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Freezing Rain, Sleet, Snow, Rain...Kitchen Sink (12/15-16)

[mention]iGRXY [/mention]

This is more in line what I expect. I would have done one of those maps, however just been busy with Christmas and work/school.

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BirdManDoomW said:
Brad P now saying could start as flake of snow for Charlotte and sleet. With ZR in the trees for Charlotte. Then cold rain.
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We're definitely not getting even so much as a flurry in Charlotte w/ these 850s. If any frozen precip occurs it might briefly be sleet near the onset before changing to rain and/or freezing rain

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Tarheelwx said:
It seems they should include parts of NW NC in the risk catagory. I think it will all boil down to how much precip falls before the temps rise to 33.
TW
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If they did then offices would be jumping the gun too quick with watches/advisories. It’s what they use. Not specific model runs that people think they do. It’s a tough call where the watch vs advisory should be. My guess is advisory everyone below Wilkes and warning north.
 
Another example of a laterally propagating frontogenetically forced band of heavy snow that couples to the rain/snow line >>> hence the old adage you have to "smell the rain (or sleet)" to get really good snow. Well, this is probably why:

Areas on the leading edge of the warm nose w/ max temps in 0-+1 C range (dark green in the first image below) experience latent heat of melting from snowflakes falling into the leading edge of a warm nose that offsets warm advection, while other places both back into the cold air and deeper in the warm sector continue warming unimpeded from advection. This enhances the local temperature gradient there and creates a natural zone of baroclinicity and enhanced frontogenesis in the region where the max column temps are near-just above freezing that allows for a frontogenetically forced band of intense snow to couple w/ the rain-snow line. This leads to the stereotypical front-end thump of snow followed immediately by mix &/or rain evolution we see so often in winter storms down here.

This storm is another good example of such a phenomenon.

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If some of these short range models are right with not even a cloud in the sky until 7am Wednesday...that would help bring a burst of SNOW along the Highway 421 corridor. All cold and dry then moisture pounces on it before the warm nose takes over. There’s other reasons to believe this too...it’s showery weather coming from offshore South Carolina...no big cloud field to keep us warm overnight. I know..the stars have to align for this to happen..but I wanted to explain why some of the in-house models that TV stations use show all snow initially up this way...even the cmc. Dry cold cad is known to eat away at cloud advancement too. A lot of factors maybe a surprise could happen further east on highway 421 near Winston.
 
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RGEM in past winter storms has a bit of a cold bias. Also if temps are in the 31-32 range like the NAM shows with moderate to heavy zr the accrual rate will not be very good. ZR accumulates best when rates are light and/or temps are 30F or colder. If I remember correctly, the heavier rates of zr can also cause latent heating which warms the surface above freezing quicker vs a lighter rain.
 
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snowlover91 said:
RGEM in past winter storms has a bit of a cold bias. Also if temps are in the 31-32 range like the NAM shows with moderate to heavy zr the accrual rate will not be very good. ZR accumulates best when rates are light and/or temps are 30F or colder. If I remember correctly, the heavier rates of zr can also cause latent heating which warms the surface above freezing quicker vs a lighter rain.
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Its lighter stuff like this which is honestly the bigger problem, it can make up for surface temps around 30-32 889FBB08-8E8F-4E96-AA27-675C3FAFB423.png
 
I wouldn't be entirely shocked to see a brief period of freezing drizzle or light freezing rain for some areas north and west of downtown Raleigh. It's really close at onset with wetbulbs around 30-32 but quickly pushes above freezing
 
snowlover91 said:
RGEM in past winter storms has a bit of a cold bias. Also if temps are in the 31-32 range like the NAM shows with moderate to heavy zr the accrual rate will not be very good. ZR accumulates best when rates are light and/or temps are 30F or colder. If I remember correctly, the heavier rates of zr can also cause latent heating which warms the surface above freezing quicker vs a lighter rain.
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Brad P made a post about ZR being a self-limiting process. Heat leaves the water as it freezes and warms the air around it, so the heavier rates would heat the ground level up much faster than lighter ZR.
 
snowlover91 said:
RGEM in past winter storms has a bit of a cold bias. Also if temps are in the 31-32 range like the NAM shows with moderate to heavy zr the accrual rate will not be very good. ZR accumulates best when rates are light and/or temps are 30F or colder. If I remember correctly, the heavier rates of zr can also cause latent heating which warms the surface above freezing quicker vs a lighter rain.
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This is correct but we aren’t looking at heavy rates with this system. Light to moderate is about the most you’ll likely get from this system.
 
FamouslyHot said:
Brad P made a post about ZR being a self-limiting process. Heat leaves the water as it freezes and warms the air around it, so the heavier rates would heat the ground level up much faster than lighter ZR.
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The lighter precip occurs near and prior to the onset of the event when global or even regional high res NWP models can’t and often don’t properly resolve it. Even when you consider sub cloud evaporation, I wouldn’t be surprised if freezing drizzle and/or light sleet started being observed in the Piedmont significantly earlier than forecast perhaps as early as midnight tomorrow night given the mid level warm advection and isentropic upglide that starts showing up by 4z. This underrepresented precipitation thru diabatic processes (mid level warming thru condensational deposition and low level evaporation) also would help “lock-in” the CAD dome ever so slightly sooner, potentially providing a few additional hours for low level cold advection to work its way down into the Carolinas


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Notice that precip on the 3km NAM doesn’t start breaking out for another 4 hours after this frame above. The aforementioned information and prior experience in these events supports this idea but we’ll see

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The NAMs warm nose is stout throughout nearly the whole event. EURO and RDPS it there but more marginal. QPF here is between .8 and 1.0 between the 3 of them. NAM may start as a little IP but seems all ZR if not going to plain rain. The other 2 are more of a mix with RDPS switching back to snow late. I am betting on the NAM but not expecting the temp to recover all the way to above freezing unless we bust high Tuesday night.
 
From past expierence. Precip always gets going sooner in these events as webb alluded to. 4-6 hrs not uncommon in 9 out 0f 10 cases from past expierence.

Also never, ever disregard the Nam thermal profiles. Look deep into it. It's not always accurate , but usually can sniff out the hidden piece of info in these events.

Those canadian maps I posted are usually your coldest, but not by much. They show the possibility if the CAD is a degree or 2 stronger or stays longer. Thats never a given but always a possibility because models have a tendancy to always underestimate both.
 
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