Low top supercell convective snow squall

rdale

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Mar 1, 2004
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I struggle to understand how a supercell snowstorm would exist on this planet... What leads you to believe it was rotating? Was it a long-lived meso? Any radar data / surface obs / etc? When did this happen?
 
I didn't have radar data at the time I saw this. I was going on the appearance of a clear updraft base, an inflow band into the updraft and something that looked like a tail cloud appendage coming off the updraft base. There is also the fact that the other squalls did not have the discrete structure this squall had. That said, other than the rotation, I was assuming that a snow squall could in fact be a low top supercell. I've seen a low top supercell over the Jemez Mountains 40 miles southwest of the Truchas. It wasn't a snow squall, but it did not have any lightning and it did have rotation that was confirmed by radar and the ABQ NWS office:

http://mesophile.blogspot.com/2007/09/late-summer-rotation.html

But, that all said, I know I could be wrong, which is why I asked for the correction.

Thanks all for reading.
 
Feb 8, 2004
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From the AMS Glossary Of Meteorology:

supercell—An often dangerous convective storm that consists primarily of a single, quasi-steady rotating updraft, which persists for a period of time much longer than it takes an air parcel to rise from the base of the updraft to its summit (often much longer than 10–20 min).
Most rotating updrafts are characterized by cyclonic vorticity (see mesocyclone). The supercell typically has a very organized internal structure that enables it to propagate continuously. It may exist for several hours and usually forms in an environment with strong vertical wind shear. Supercells often propagate in a direction and with a speed other than indicated by the mean wind in the environment. Such storms sometimes evolve through a splitting process, which produces a cyclonic, right-moving (with respect to the mean wind), and anticyclonic, left-moving, pair of supercells. Severe weather often accompanies supercells, which are capable of producing high winds, large hail, and strong, long-lived tornadoes. See also convective storm, thunderstorm, splitting convective storm, cell, bulk Richardson number.
Essentially, there is no such thing as a snow-producing supercell. It's meteorologically impossible. Those photos show typical shallow winter convection.
 
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A few thoughts on this topic:

1. Nick, what is it in that definition that would make a snow-producing supercell impossible? It seems to me that the updraft of a convective snow squall could rotate just like any other convective updraft, if the wind shear profile is right. Seems to me that if it rotates long enough for a parcel to make it from the surface to the storm top, it is a supercell per that definition.

2. Jody, do you mean that the time was 0037Z March 1 (i.e. 5:37 MST Sunday, Feb 28)? Looking at the radar archive, it doesn't seem to show much snow shower activity near Truchas - seems to be a bit farther southeast at that time. Could the photo have been taken earlier than that?

3. Regardless of whether the cell Jody saw was a supercell, there was some VERY intense convective snow squall activity over NM late Sunday afternoon and evening. Take a look at this picture - be sure to read the description and then click to enlarge the picture:

http://www.younewstv.com/areas/kob/85786267.html

Also available here:

http://www.flickr.com/photos/34068123@N07/4396313359/

This was shown on Channel 4 in Albuquerque on the 10 p.m. news Sunday night. This is a shot I have always wanted to get, but not yet managed to - a CG strike (or indeed any shot showing an actual lightning streak) associated with thundersnow. IMHO this is a tougher shot to get than a tormado - thundersnow is a rare event (though NM is a good place to experience it - it has occurred on at least 3 days so far this winter, maybe more), and usually the actual streak of lightning is obscured by snow. Kudos to David Arnold on a great shot. And Jody, whether or not the cell you observed was a supercell, your pictures are very nice, too.
 

Jeff Duda

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I suppose theoretically it is "possible" to get a supercell that precipitates only snow, but it would very VERY unlikely. You'd have to have temperatures aloft to be cold enough to get enough CAPE to get a strong enough updraft to generate such a storm. The Albequerque, NM sounding from 00z 1 March shows only 145 J/kg of SBCAPE, but less than 20 kts of 0-6 km shear and less than 20 m2/s2 of SRH. I doubt such instability and shear could create a supercell. Not to mention, the freezing level was 3300 ft AGL, so any actual snow falling from under the cloud base would certainly have melted into rain or evaporated completely by the time it reached the ground. So technically, that wouldn't even be a snow-producing supercell.
 
2. Jody, do you mean that the time was 0037Z March 1 (i.e. 5:37 MST Sunday, Feb 28)? Looking at the radar archive, it doesn't seem to show much snow shower activity near Truchas - seems to be a bit farther southeast at that time. Could the photo have been taken earlier than that?
If it was earlier, it's only by an hour. My phone was in MST when the picture was taken, but could have been on PST.

It may be that the Truchas are in the shadow of Santa Fe Baldy or Lake Peak. That would explain why the storm didn't show up on radar.

I'm glad you like the pics, John. They were taken with my iPhone from Highway 68 on the way back from Taos, lighting courtesy of Mother Nature.
 
I suppose theoretically it is "possible" to get a supercell that precipitates only snow, but it would very VERY unlikely. You'd have to have temperatures aloft to be cold enough to get enough CAPE to get a strong enough updraft to generate such a storm. The Albequerque, NM sounding from 00z 1 March shows only 145 J/kg of SBCAPE, but less than 20 kts of 0-6 km shear and less than 20 m2/s2 of SRH. I doubt such instability and shear could create a supercell. Not to mention, the freezing level was 3300 ft AGL, so any actual snow falling from under the cloud base would certainly have melted into rain or evaporated completely by the time it reached the ground. So technically, that wouldn't even be a snow-producing supercell.
Thanks for posting the sounding information, Jeff. I would agree that those parameters would seem pretty weak for a supercell, although I am not entirely sure exactly what it would take in a cold environment.

As for snow making it to the ground, I can tell you that it snowed in Santa Fe at 0Z (I was out in it), and Santa Fe is at a lower elevation than Truchas (but 2000 feet higher than ABQ). It is not at all unusual in New Mexico and other places in the west for it to snow with the temperature above freezing. Yesterday, for example, it snowed in Santa Fe with the temperature in the mid-40s. If there is much snow, the temperature will often fall rather quickly as the downdraft associated with the snow brings colder air down to the surface. So, a sounding showing above freezing temperatures at 3300 AGL does not mean it couldn't snow, and also, 3300' above ground level in Albuquerque would be near ground level in Truchas.
 
Apr 29, 2004
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There are going to be some major microphysical differences due to the phase of precipitation in a snow thunderstorm which may affect the distribution of the precipitation and subsequent formation of a low-level mesocyclone and rear-flank downdraft. How? I have no idea, but perhaps someone tried to model this in the past for s**ts and grins and could elaborate.
 
I sent the photos and my description to the Albuquerque NWS office last night. The head forecaster Ken got back to me this morning. He's passing the images around the office to garner opinions, but he didn't reject my low top supercell theory outright. I'll let y'all know what he comes back with.
 

rdale

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That said, other than the rotation, I was assuming that a snow squall could in fact be a low top supercell.
Just wanted to clarify - you realize that a storm has to be rotating in order to be a supercell? Being low-topped and with good structure is not enough, it must have persistent rotation.
 
Dec 10, 2003
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Just wanted to clarify - you realize that a storm has to be rotating in order to be a supercell? Being low-topped and with good structure is not enough, it must have persistent rotation.
And that's the key.

It's just like spotting a tornado. You can have low scud features, inflow features that resemble a tornado, but if it's not rotating, it's not a tornado.

A supercell has to be treated the same way.

For this particular instance, there would have to be concrete confirmation of not just rotation, but persistent rotation - in this snowshower to be classified a supercell. Otherwise, it's just a darn good looking convective-type snowshower.

Love the pic, regardless of it's meteorological classification.
 
Jan 28, 2005
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Almost for sure, low top supercell convective snow squalls due occur in Michigan and other great lake states.

There is no doubt that the proper ingredients such as moisture, shear and CAPE would be in place during the cold season near the lakes. The lakes can generate tremedous CAPE from heat/moisture coming off the lakes and very steep lapse rates. (Just take a look at BUFKIT soundings during the late Fall. 1500-2000 lake induced CAPE values happen) Proper shear is more difficult to obtain because it would tend to spread available moisture out too much and weaken convection. Also adequate shear would generally cause lake induced convection to quickly move away from its source of heat/moisture so persistence requiements of a supercell wouldn't be met.

I can think of at least one situation where something like this would mostly likely occur.

Consider Lake Superior and the North shore of the Upper Pennisula which runs basically in a West to East Fashion. NW winds coming off Lake Superior..supply a continous supply of the required heat and moisture...and a West to East boundary that exists inland from the Lake caused either by a lake breeze or some other type of localized convergence..could create a persistent low topped supercell moving along the boundary.

Is there an example of this? Per "Michigan Weather" by Richard A. Keen...in 1987. "A lake-effect "snowburst" dumps 27.5" of snow on Munising on April 1-2, burying cars in 6-foot drifts. Less than 40 miles away at Grand Marais, only 1" of snow falls. As the storm gears up on the afternoon of the 1st, a brief tornado drops out of a snow squall and damages a mobile home on the south shore of Whitefish Bay".

Couple of things to note about this. The tornado is at least suggestive of a supercell. I don't think that this tornado would have been of the waterspout variety as Whitefish Bay would have likely still been completely froze over on April 1 and the South shore is at least 25 miles from Lake Superior. Munising and Grand Marias are in a line West of Whitefish Bay. Another likely necessary ingredient..no snow on the ground. Afternoon heating on bare ground would definately help the cause for more scattered/intense convection.
 
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Setting aside labeling for a bit, my understanding is that there is a basic underlying mechanism in question. That is where the downdraft associated with precipitation from a Cb is laterally displaced due to vertical shear enough so that it doesn't cut off the updraft feeding the cell. With or without directional shear IMO this mechanism is sufficient to perpetuate something that looks and acts like a single cell.

Now the downdraft pool created by liquid precipitation in non-supercellular vertical shear conditions pretty much always (from my amateur observation) eventually chokes off the updraft. However this isn't inevitably the case and what I'd call "training uni-cellular" storms can and do occur. I think the tail-end Mike of some squall lines has these characteristics, for example.

Frozen precipitation doesn't generate a downdraft nearly as intense and cold as liquid precipitation. So I'd suggest that such training uni-cellular storms aren't all that uncommon. I think I've seen them many, many times over the years, including a few years ago on my way from Santa Fe to Albuquerque one winter day. They can and do have enough shear to generate elevated funnels sometimes, but almost never have the right dynamics -- associated with supercells -- to produce a tornado. FWIW.
 

Jeff Duda

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Is there an example of this? Per "Michigan Weather" by Richard A. Keen...in 1987. "A lake-effect "snowburst" dumps 27.5" of snow on Munising on April 1-2, burying cars in 6-foot drifts. Less than 40 miles away at Grand Marais, only 1" of snow falls. As the storm gears up on the afternoon of the 1st, a brief tornado drops out of a snow squall and damages a mobile home on the south shore of Whitefish Bay".

Couple of things to note about this. The tornado is at least suggestive of a supercell. I don't think that this tornado would have been of the waterspout variety as Whitefish Bay would have likely still been completely froze over on April 1 and the South shore is at least 25 miles from Lake Superior. Munising and Grand Marias are in a line West of Whitefish Bay. Another likely necessary ingredient..no snow on the ground. Afternoon heating on bare ground would definately help the cause for more scattered/intense convection.
There are plenty of tornadoes that can be produced by non-supercell convection. Hell, you can get vorticies in clear air. If you get a tornado touching down along a lake shore when there is such a temperature gradient as to get convective lake effect snow, I sense there is a non-supercellular, non-mesocyclonic entity at work. I must agree with Mr. Dale and remain a skeptic until shown a concrete example.
 
I heard back from the Albuquerque NWS office:

As it turns out, my family and I were returning from Red River this past Sunday afternoon and witnessed several well-developed convective bands of mixed IP/SHSN just south of Taos, and then along North 14 between Madrid and Sandia Park. What struck me most was moderately strong outflow (enough to shake the Explorer) and a visually striking shelf-like cloud structure on the cells just east side of the Sandias. I snapped a pic on my cell phone, but it did not come out very good. The convection from my perspective was indeed shallow/squatty but well-developed. I did hear a few rumbles of thunder between 530-600pm. Hardly a low topped supercell.

Responses from other staff members also corresponds more toward low topped convection/convective snow bands in a shear environment but not supercellular.​
 
Jan 28, 2005
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Any radar examples? I just don't see the ability for a full-fledged supercell with snow. Could there be a little spin in a snow squall? I'd say so. But persistent? Doubter until proven otherwise ;)
That sounds likes a challenge! :) I will keep my eyes open for an example going forward.

Looking at this paper on waterspouts..it classifies waterspouts as being "tornadic" when the convective cloud depths exceeds 25,000 feet or so. I believe the author means supercelluar tornadoes are possible with cloud depths of that height along with the cooresponding temperature difference between the lake and water. http://www.essl.org/ECSS/2007/abs/07-Climatology/1178114308.keul-1-sec07.oral.pdf

That might be a little tough to meet except perhaps during the Fall..of course..then you might have the problem that our SP Supercell ( :) ) might actually be producing rain instead of snow!
 

Dave Marshall

I live on the coast of Lake Erie, where we get some pretty spectacular winter convection from time to time. Thunder snow/sleet is not uncommon here. I've always wondered about something like this, and this is how I've got it figured from my limited forecasting experience.

Convection can occur at any temperature, you only need an appropriately strong vertical temperature gradient. For a supercell, besides that convection, you need moisture (a problem in winter but not impossible. Lake effect, for instance) and a properly sheared wind field. If any of you guys have ever looked at a really wicked winter storm, the hodographs are often some of the most beautiful and deep curves I've ever seen, so I would think that's also possible. There are lots of other variables, but those are the big ones, so just for argument's sake, lets say it is theoretically possible.

My one big question is, given the fairly vigorous nature of the updraft in a supercell, even if the entire storm existed in a below-freezing environment, would it really be snow? I would almost think that the updraft would result in sleet or some other form of heavier precip, though I suppose if some of the precip formation happened closer to the top of the storm, it might not be quite so nasty an environment for a delicate snow fake, and could result in some snow/sleet/hail mix.

I've seen some pretty spectacular thunderstorms producing copious amounts of sleet, complete with CG and a gust front. What says it couldn't rotate in the right wind fields?

As an interesting side note, in December we had a pretty nasty winter storm come through. It went from the high 40s to the low 30s in less than 2 hours. A tornadic supercell fired on the leading edge of the cold front, just inside the warm sector, followed closely by a *violent* squall line that produced hail, sleet, and rain, along with tons of lightning and winds approaching 70mph. In the cold pool below the squall line, temps dropped into the low 40s/high 30s almost instantly. Pretty spectacular cold core sort of setup ;)
 
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Jan 28, 2005
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Haslett, Michigan
I live on the coast of Lake Erie, where we get some pretty spectacular winter convection from time to time. Thunder snow/sleet is not uncommon here. I've always wondered about something like this, and this is how I've got it figured from my limited forecasting experience.

Convection can occur at any temperature, you only need an appropriately strong vertical temperature gradient. For a supercell, besides that convection, you need moisture (a problem in winter but not impossible. Lake effect, for instance) and a properly sheared wind field. If any of you guys have ever looked at a really wicked winter storm, the hodographs are often some of the most beautiful and deep curves I've ever seen, so I would think that's also possible. There are lots of other variables, but those are the big ones, so just for argument's sake, lets say it is theoretically possible.

My one big question is, given the fairly vigorous nature of the updraft in a supercell, even if the entire storm existed in a below-freezing environment, would it really be snow? I would almost think that the updraft would result in sleet or some other form of heavier precip, though I suppose if some of the precip formation happened closer to the top of the storm, it might not be quite so nasty an environment for a delicate snow fake, and could result in some snow/sleet/hail mix.

I've seen some pretty spectacular thunderstorms producing copious amounts of sleet, complete with CG and a gust front. What says it couldn't rotate in the right wind fields?

As an interesting side note, in December we had a pretty nasty winter storm come through. It went from the high 40s to the low 30s in less than 2 hours. A tornadic supercell fired on the leading edge of the cold front, just inside the warm sector, followed closely by a *violent* squall line that produced hail, sleet, and rain, along with tons of lightning and winds approaching 70mph. In the cold pool below the squall line, temps dropped into the low 40s/high 30s almost instantly. Pretty spectacular cold core sort of setup ;)
I think when you are talking about "sleet" in regards to lake-effect..I think you are really talking about "graupel".

In the Fall, almost all the convective type snowshower activity I see includes at least some graupel if you look closely enough..and at times 100% graupel.(Graupel forms in supersaturated cold air near -13C..exactly what you have from lake induced convection.) - It wouldn't surprise me that anything convective enough in a cold pattern that it approachs supercellular characteristics-it would consist almost entirely of graupel.
 
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