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Area Forecast Discussion (AFD)

Issued by State College, PA (CTP)

                            
000
FXUS61 KCTP 200945
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
445 AM EST Wed Feb 20 2019

.SYNOPSIS...
A winter storm will bring snow, ice and difficult travel to 
central Pennsylvania on Wednesday. Conditions will improve into
Thursday as a breezy west wind directs drier air into the region.
Fair weather will continue into Friday before another round of 
precipitation over the weekend. It will turn windy and colder 
Sunday night into Monday with little to no precipitation
expected early next week. 

&&

.NEAR TERM /THROUGH TONIGHT/...
*Significant snow and ice accumulation for south-central PA 

Robust isentropic lift/theta-e advection behind very moist 
southerly flow and synoptic ascent associated with 150KT jet 
streak to the north will produce widespread wintry precipitation
across central PA today into tonight. The leading edge of 
precipitation (snow) at 0930Z has lifted north of MD line to 
the PA TPKE over the southwest mtns and should reach the US-322 
corridor around 13Z.

Thermal profiles are cold enough for all snow to start the 
event. Hires model guidance continues to indicate a heavy burst
or "thump" of WAA snow across south-central PA this morning. 
This initial, ~3-6hr snow burst will be driven by a brief but 
intense period of low to mid level FGEN. The 00Z HREF 
confirms high probability of 1-2 in/hr snowfall rates in the 
12-18Z window. The duration of intense snow will be somewhat 
limited, as the warm nose aloft will lift northward turning 
precipitation from snow to ice (sleet/freezing rain) in most
places this afternoon. 

One concern is guidance may be too quick to warm the column due
to reinforcement of the wedge by initial evaporative cooling 
(09Z dewpoint depressions running 10-15+ degrees) and falling 
precipitation (moderate to heavy snow). This would be a boom 
scenario for snow totals and would add a couple of inches on to 
current fcst. This is also important from a winter headline 
perspective as it could push storm total amounts from advisory 
to warning levels. Conversely, a quicker change or mix with 
sleet would trim snow totals. Precipitation will changeover from
snow to ice from southwest to northeast leading to a prolonged 
period of freezing rain across a good portion of south-central 
PA especially the higher terrain/ridges from the Laurel 
Highlands into the southern Alleghenies. 

The heaviest snowfall of 6-8" in fcst over far south-central PA
just north of the WV/MD panhandles. 3-6" is projected 
throughout the lower Susquehanna Valley and along the I-80 
corridor. 1-3" is fcst over the northern tier with the lowest 
amounts along the NY border. The heaviest freezing rain is 
likely in the terrain zones mentioned above, where a risk 
exists for >0.25" of ice accretion even after the snowfall. 

A second wave of heavier QPF appears likely in the 00-06Z Thu
timeframe which would fall mainly as freezing rain. This could
push some of the advisory area close to warning level freezing 
rain/ice accumulation. Precip intensity will decrease btwn 
06-09Z with west wind shift drying things out east of the 
Alleghenies. The shift to upslope flow favors the highest POPs 
over the western ridges into the predawn hours Thursday. 
Temperatures will slowly climb above freezing overnight with 
light rain becoming the most likely/dominant ptype by 12Z 
Thursday.

&&

.SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/...
Breezy and dry wx expected on Thursday with temps rebounding
into the 40s. High pressure returns Thu night into Friday
bringing fair and dry weather. 

Another shot of WAA precip into a retreating cold sector 
with departing sfc high pressure is likely heading into the 
weekend. Timing will be important with a period of snow or ice to
start the weekend over south-central PA. 

&&

.LONG TERM /SATURDAY THROUGH TUESDAY/...
Aside from an initial period of mixed precipitation early
Saturday, expect periods of rain throughout the weekend.
Temperatures will trend milder Saturday into Sunday with
fcst highs in the upper 40s north to near 60F in the LSV. 

It will turn windy and colder into early next week. Model
guidance suggest advisory level wind gusts are possible Sunday
night into Monday. The colder NW flow should produce snow
showers downwind of Lake Erie. Otherwise, the wx looks mainly
dry Monday and Tuesday. 

&&

.AVIATION /08Z WEDNESDAY THROUGH SUNDAY/...
No big changes to the late evening TAF updates.

Earlier discussion below.

For the 00Z TAFS, looking at VFR conditions overnight with
mid and high clds.

Snow moves in from southwest to northeast from just before
sunrise at JST, to other areas by mid to late morning.

Sleet and freezing rain becomes the problem by afternoon, as 
the warm front nears. Conditions will become poor with LLWS
as well.

The precipitation becomes lighter Wednesday night into early
Thursday morning.

Outlook...

Wed...Snow (ocnly heavy during the late morning and early
afternoon) changing to to a wintry mix, then mainly freezing 
rain before nightfall.

Thu...AM light freezing rain/low cigs possible early.

Fri...AM low cigs possible W Mtns.

Sat...PM rain/low cigs possible.

Sun...Showers, mild, and windy. Low conditions possible.

&&

.CTP WATCHES/WARNINGS/ADVISORIES...
Winter Weather Advisory from 10 AM this morning to 6 AM EST 
Thursday for PAZ004>006-037-042.
Winter Weather Advisory from 7 AM this morning to 6 AM EST 
Thursday for PAZ010>012-017>019-041-045-046-049>053-058.
Winter Storm Warning until 6 AM EST Thursday for PAZ024>026-
033>036.
Winter Storm Warning from 6 AM this morning to 6 AM EST 
Thursday for PAZ027-028-056-057-059-063>066.

&&

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