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Area Forecast Discussion (AFD)

Issued by State College, PA (CTP)

                            
000
FXUS61 KCTP 200329
AFDCTP

Area Forecast Discussion
National Weather Service State College PA
1029 PM EST Tue Feb 19 2019

.SYNOPSIS...
A winter storm will bring significant snow and ice to central 
Pennsylvania on Wednesday. Dry and windy weather is forecast 
Thursday, with fair weather continuing into Friday. The next 
weather system will return precipitation to the area over the 
weekend.

&&

.NEAR TERM /UNTIL 8 AM WEDNESDAY MORNING/...
Look at newest NAM and HREF info gives a little more concern for
a possible move upward in snowfall and ZR totals, esp in the 
SW. NAM liquid between 15 and 18Z is >0.60" but that is right 
around the time the transition to sleet should be occurring 
there. Also, the 0.30" mean QPF in the 00Z-03Z time frame is 
concerning, since much of that could be ZR. Will hold on any 
changes for the time being and allow next shift to get more of 
the 00Z runs in and confab with the neighbors before making 
moves.

Prev...
High clouds thickening up. Precip still 300mi to the south. But,
it is on track. Nudged the snow numbers before 12Z up just a
little, as all models very wet and 1"/hr rates are possible  
across the Laurels just before sunrise. Left the rest of the 
periods alone, but latest HREF progs have things staying snow 
just an hour or two longer, and that could mean another 1/2" to 
2" over what we have going in the srn tier before the sleet 
comes along to compact it. 

Prev...
A sprawling area of high pressure (1038 mb) was centered over
southern New York and stretched westward to Wisconsin this 
morning, with varying amounts of thin cirrus streaming NE across
the Commonwealth. High clouds will thicken-up from the 
southwest through early tonight.

1930Z temperatures are within a deg F or two of their 
forecast maxes ranging from the mid 20s across the north to the
mid 30s in the Lower Susq Valley. The lack of wind under the 
dome of high pressure made it feel a bit warmer in the sun
today, compared to Monday that featured a brisk wind.

Clouds will gradually lower and thicken tonight as high 
pressure retreats into New England. A strong llvl cold air
damming signature will develop east of the Alleghenies south of
the high center. Hires models and ensembles spread WAA front- 
end snow into the Laurel Highlands and south central mtns by 
daybreak Wednesday with slippery travel for the AM commute.

The leading/northern edge of the snow shield will be near the RT
22/322 corridor in South-Central PA around 12Z Wednesday.

&&

.SHORT TERM /8 AM WEDNESDAY MORNING THROUGH WEDNESDAY NIGHT/...
*Snow and icy mix will result in travel disruptions Wednesday 
 into Wednesday night 

The CAD pattern will set the stage for significant snow and ice
event for south-central PA on Wednesday. Strong low-level
theta-e advection of rich GOMEX moisture and favorable mid to 
upper level dynamics will support a broad area of wintry precip
overspreading the area Wednesday morning. 

Across the southern half of Pennsylvania, any light snow within
an hour of the onset (between 11Z-14Z Wed) should transition to
a 3-5 hour period of moderate to heavy snow with snowfall rates
of 1 to 2 inches per hour, thanks to strong upper divergence 
and low-mid level FGEN. This area of high prob GRTN 1 inch per
hour lingers the longest across the South-Central Mtns and Lower
Susq Valley, before shifting east of our SE CWA during the mid 
to late afternoon hours. 

Somewhat similar to the Nov 15th, 2018 Winter Storm, this
upcoming one features hefty amounts of moisture, and Max Wet 
bulb temps only creeping a deg or 2 C above zero in an approx 2
kft thick layer (up around 7-9 kft AGL) for the early and mid 
afternoon. Strong UVVEL may cool this slightly above freezing 
layer (or it may be only deep enough to make the snowflakes 
sticky) and delay the changeover to sleet during peak intensity.
This creates a potential error in snow amounts of 2-3 inches on
the low side should the changeover not occur until late 
afternoon across Central PA and the Susq Valley. 

The lower probability of a quicker changeover could trim 
current snow totals by an inch or so. 

Hires model data and NBM maxTw aloft indicate that snow will 
transition to an icy mix of sleet and freezing rain during the 
afternoon/evening before ending (possibly as freezing drizzle) 
late Wednesday night as the thicker mid/high seeder-feeder cloud
deck peels off to the east.

&&

.LONG TERM /THURSDAY THROUGH TUESDAY/...
Any lingering precipitation should end Thursday morning as 
increasing westerly flow directs drier air into the area. There
remains a slight probability for lingering showers early
Thursday though any amounts will be light. Colder air drops back
down into the region and high pressure builds in for Thursday 
night.

The regional temperatures will be around normal through the 
latter half of the week. Dry wx continues on Friday before the 
next Plains- Great Lakes storm system spreads precipitation back
into the area over the weekend. The models have a similar track
and consensus, especially given the strong high pressure out 
ahead of it. The storm track/pattern looks somewhat similar to 
the midweek event, but with stronger/primary low and weaker sfc 
high/CAD. This would favor FZRA or plain rain but will wait and 
see how the ptype details evolve in the coming days. 
Temperatures could get quite mild and break into the 50s on 
Sunday before turning breezy and cooler by early next week.

&&

.AVIATION /03Z WEDNESDAY THROUGH SUNDAY/...
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 Wednesday to 6 AM EST 
Thursday for PAZ004>006-037-041-042-046.
Winter Weather Advisory from 7 AM Wednesday to 6 AM EST 
Thursday for PAZ010>012-017>019-045-049>053-058.
Winter Storm Warning from 4 AM Wednesday to 6 AM EST Thursday 
for PAZ024>026-033>036.
Winter Storm Warning from 6 AM Wednesday to 6 AM EST Thursday 
for PAZ027-028-056-057-059-063>066.

&&

$$
SYNOPSIS...Lambert
NEAR TERM...Dangelo/Lambert
SHORT TERM...Lambert
LONG TERM...Ceru/Steinbugl
AVIATION...Martin