Severe Thunderstorm Snow

[Brandon Schmidt]

Metar's in the central plains are showing "thunderstorm snow" with various severe thunderstorm warnings in affect. Wet bulb and surface temp both well below freezing. SPC has also outlined an area within an MCD noting this. Severe storms in a sub freezing environment, 0 cape, 20-50 lifted indexes...strong forcing though. Also the temperature gradient is extremely tight in through Oklahoma. How common it this...this is the first time I've seen anything like it as I've always imagined an actual severe storm but with snow but this is the first time I've seen it. Anybody seen it before?

 

[Jeff Duda]

I don't think I have ever witnessed widespread persistent severe thunderstorms behind a front, but elevated thunderstorms in the cold season (especially towards the ends of the cold season - November and Feb/Mar/Apr) are not at all uncommon. Keep in mind that these storms are not surface based, meaning the air parcels fueling the storms are not those coming from the surface. Surface parcels behind the front probably entirely have zero CAPE (or very little with a TON of inhibition). Thus, storms like this above a shallow and sharp polar/arctic air mass are being fed by parcels coming from the warm sector and being lifted over the cold air. As the cold air builds in like a wedge, the warm/moist parcels finally lift enough to condense and become buoyant. If these parcels have enough CAPE and some shear to work with, they can still become supercellular and drop very large hail. In a situation such as this, very little, if any, hail melt will occur as the stones fall through a cool, if not entirely sub-freezing, profile below the storm.

I personally have never seen observations of, nor eyewitnessed, snow falling from a severe thunderstorm, however. I suppose snow is created near the tops and anvil regions of supercells, but they always either melt or sublimate given the deep layer below the anvil that the hydrometeors must fall through to reach the surface.

 

[Elaine Spencer]

"Anybody seen it before?"

Don't know if this situation was exactly the same, but I do remember in February of 2003 a "squall line" of thundersnow passing through central Illinois along a cold front, and ILX issuing SVR warnings due to the high winds. I was sitting in a restaurant looking out a big picture window at the snow, wind and lightning when it passed through.

This particular storm also caused snow rollers to form along its path... there was a layer of existing snow on top of some ice, and the wind actually rolled some of that into snowballs. There are pictures of the snow rollers here:

http://www.crh.noaa.gov/ilx/events/roller/roller.php

 

[Stephen Levine]

I have seen such snow squall lines upon rare occasion along a strong arctic front. The most dramatic was decades ago when I lived in Pittsburgh and was in school. The sky turned green outside and whiteout conditions descended for about 5 minutes. Occasional lightning flashes decorated the scene. I also saw this once in S.E. Ohio and in the Boston area along the arctic front.

"Anybody seen it before?"

Don't know if this situation was exactly the same, but I do remember in February of 2003 a "squall line" of thundersnow passing through central Illinois along a cold front, and ILX issuing SVR warnings due to the high winds. I was sitting in a restaurant looking out a big picture window at the snow, wind and lightning when it passed through.

This particular storm also caused snow rollers to form along its path... there was a layer of existing snow on top of some ice, and the wind actually rolled some of that into snowballs. There are pictures of the snow rollers here:

http://www.crh.noaa.gov/ilx/events/roller/roller.php

 

[scott r currens]

Severe storms in a sub freezing environment, 0 cape, 20-50 lifted indexes...strong forcing though.

Those are surface base indexes so they don't tell the real story for elevated storms. I recall seeing MUCAPE (Most Unstable CAPE) values in the 500-1500 range several hundred miles west of the surface cold front.

 

[Michael Towers]

How common it this...this is the first time I've seen anything like it as I've always imagined an actual severe storm but with snow but this is the first time I've seen it. Anybody seen it before?

On the day you referenced I encountered a thunderstorm elevated above a shallow arctic air mass featuring a simultaneous mix of sleet and snow, marble hail (with a warning for quarter hail), frequent CGs and winds very likely in excess of SVR thresholds while driving on I-80 near York, NE. It really was an amazing combination of weather to witness, I’ve experienced “thundersnowâ€￾ before but never snow accompanied by hail & CG’s. This experience along with what happened that morning taught me a new lesson…be prepared for both winter and SVR weather phenomena (sometimes simultaneously) when chasing certain High Plains setups in early spring. That morning in Colby my truck was entombed in ice, the air temperature was 21F and freezing rain was being driven by winds gusting well over 40 mph. With no winter gear I was totally unprepared for what I encountered (an ice scraper & gloves would have certainly come in handy). The subsequent drive home was a treacherous mix of frosted windshields, icy/snowy roads and the aforementioned wintry thunderstorm...if only the chase the day prior had been so eventful…

 

[Dave Kaplow]

Not sure if this is relevant, but I remember a late season CT squall line back in the early ‘90s in which at the very height of the storm the precip suddenly flipped from heavy rain to snow. It snowed quite heavily for a few minutes, amidst some very strong wind gusts, and then switched back to rain for the remainder of the event. I don’t remember a lot about the details of this storm, not even the exact date, but it was notable for me because this was the ONLY time I’ve ever seen this happen. What I do remember is that it occurred at the very end or maybe even after the typical Southern New England thunderstorm season usually ends, in temperatures that were on the low side for severe weather. I know the event produced lots of wind damage across the region, and some people reported tornadoes, but IIRC all of those reports were found to have been caused by straight line winds. I don’t know for sure but I’m guessing this was a strongly forced convective line occurring in a very low CAPE environment, I say this because of the relatively cold temps and because I don’t remember hearing much thunder, although there definitely was some. I doubt the snowfall part of the event was very widespread, indeed it may have been limited to those few minutes I personally observed, possibly occurring in my location only. I don’t remember seeing anything about snow in any of the accounts that appeared in the local papers. I do recall the event got labeled a “freak storm†in the local press, and I suppose I can’t argue with that designation. It remains the one single time that I have ever witnessed genuine snow falling out of a warm-season severe thunderstorm.