM...Ross/DeVoir
SHORT TERM...Ross/DeVoir
LONG TERM...La Corte/Gartner
CLIMATE...Steinbugl is the U.S. government's official web portal to all federal, state and local government web resources and services.