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Wintry Machine Learning Mauler 1/30-2/1

WxDawgWx said:
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KCHS's new graphic for those interested. Admittedly a bit perplexed to see me at only 20% of .1 inch or greater, but whoever made this is a lot smarter than me.
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Thanks for posting that! I think the odds based on both history and the dryness/hit and miss setup mean that <50% chance for measurable makes sense with 20% seemingly reasonable. Model snow maps tend to overdo. They can show a few tenths and end up as just a T. Also, the ones that have snow nearby are dependent on SE moving narrow streams of snow that tend to dry up as they move SE due to lack of moisture source plus any one area has to get lucky to be under one of the bands.
 
Icon coming in much stronger with ULL
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Almost time to start dialing in liquid amounts with the CAMs. I’ve been in a handful of occlusions in the MA and NE, ~970 mb into Long Island Sound, subs 980s inside the B/M, and typically ~2" liquid equivalent is about the upper end. I certainly wouldn’t fret if I were in central or eastern NC, there’s going to be one hell of a deformation axis setting up shop, and wherever that pivots, jackpot.

Climo usually favors RWI–ECG–ORF, but to be honest there aren’t many case examples of an occlusion occurring ivo HAT, so we’re charting into some real unknowns. Dare I say it, 1980 may be the best analog.
 
GaWx said:
Thanks for posting that! I think the odds based on both history and the dryness/hit and miss setup mean that <50% chance for measurable makes sense with 20% seemingly reasonable. Model snow maps tend to overdo. They can show a few tenths and end up as just a T. Also, the ones that have snow nearby are dependent on SE moving narrow streams of snow that tend to dry up as they move SE due to lack of moisture source plus any one area has to get lucky to be under one of the bands.
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Yeah, can't blame them for being conservative. Would think that the ceiling has been raised for those of us on the periphery since 20:1 rates have been floated (assuming things stay modeled as is). Do you think that the best-case scenarios for us actually result in quite a bit of accumulation, albeit unlikely?
 
Expanding on convective feedback, something I've noticed on most models so far. I think it's a systematic error that is artificially depressing qpf values along the eastern half of the Carolinas/Virginia. 12z NAM has a great example:
1769632782359.png
That feature as modeled gunks up moisture transport and generally robs the eventually SLP (off myrtle beach) of ascent. SLP is much weaker in response.

The SLP and 500 mb trough are interlocked at a fundamental level in a feedback loop. A stronger SLP increases both warm air advection (raising heights ) to its east and cold air advection (lowering heights) to its west. With a stronger ridge and trough, more vorticity is generated, ascent is enhanced, the SLP continues to strengthen, and so on. I pay attention to it a lot during coastals.

I think January 3rd, 2018 is an instructive example. This is the last storm I can remember that had a "Bahamas low" problem. This was also a storm whose westward precipitation field was an open question (it was further west than modeled). I unearthed an old satellite loop.



You can see via the coldest cloud tops that convection surrounding the SLP won out pretty quickly. Getting that convection right quickly is important- stretching the low level vorticity across the baroclinic zone helps the SLP get a jump start. (I think this is a tip I learned from Webb, he linked a paper about it many years ago.)

I think CAMs will be better but even up until gametime will have some hot towers lead them astray. If this behaves like the storm in 2018, we should see more coastal enhancement at verification as the convection closer to the coast wins out. If the shape of this look holds out, then my bias at the coast will be aggressive. Hope I'm right.
 
18z ICON definitely took a step in the right direction. 6-10" for the Charlotte area and it looks like Moyock gets 15" on the 10:1 clown, so probably 20-30" with ratios. NE NC / SE VA gets crushed. It does kind of "skip" over the eastern Piedmont, which results in comparatively lower totals in the Triangle, however.
 
Bigedd09 said:
Brad P was talking about a band of heavy snow on the backside as the low goes off the coast


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Yeah that would make sense with the deepening coastal to see a deformation band. If you’re fortunate enough to get under the pivot point of it, you can get dumped on in a very short amount of time
 
WeatherNC said:
Almost time to start dialing in liquid amounts with the CAMs. I’ve been in a handful of occlusions in the MA and NE, ~970 mb into Long Island Sound, subs 980s inside the B/M, and typically ~2" liquid equivalent is about the upper end. I certainly wouldn’t fret if I were in central or eastern NC, there’s going to be one hell of a deformation axis setting up shop, and wherever that pivots, jackpot.

Climo usually favors RWI–ECG–ORF, but to be honest there aren’t many case examples of an occlusion occurring ivo HAT, so we’re charting into some real unknowns. Dare I say it, 1980 may be the best analog.
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For those interested, here is a link to the March 1980 storm WeatherNC mentioned above. Good read.

 
packfan98 said:
The map I posted was from their special weather briefing video that they put out to local agencies. Not sure if RNK did a special briefing.
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I was surprised they said they’re not going to put out a watch until tomorrow.
 
BIG FROSTY said:
RNK doesn't even have a snowfall map for this time period. There's ends at 7am saturday, Why is that??
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Without hitting banter territory. RNK is often very conservative and only updates graphics when the forecast period switches (usually 5pm it switches from this afternoon tonight).

Usually when there us big snow it goes from a “Chance” to 7-8” or whatever about 24 hours out.
 
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