Pre-Thanksgiving Snow, Central PA Styley

If you ask any meteorologist what type of weather phenomenon influenced their decision to pursue a career in the field, they usually answer one of snow storms, hurricanes, or thunderstorms and tornadoes. For myself, it was definitely winter snow storms. As a kid living in central Pennsylvania, I always hoped for the massive 1ft + snowfalls we saw once every five years or so. There were many disappointing seasons, but nonetheless I would stay up all night watching the weather channel, just hoping for that epic storm. My dreams came true during the 1996 and 2002 storms where our area saw closer to 2 feet of snow. I can remember waking up the next morning to snow drifts up to my roof. I was absolutely amazed. In my high school years we had a few storms that produced 8 inches or so but it wasn’t until I moved on to college that central PA got hit with big storms again. I was pissed. I did however get lucky last year when a low dumped 22 inches on top of Blacksburg, VA.

Interestingly, I am now partial to tornadoes, tropical systems, mesoscale, and climate controllers, with little interest in pursuing research related to winter storms. However, I never forget my roots. I still get giddy about the potentials for snow fall.

And thus I am psyched about the first serious potential snowfall on Wednesday. Although still some uncertainties on how far west the low tracks, where the rain/snow line sets up, and where the heaviest bands set up, I am going to make my predictions.

A high amplitude upper level trough is situated over the central US, diving as far south as the Gulf of Mexico, providing strong dynamics for a surface low to blow up off the southeastern coast. A potent shortwave from the Mississippi Valley will aid in the development. The low will track up the coast Wednesday spreading precipitation into PA starting around sunrise.

Screen Shot 2014-11-25 at 10.53.20 AM

Forecasted pressure and fronts for 7 am Wednesday. Low has developed off the coast. It will migrate northward throughout the day.

If you want snow, you want to be NW of the low pressure. This is the “cold” sector of the system. As winds whip counter-clockwise around the low, colder air is advected in from the NW. Central PA is perfectly situated in this sector.

The models have been in a little bit of disagreement as far as the track of the low pressure. The EURO model has been most robust on a more westward track of the low, and consequently has the most intense projected snowfall accumulations.  The NAM has recently shifted very far west and seems to be predicting a quicker transition to snowfall than the GFS model runs.

Regardless of technical model jargon and disagreement, the models are in enough agreement that we are guaranteed to see at least 1 inch of snowfall in central PA.

Here is the NAM model total snowfall depth forecast through Wednesday evening. This model run is based on a 10:1 snowfall ratio(10 inches snow for 1 inch rainfall). Because things may start off as rain/mix with a lag in the cold air advection, the chances of a 10:1 ratio are slim. This will be a heavy, wet snow, and we’ll more likely see ratios of 7:1 to 9:1. Furthermore, the models are in disagreement with how much moisture will be available. The NAM has been slightly more robust on PWAT values than other models.

12Z NAM 36 hour snowfall accumulation based on a 10:1 ratio

12Z NAM 36 hour snowfall accumulation based on a 10:1 ratio

Contrast the NAM model with the GFS snowfall output, also a 10:1 ratio.

12z GFS snowfall depth forecast through Wednesday evening using a 10:1 ratio.

12z GFS snowfall depth forecast through Wednesday evening using a 10:1 ratio.

The GFS is not as robust as the NAM, but still has us in a 4-6 inch range through Wednesday evening.

Some things to consider:

1. If the low tracks slightly off the coast, counties to the SE of the central PA will have better chance for more snow.
2. If the low hugs the coast a bit more, heavier bands could set up over central PA and we could see the higher amounts associated with the NAM model. Although, keep in mind, 10:1 ratio will most likely not be the case.
3. Will colder air than what is being depicted on models advect into the area earlier? If so, we could see a quicker transition to snow, thus upping the total amounts.
4. How much evaporational cooling and dynamic cooling will occur?
5. Will dry air aloft effect snowfall totals?
6. Temperatures were 70 degrees on Monday. Ground temps will be relatively warm, which would lend to wet roads at first, decreasing total depth amounts.

My prediction:

I am leaning towards the GFS with Central PA(Mechanicsburg, Harrisburg, Carlisle, etc.) experiencing 3-5 inches. I think early morning temperature profiles will lack cold air depth to support early morning snowfall, thus I’d expect a period of early morning rain/snow mix before transitioning to all snow in the late morning/early afternoon period. Furthermore, I don’t see 10:1 ratios holding steady, and I am leaning towards the lower QPF/PWAT values. I do however believe that some heavier bands will set up over our area early afternoon, dumping several inches of snow in a quick few hour burst.

You can live and die with the models. Hopefully the public will understand that winter forecasting is very difficult, especially when the models are not in agreement. This set up is relatively complex, and this leads to a difficult forecast. Hence why the NWS is going with 3-7 inches in our area.

The takeaway: If you have to travel tomorrow, do so as early as possible, or consider waiting until Thursday to travel. Any snow that falls tomorrow will quickly be gone on roadways as temperatures reach 40 on Thursday.

Be safe, and enjoy the first snowfall!



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