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Area Forecast Discussion (AFD)

Issued by State College, PA (CTP)

                            
000
FXUS61 KCTP 181052
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
552 AM EST Fri Jan 18 2019

.SYNOPSIS...
A major winter storm will impact central Pennsylvania over the 
weekend, followed by an arctic blast into early next week.

&&

.NEAR TERM /THROUGH TONIGHT/...
A weak trough of low pressure at the surface and a flat wave in
the mid and upper levels of the atmos will push through the
region by late this morning. Expect some very light snow or  
flurries to persist in many places until 13-14Z...followed by
slow improvement during the day as weak WNW llvl flow tries to
mix out the fairly deep moisture/thick cloud cover extending up
to 7-8 kft agl. 

Additional snow accumulation in the valleys will be at best 
1/4 inch in a few locations across Central PA and the Susq 
Valley, while the NW Mtns and Laurel Highlands could see another
inch by late this afternoon .

The strato cu deck will finally see some significant holes poked
through it late this afternoon and evening before the high, 
then mid-cloud layer from the impending weekend storm spreads 
in overnight tonight. The strato cu across the NW mtns and 
Laurel Highlands may not dissipate until the aforementioned 
higher cloud deck spreads in.

Afternoon temps will reach around the freezing mark across the 
northern and western mtns, while the central and SE valleys warm
into the upper 30s to around 40F (which will be 4-5 F above 
normal).

Lows tonight will vary from the mid-upper teens up north, to 
the mid and upper 20s in the south.


&&

.SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/...
*Major winter storm will impact central PA over the holiday 
 weekend followed by arctic blast into early next week
*Heavy snowfall of 1 foot or more likely north of I-80
*Significant snow and ice accumulation possible across south
 central PA 
*Dangerous wind chills between 10 to 25 degrees below zero 
 Sunday night into Monday 

Major winter storm system will take a Colorado low track from
the southern Rockies this morning to the New England coast by
Sunday. The storm will bring heavy snow and significant icing to
central Pennsylvania over the MLK holiday weekend. Frigid cold 
will develop behind the storm with dangerous wind chills Sunday 
night into Monday. 

18/00Z model spread continues to narrow, although slight errors
in storm track will translate to significant forecast challenges
regarding precipitation type and amount. The remaining speed and
track uncertainty is tied to the evolution of energetic southern
and northern stream troughs that will eventually phase and yield
a full latitude trough along the East Coast by Sunday. 

The latest GFS/NAM models are still a bit on the faster/more
progressive side of the guidance envelope and slightly north of
the ECMWF/CMC. A multi-model ensemble blend (NBM+WPC) is still
favored for temperatures/thermal profiles and precipitation 
(QPF, snow and ice) grids. Minor adjustments were made to storm
total snow and ice accumulation, with a slightly faster motion
and mixing potential reducing amounts by an inch or so in some 
areas. The track of the storm will play a critical role in 
determining ptypes and snow/ice amounts for this storm. 

There continues to be a very strong signal for heavy snow (12+
inches) in north central PA this weekend, and have upgraded 
counties north of I-80 to winter storm warning. We were keen to
ride the watch elsewhere. The high-end of the snow risk range 
in north central PA is between 15-20 inches based on the 90th 
percentile. 

The area of greatest uncertainty is on the southern periphery 
of the heavy snow between I-80 and I-70/76/81. The transition 
zone and is at most risk for boom/bust scenarios depending on 
the northward extent of warm air aloft. The far southeast zones 
especially southeast of US-30 should see the lowest snowfall in
the CWA and will likely change plain rain for a time late 
Saturday night. Significant icing is most probable over the 
southern tier of central PA with up to 0.25 possible in some 
areas. 

Brutally cold, arctic air will blast into the area behind gusty
25-35 mph northerly winds with falling temperatures during the 
day on Sunday. Low temps Sunday night in the single digits 
above and below zero. Gusty winds combined with the frigid 
temperatures will likely result in dangerous wind chills in the 
10 to 25F below zero range. Windy conditions may also 
contribute to significant blowing and drifting snow especially 
over the norther tier. We mentioned these additional hazards in 
the warning headline, but wind chills will probably need to be 
addressed separately once the time comes. The plummeting 
temperatures could result in a rapid freeze up and lead to 
additional travel problems.

&&

.LONG TERM /MONDAY THROUGH THURSDAY/...
Very cold temperatures continue on Monday with highs in the 
single digits to mid teens. NW winds will still gust into the
upper teens to lower 20s (KTS) through Monday afternoon which
will keep the wind chill 5-10 below zero across the mtns, and
around Zero C in the Central Valleys and Susquehanna Valley.

Temperatures will trend back to climo averages by midweek. 
Other than some lingering lake effect/upslope snow showers, 
Monday and Tuesday look fairly quiet with the next system fcst 
to arrive on Wednesday.

00Z EC appears to be latching onto/emphasizing a digging and
deepening southern stream upper trough diving into Texas 
and then the Gulf Coast Region during the midweek period before
turning quickly NE and taking aim on the mid atlantic region 
with another potential Nor'easter. 

00Z GFS and its EFS reflect more of a dominant and weaker 
northern stream wave bringing periods of lighter QPF across the
state.

Another blast of very cold air appears to visit us again for
next weekend. 

&&

.AVIATION /10Z FRIDAY THROUGH TUESDAY/...
09z update... 

Periods of very light snow will persist in many locations this
morning (and into the afternoon hours at KBFD and KJST). 

Flight categories will be MVFR (in the Central Valleys) while 
the Northern and Western Mtns will stay mainly IFR (Cig-wise) 
through 23Z today with generally MVFR visibility.

Surface winds will turn be at 5 kt early today, before shifting
to the W and NW this afternoon at 8-10 kt.


Outlook...

Sat...Widespread IFR in snow and mixed precip. 

Sun-Mon...IFR/snow early Sun ern 1/3. Bcmg windy. MVFR cigs and
snow showers NW 1/2. MVFR to low VFR cigs SE 1/2.

Tuesday...Mainly VFR.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Winter Storm Warning from 1 PM Saturday to 4 PM EST Sunday for 
PAZ004>006-010>012-037-041-042-045-046.
Winter Storm Watch from Saturday afternoon through Sunday 
afternoon for PAZ017>019-024>028-033>036-049>053-056>059-
063>066.

&&

$$
SYNOPSIS...Steinbugl
NEAR TERM...Lambert
SHORT TERM...Steinbugl
LONG TERM...Lambert
AVIATION...Lambert