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PHL CWA Feb 1-2 Storm OBS and Discussion


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It comes down to the depth/strength of the warm layer, and whether it can fully melt all solid parts into a rain drop. If that happens, and the bottom cold layer is never below -8ºC, then you will maintain a supercooled droplet since it cannot form back into a solid. If a little solid ice makes it through the warm layer, then whatever part has melted to liquid can refreeze around that into a sleet pellet when its below 0ºc. In the case where it fully melts in the warm layer to a droplet, it can refreeze into an ice pellet only if the surface cold layer reaches below -8ºc (pretty rare in these setups).

Technically incorrect. You're confusing kinetics with thermodynamics. Supercooled water droplets will eventually freeze into the thermodynamically-favored lower energy state of ice at 32F, but it typically takes a long time (slow kinetics or rate of freezing) for it to do that in the absence of a cold (<32F) solid surface or ice crystal, i.e., far in excess of the time it takes for the supercooled drop to traverse the cold layer of air above the surface. The drop will, however, freeze instantly upon contact with a solid surface that is below 32F, as the solid surface acts exactly like an ice "seed" particle for the supercooled water droplet to reach the thermodynamically favored solid state of ice.

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Snow/sleet amounts dropped south of I-78 (and rightfully so) by the NWS...they can probably cut those numbers in 1/2 again in those areas.

too much mixing or not enough precip altogether?

FWIW the HRRR keeps us all in 1-2" overnight. The main WAA show wasn't even supposed to start until after 06z.

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too much mixing or not enough precip altogether?

FWIW the HRRR keeps us all in 1-2" overnight. The main WAA show wasn't even supposed to start until after 06z.

I think a little of both (for us near and south of the PA Turnpike). I'd be shocked to see 2 inches in our necks of the woods honestly, but I can and have been wrong plenty this winter. I could see a solid covering to an inch though if the radar stays like it is off to the west. That main slug of precip. in the Ohio Valley should pass mainly to our north overnight.

Here's the 00z GFS between 6z-18z

gfs_p12_018s.gif

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I think a little of both (for us near and south of the PA Turnpike). I'd be shocked to see 2 inches in our necks of the woods honestly, but I can and have been wrong plenty this winter. I could see a solid covering to an inch though if the radar stays like it is off to the west. That main slug of precip. in the Ohio Valley should pass mainly to our north overnight.

Here's the 00z GFS between 6z-18z

Yeah I'm leaning closer to the 1" for MBY (and my 2-3" is going to bust badly but oh well) but I think 0.7" or wtv the NWS has might actually be too low...

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It'll probably be 3-6" north of I-78 and 1-3" south of I-78. Of course I'm right along I-78 where bust potential is higher than most places.

If you get that slug of moisture from the Ohio Valley later tonight, you should get several inches, if it grazes by to your north.....maybe not.

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bufkit ice totals for select locations for 00z NAM

RDG- .84" frz rain, .02 sleet, .2 rain

ABE- .96 frz rain. .04 sleet, .12 rain

PHL- .22 frz rain. .04 sleet 1.05" rain

AVP- .85 frz rain, .15 sleet no rain

That would be the biggest and most damaging ice storm in years for this area taking the NAM verbatim.

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