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Central PA - Jan/Feb 2019 Obs and Discussion


MAG5035

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Alight y'all. It's that time where I come in and make a forecast for my PA brethren. First off, this storm is quite the juicy storm with PWAT's running 1-2 standard deviations above normal across the south and into the Tennessee Valley. This moisture will advect northward into a solid polar airmass with low dew points capable of producing a sufficient wet bulb once the precip arrives. This will keep the entire region as snow for the morning with the only warm spot in the state in the SW corner due to the strong WAA on the western side of the Apps. Between 10-14z, snow will break out across areas from west-southwest to the east with the last spots to see snow east of the 83 corridor. Strong low and mid level frontogenesis will move into south-central PA and the Southern Laurels in the morning with heavy snowfall rates for several hours expected. The higher terrain around 99 will be the best area for coupled frontogenic forcing, as well as the orographic enhancements of the Allegheny's. Strong mid level front will motion to the northeast into CPA, but there will be a cutoff of the greatest forcing potential over the area, and that looks to lie just north of I-80. Further to the east, WAA regime will supply a solid area of snowfall, but the strength of the jet between 850-600 mb will eventually shift the focus over to sleet, and the question lies, how quickly can this occur. Now, given the rates associated with the 700mb frontogen, heavy snowfall might counter-act the initial warm surge aloft, allowing for a prolonged period of rimed aggregates to fall across Southern PA up to the turnpike. Models are beginning to the potential with even the longer range HRRR indicating the evolution further south into Central MD. I think what will occur is a very prolific thump for areas like HBG/THV/LNS with the best focus west of the Shenandoah. Sleet and ZR will ultimately win out by late afternoon and continue through storms end with plain rain a distinct possibility for areas east of I-83 and south of the turnpike.

My forecast is a gauge on the current trends in guidance and the overall synoptic setup evolving. I might be conservative in some spots, but I'd rather bust a little low. Accumulations will be snow/sleet and ice accretion. 

Laurels: 6-12" with 0.25-0.50" ice (Jackpot of up to 14" possible)

HBG down the 81 corridor to Chambersburg: 5-9" with 0.1-0.3" ice

MD Line from north of Hagerstown over to SE York Co: 5-8" with 0.1-0.25" ice

THV: 5-8" with 0.1-0.25" ice

Lancaster Co South of Pike: 3-6" with 0.1-0.25" ice

Lancaster Co North of Pike: 4-7" with 0.1-0.3" ice

Central PA north of Turnpike over to 99: 5-9" with 0.1-0.2" ice

I-80 corridor: 3-7" with T-0.1" ice

 

Hope everyone enjoys the snow! 

 

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27 minutes ago, Blizzard of 93 said:

The latest run of the HRRR 3k puts all of CTP in a great spot. This run ends at 18z tomorrow,& the simulated radar shows a few more hours of snow to go for most of the LSV.

4FC3A0FC-B7E9-45F0-8AAE-CD1C2D890A7A.png

68884D48-E18B-4CD3-9066-38CB13BF4F0E.png

now thats what Im talkin about.....

upper graphic looks best ive seen for LSV. 

Keep talkin dirty to me.....it's workin :lol:

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Im approve of this map.  Friggit.  Good enough for me.  I was never in the best region for snow, so lets do it.  This while including sleet, is still a nice CTP special.  And fwiw, you can take totals by 60% and thats about what I'd guess happens.  Time to go to meso/short rangers and watch da radar and look outside and enjoy every flake...whatever works for you. 

talk to ya'll in the morn.  Looks like were trackin more for next week and possibly beyond as well. Enjoy gang.

namconus_asnow_neus_12.png

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CTP disco the 50 Shades of Gray for weather nerds.

SHORT TERM /7 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.
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