Area Forecast Discussion
Issued by NWS State College, PA

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FXUS61 KCTP 050019
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
719 PM EST Sun Dec 4 2016

.SYNOPSIS...
High pressure will slide quickly off to the east of the region
this afternoon and evening. A weak frontal boundary but rather
potent upper air disturbance will pass through tonight. Another
storm system will affect the area during mid-week.

&&

.NEAR TERM /UNTIL 6 AM MONDAY MORNING/...
850 mb ridge will be drifting across central and eastern PA late
this afternoon and early this evening. The waning boundary layer
northwest flow around that level will mean a brief erosion of the
lower layer of stratocu beneath an increasing/thickening shield
of mid and high level clouds. So, mostly cloudy to ovc skies, dry
conditions and light variable sfc winds will persist for the rest
of the day - through a few hours after dusk.

Temps late this afternoon will range from the mid and upper 30s
across the Alleghenies...to the lower-mid 40s in the central and
southern valleys.

Latest 09Z/12Z Ensembles and Oper Models have been quite
consistent with the onset timing (and type) of precip that will
advance quickly NE later this evening within an area of robust
llvl warm air advection aloft (and along/ahead of a potent
mid/upper level vort max).

The first few hours of the precip (after it arrives in the west
this evening...and across the east around midnight) will be snow
in most areas (with perhaps some sleet/rain mixed in across the
south where the wet bulb 0C level will be AOA 1500 ft AGL).
A longer period of snow will occur across the central and
northern counties of the state...leading to a light accum of a
coating to 2 inches.

The Laurels may drop near or just below freezing for a period in
the middle of the night as the temps begin to rise above 0C aloft
and turn the precip to liquid form. This could allow for some
FZRA in the southwest - most likely confined to the (highest)
elevations. Still not enough confidence in temps being below
freezing for very long where it does turn to rain in order to post
a FZRA (or winter wx) advisory. Have mentioned it in the grids,
and HWO but kept out mention of specific FZRA accretions.

There will be enough precip falling through the initially deep
layer of dry air and decent wet-bulbing via the 20-something
dewpoint air to make a light coating of snow/sleet across much of
the area.

Low temps will be mainly in the 28-32F range across the
north where the cold air is deep enough to support mainly
snow...while readings in the l-m30s are expected across the
central and srn valleys

Will maintain a general 1-2" in the north and an inch or less in
the central and a little less still in the south. The precip could
turn to rain at the very end, but will likely just fizzle into
some patchy drizzle. This, too, could spell trouble if it lingers
into rush hour Monday morning and if temps can stay below
freezing.

&&

.SHORT TERM /6 AM MONDAY MORNING THROUGH 6 PM MONDAY/...
The fast zonal flow aloft takes the best forcing for precip off
to the east of the area early Monday. Will linger some sct
-SHSN or --RA at best in the east and along the west/high
elevations through about 15z Monday, but dry elsewhere.

Low clouds and the patchy, very light precip early will break for
some afternoon sunshine as a bubble (1020mb) of sfc high pressure
slides quickly east from the Ohio Valley.

High temps Monday afternoon will vary from about 40 across the
higher terrain of the north and west to near 50F in the SE.

&&

.LONG TERM /MONDAY NIGHT THROUGH SUNDAY/...
Several successive systems will be moving through the region this
upcoming week. This will allow for a more tumultuous weather
pattern. The first will have a former cutoff low from old Mexico
as it`s forcing mechanism. The low will cross the border early
Monday and open up as it slides NE into the OH valley Mon night
and overhead on Tuesday. A broad but weak low will form over the
lower MS valley and perhaps do a split with one spot of lower
pressure going up the OH river and another one more to the south
which would eventually move over the DelMarVa peninsula.

The 850 MB temperatures show warm air advection aloft. The big
forecast question is the timing of frozen precipitation Monday
night through Tuesday. The current models have a weak upper level
trough that should only provide weak forcing. Without a very solid
low moving to our south, there still remains some question whether
all the precip will remain frozen. Similarly, without a distinct
low going to the west of us, there is a question that the warm air
advection will push over the entire area and turning things to
all rain. Have transition periods of change precipitation from
09Z Tuesday to 18Z Wednesday. The main area of mixed precipitation
remains over the central mountains. The southeast should have a
period of mixed precipitation before switching to rain. The NE
third of the area may stay cold enough to allow for an inch or so
of snow. There is a chance for freezing rain, though the precip
will be falling during the day, and temps should rise above
freezing after the precip begins but probably before noon passes.
Even the northern mountains should get into the mid to upper 30s.

The end of the week system brings another chance for
precipitation. This system is out ahead of a deep upper level low
moving through the Upper Great Lakes. There is some model
disparity on the timing however have increased POPS into Friday
night as the precip looks like a good bet around the Thu/fri
period and all solutions eventually bring this trough through. So
predictability is increasing, but timing issues remain. Best bet
for precip at this point is mainly rain over the SE as decent push
of warm air lifts ahead of the system, with a rain/snow mix then a
switch to snow as 850 mb temps are trending from -8 to -12 C.

Behind the trough, looks like a chilly weekend in store with a
potential sharp drop off in temps.

&&

.AVIATION /00Z MONDAY THROUGH FRIDAY/...
Brief period of VFR conditions across all of Central Pennsylvania
at 00z, but will be short lived as snow/rain system rapidly
approaching from the west.

Snow moves into northern mountains and higher elevations
tonight, where a period of IFR conditions are expected. Rain and
snow for central airfields, while a mainly rain event in the
southeast.

System exits early and quickly on Monday from west to east.
Lingering lower stratus expected in the mountains of the west and
north, but elsewhere expect conditions to improve to VFR by
mid/late morning.

Outlook...

Mon afternoon/evening...No sig wx expected.

Tue...Mixed precip/low CIGs possible, especially late.

Wed...Lingering low CIGs possible.

Thu...Mixed precipitation event - low CIGs possible.

Fri...Blustery and much colder with NW mtn and Laurel Highland
restrictions in SHSN and areas of enhanced Lake Effect Snow.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$
SYNOPSIS...La Corte/Lambert
NEAR TERM...La Corte/Lambert
SHORT TERM...Dangelo/Lambert
LONG TERM...Ceru/Gartner
AVIATION...Jung/Lambert



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