Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 221638
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
1138 AM EST Sun Jan 22 2017

.SYNOPSIS...
A multifaceted storm will bring drenching rain, elevation-
driven snow and gusty winds to Central Pennsylvania early this
week. High pressure and unseasonably mild conditions will
briefly return for midweek before a pattern change brings colder
temperatures and lake effect snow to close out the month.

&&

.NEAR TERM /THROUGH TONIGHT/...
Much like yesterday, fog and low clouds remain entrenched from
the central ridge/valley region into the Susquehanna Valley. The
sustained very low visibility prompted another extension to
dense fog advisory until 1pm, but current observational trends
suggest this may be cancelled before 1pm EST expiration time.

Visible satellite loop does show a few peaks of sun developing
across the NW Alleghenies and Laurel Highlands where
temperatures have surged into the low 50s. This will not last as
clouds quickly spread back into this area ahead of dynamic storm
system evolving over the Deep South. Cut back a few degrees on
max temps in the fog/low cloud areas where daytime readings will
likely hold in the mid-upper 40s.

Cut back on pcpn potential for today and early tonight based on
HIRES NCAR/HRRR/SSEO blend. POPS trend upward from south to
north later tonight into early Monday morning as moisture
increases ahead of the aforementioned storm system which should
reach western SC/NC by 23/12z.

A tightening pressure gradient between the approaching storm and
high pressure in eastern Quebec will produce strong east winds
early Monday morning and continue through Monday afternoon. Max
gusts 40-45mph are likely in the Lower Susquehanna Valley. A
wind advisory may be needed later today.

&&

.SHORT TERM /MONDAY/...
1130AM update: Minor changes were made to storm total snowfall
maps with greater emphasis on snow accumulations in the higher
terrain from north-central PA down along the Allegheny Front
and into the Laurel Highlands. The overnight HIRES models were
rather bullish on several inches of snow accumulation over the
higher terrain, but marginal boundary layer thermal profiles and
potential warm nose aloft (sleet?) add several layers of
uncertainty. The greatest risk for several inches of - heavy wet
snow - is over the north-central mountains to the north of
I-80. This is also the area that may need a winter weather
advisory and will be considering/collaboration this potential
hazard this afternoon. A period of heavy rain is possible on
Monday but flood risk will be mainly localized and confined to
poor drainage areas. The rain will help to ease dryness across
east-central PA.

Previous...
* Surface low taking a very favorable track for winter storm.
* Monster easterly jet 60-75 kts forecast.
* Number of SREF/GEFS plumes showing snow is on the increase.
* Dynamic cooling under intense forcing will bring a rain to
  snow scenario.
* Higher elevations under most threat for significant snow.
* Central ridge-valley areas not out of the woods.
* ECMWF/GFS develop steep mid level lapse rates...thunder snow?

Monday will be the most complicated and active day of the
forecast period. In a normal winter with normal cold air in
place, the expected track of a deep low up along or just off the
coast would be cause for joy among snow crows. But we are
abnormally mild ahead of this storm and that complicates the
forecast enormously.

During the day the models agree in intensifying the easterly
low level jet, as strong as 75 kt in some of the guidance.
SREF/GEFS show this to exceed 6 STD DEV in the anomaly plots.
Not a surprise with such an intense jet, ensembles also show a
high likelihood of more than an inch of QPF over much of
southern and central PA during the day Monday.

The complicating factor will be the amount of dynamic cooling
that is forecast to occur at the height of the storm as strong
warm advection and frontogenetic forcing support a period
intense upward vertical motion Monday morning into the
afternoon.

Initially we will see rain in all locations, but by mid morning
the higher elevations could mix with or change to wet snow that
will continue into the afternoon. Forecast soundings suggest
the snow could actually become more than just a higher
elevation threat by mid afternoon with thermal profiles even as
far east as AOO/UNV cooling sufficiently for all snow. If the
next run of models continues this idea of explosive dynamic
cooling as precip increases under the intense forcing, I would
anticipate some sort of headline to become necessary. The threat
is for several inches of heavy wet snow, especially favoring
the higher elevations from the Laurels northward, but possibly
even eastward into the central ridge-valley region of the CWA.

Another concern is the potential for flooding. Target area for
the heaviest rain is across the south central part of the state,
where orographic forcing at nose of powerful easterly low level
jet will play a role. Mean QPF from both SREF and GEFS max out
around 1.5 inches in this region, while FFG/FFH is around 2
inches/6hr. Have to stay alert for locally higher amounts such
as hinted at by the higher resolution NAM. Right now it looks
like a relatively fast moving event so will mention the
possibility of minor flooding in the HWO for this area late
Monday.

It`s all going to make for a busy day for sure.

&&

.LONG TERM /MONDAY NIGHT THROUGH SATURDAY/...
The storm will reach our latitude between about 6-12Z Tuesday,
an old rule of thumb that dictated when the steady precip tends
to taper off. Upper ridging will quickly build into the region
later Tuesday into early Wednesday, bringing a short period of
fair and mild wx.

A pattern shift is advertised for the second half of the week
as low pressure takes up residence over eastern Canada, and
despite a tenacious ridge over the northern Caribbean and
Bahamas, models carve out a long wave trough over the eastern US
with a return to seasonable cold by the end of the week into
next weekend.

The ridge in the west and trough in the east is also usually a
good one for winter storminess, but the deterministic ECMWF and
GFS don`t show much more than a series of clipper type systems
that promise an extended period of clouds and scattered mainly
mountain snow showers that should continue through next weekend
into the week after.

&&

.AVIATION /16Z SUNDAY THROUGH THURSDAY/...
Solid LIFR-VLIFR continues across the central and eastern
airfields while KBFD/KJST enjoy VFR conditions. Remain very
pessimistic on significant improvement and will likely continue
IFR or lower through tonight with the 22/18z TAFs. For KBFD/KJST
will gradually trend lower tonight as mid clouds increase from
south to north. Widespread sub-VFR is near certainty from later
tonight through Monday as rain overspreads the airspace and
mixes with/changes to snow in the higher elevations along the
Allegheny Front. Strong gusty winds will impact southern
airfields especially KMDT/LNS with 30+KT gusts by daybreak
Monday.

Outlook...

Tue...Widespread sub-VFR with some improvement by aftn. Rain/snow
ending in the morning. Gusty winds from NNW decreasing late.

Wed...Low cigs/rain possible western 1/2. Breezy late.

Thu...MVFR/IFR in rain/snow showers west. MVFR to VFR east.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Dense Fog Advisory until 1 PM EST this afternoon for PAZ006-
012-018-019-025>028-034>037-041-042-045-046-049>053-056>059-
063>066.

&&

$$
SYNOPSIS...Steinbugl
NEAR TERM...Steinbugl
SHORT TERM...La Corte/Steinbugl
LONG TERM...La Corte
AVIATION...Steinbugl


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