Low top supercell convective snow squall

[Jody Radzik]

Please correct me if I'm wrong, but I believe this to be a low top supercell snow squall that formed over the Truchas Mountains in the Sangre de Christo range of the Southern Rockies, 40 miles north of Santa Fe, NM.

 

[rdale]

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?

 

[Lanny Dean]

Jody,
can you tell us why you believe that or maybe provide some dates/data.

 

[Jody Radzik]

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.

 

[nickgrillo]

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.

 

[Robert Dewey]

It's definitely a nice looking structure, but I agree with Nick and Rob about the actual classification of it being a 'supercell'

 

[Clarence Bennett]

Give us the approximate time and date of this photo. We can then pull the radar data and look at velocity. Makes it real simple...anything is possible.

 

[Jody Radzik]

The second picture was taken 12:37:06 a.m. Monday March 1, 2010 UTC.

 

[John Farley]

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]

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.

 

[Jody Radzik]

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.

 

[John Farley]

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.

 

[Greg Stumpf]

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.

 

[Jody Radzik]

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]

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.