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Area forecast discussion 
National Weather Service State College PA
250 PM EST Thursday Dec 18 2014

Synopsis...
northwest winds will keep flurries and scattered light snow
showers over the north and west into Friday morning. High
pressure will slide down from the upper Great Lakes and provide a
generally fair period for most of the weekend. An unsettled pattern
will set up in the first part of the new week. A weak low pressure
area could bring light rain or snow to the area on Monday. Then a
significant area of low pressure is expected to develop over the
Midwest and upper Great Lakes. This will likely produce widepsread
rain for the state on Wednesday...followed by strong west winds
and scattered mountain snow showers for Christmas day.

&&

Near term /through tonight/...
seasonably cold northwest flow across the eastern lakes will
remain in place through Friday with the typical downwind stratus
across most of central PA. Cyclonic flow aloft on the backside of
Newfoundland Upper Cutoff/Rex blocking pattern will keep weak or
nearly neutral cold advection over the region. A southern stream
wave streaking eastward from the Tennessee River valley Thursday afternoon
will produce areas of very light snow across the Laurel Highlands
into the south central mountains this evening with little if any
accumulation expected. West facing slopes of the west central
mountains and Laurel Highlands may see a localized coating to an
inch or two of snow as the upper and boundary layer flow align
for a period late tonight into Friday morning.

Otherwise expect widespread stratus to persist across most of
central PA. The southeast will likely see some dissipation by
Friday morning. Surface high centered over the northern Great
Lakes and eastern Quebec will keep north/northwest boundary layer
flow going through Friday morning...translating to surface winds in the 7
to 11 miles per hour range.

&&

Short term /Friday/...
upper forcing slides east of the region early Friday morning...
with expansive surface high pressure building over the area in the
afternoon. Thus...the amount/coverage of the snow showers and
flurries should diminish completely on Friday. Mins will stay a
little milder than climatology with clouds around. Maxes on Friday very
similar to today...with 30s for the mountains and near 40 in the
low lands with the help of some downslope warming.

&&

Long term /Friday night through Wednesday/...
the flow remains rather flat over the southeastern U.S. And Middle Atlantic
States this weekend. So the weak low which has been projected to
pass just to the south of the area this weekend should do so...and
even well south of the area at that. All medium range guidance keeps
the surface low along a path from katl to kchs. Therefore...will
remove any remaining probability of precipitation in the Sat-sun time frame. Temperatures will
stay very similar to what we see in the near term...generally
cooler than normal during the days. Friday night should be the
coldest night until after Christmas with mins in the teens west
and 20s elsewhere.

The pattern turns very complicated/convoluted for the first part
of the week. A big trough starts to develop over the upper
plains/Midwest and the resulting heave to The Heights off the
eastern Seaboard will allow another area of low pressure to roll
to the northeast from the Gulf Coast on Monday. The low will still
stay decently far away from the local area. But the Atlantic
moisture will probably at least brush the southeastern half of
the area. Timing differences between the GFS and ec are about 12
hours - with the ec faster in moving the wave to the east. Precipitation
type at this point looks like it could be light snow...if it falls
during the night. But a daylight event would be more of a mix or
snow on the northern end of the precipitation shield and rain into the
lower susq. Probability of precipitation at this point are not Worth much more than
chances...due to timing differences and the fact that the weekend
system is now prognosticated to go well off to our south. This system may
end up rowing down the same stream...and have hedged forecasts of
the precipitation shield to the southeast a bit. Easterly winds may pile the
low level maritime air up into the central PA mountains for Tuesday and a
dark day with light showers or drizzle is possible. The temperatures
start warming up significantly aloft....though.

The big story is the expected development of a very significant
low pressure area during the middle week over the middle MS valley and
upper Great Lakes on tues/Wed. Some models had placed a 960hpa low
over Lake Huron on Wednesday. But the most recent runs of the GFS
and European model (ecmwf) place a 990hpa low under the main upper low over the
upper Great Lakes...and a rapidly deepening but initially equal-
strength surface low running up the Piedmont east of the Appalachians
on Wednesday morning. This second low will throw copious moisture back
into the state as it whips northeast on Christmas evening. The temperatures
look like they will be warm enough for mainly rain over the entire
area...but if the track is slightly more to the east - the temperatures
will be just a hair colder and the mountains may have snow
accumulations. After the secondary low zips away...the cyclonic
flow around the closed upper low and strong cold advection will
certainly make widespread snow showers over the western
mountains...and very gusty winds.

Timing differences with the main slug of precipitation are minor for
this range. HPC guidance of likely probability of precipitation seems very plausible...and
will hold it to a 70 pop at this point for Wednesday during the day. Quantitative precipitation forecast
in the gefs plumes is on a giant spread at this point...but the
mean numbers are around an inch and the medians slightly higher.
Will mention the storm and the most likely outcomes in the severe weather potential statement
at this point...due to the fact that the timing looks to be on one
of the busiest travel days - didn/T we just do this a month ago -
and a small change to the east in track of the secondary low could
mean snow - at least for the alleghenies.

&&

Aviation /18z Thursday through Tuesday/...
low level moisture trapped beneath inversion and northwest flow over
the eastern glaks will keep strato cumulus in place across most of
central PA through tonight. Latest model soundings point toward
predominantly MVFR ceilings/visibilities at kbfd. Elsewhere...model soundings
support ceilings around 1500ft at kjst into this evening...while
drying/downsloping flow yields progressively higher ceilings further
east.

A trough of low pressure will push through the region tonight...perhaps
producing a period of steadier light snow at kbfd and kjst between
00z-06z Friday with visibilities in the 1-2sm range.

Outlook...

Friday...early am -shsn/reduced visibilities poss at kjst.

Sat-sun...no sig weather expected.

Monday...low ceilings with light rain/snow possible.

&&

Ctp watches/warnings/advisories...
none.

&&

$$
Synopsis...devoir
near term...devoir
short term...devoir
long term...dangelo
aviation...devoir

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