Differences in Severe Weather Forecasting at Higher Elevations

[Mikayla Norris]

This is my first chase season in Colorado, having lived on the Southern Plains for the last several years. I finally got out for my first chase up here on 23 April and saw a beautiful supercell that even garnered a tornado warning (target area writeup should be up in the next day or 2 as of the writing of this post). The questions I have are concerning severe weather forecasting up here at 6000+ feet. On the 23rd, I relied almost exclusively on CAMs rather than individual parameters, mostly because when I looked at the individual parameters, none of them looked particularly impressive, at least not compared to what I am used to in the Southern Plains. If I had seen similar parameters when living on the Plains, I would have likely stayed home. I more or less taught myself what to look for in the models through this forum and youtube. In fact, I still review Skip Talbot's forecasting videos periodically. Obviously the main ingredients for supercells would be the same here as anywhere else, but I suspect that the actual amounts of said ingredients (and the sources of those ingredients) to get the job done may be different. I'm mainly trying to figure out things like dewpoint, lift, and shear thresholds, forecasting the effects of terrain and upsloping, etc. So does anyone have any good insight or resources into what to look for in the model data for higher elevation chasing? Or am I just way overthinking this? I appreciate any help y'all can offer!

 

[Harlan U]

I'm NOT an authority on the matter but have studied up on a lot of Colorado severe weather climatology. The SPC's Tornado Environment Browser (Tornado Environment Browser) has some great info on the distribution of parameters for tornado events, although it is a bit outdated, with the most recent end of its database only being in 2011. You're absolutely right about the "lower threshold" parameters for severe weather being much lower here, as you probably experienced yesterday under a fairly legit supercell with only ~45 degree dewpoints.
A lot (perhaps even a majority) of Colorado tornadoes are landspouts. They can form under much different conditions than supercell tornadoes and occur commonly with high-based storms. I suspect that Colorado's severe parameters are skewed more to the marginal side as a result of the prevalence of landspouts.
In my personal opinion, based on an brief analysis of past tornado events, 45 degree dewpoints seem to be the minimum required value for any chance of tornadoes. I personally like to see dewpoints in the 50s before I'll chase anything but you can get some stuff at the edges of the season (like now, in April) with very marginal moisture. A video by Pecos Hank

provides some useful info for chasing in Colorado, and also establishes a "minimum" of ~50 degrees for dewpoints. Also, you should probably look for dewpoint depressions less than 25 degrees, and ideally below 20 degrees.
I personally like to see effective shear greater than 35 knots, although Colorado has certainly had tornadoes with much less.
I must admit that I don't give very much weight to CAPE of any kind when chasing here. The Tornado Environment Browser shows a tornado event occurring with ~0 J/kg of MLCAPE at some point in the historical record. 500 J/kg of MLCAPE is generally enough to get things done here in my opinion.
I pay special attention to LCL height as it can be useful for determining if storms will be high based or not. If I recall correctly, it has also been identified by researchers in the past that low LCL heights favor tornadic supercells. In Colorado, you may want to see an LCL height of <1750m for tornado potential, and ideally below 1500m.
Since you're in Colorado now, you should know about the DCVZ/Denver Cyclone and the various terrain effects of the Palmer Ridge. Some good reading can be found at The Denver Convergence-Vorticity Zone - From A Storm Chaser’s Perspective and Tornadoes in Denver: The Denver Convergence-Vorticity Zone (DCVZ) and Denver Cyclone.
Some further info can be found at the Colorado Climate Center's severe weather climatology page (Colorado Climate Center - Severe Storm Reports) and in a few papers by local atmospheric scientists (https://journals.ametsoc.org/view/journals/apme/58/10/jamc-d-19-0098.1.xml) (https://climate.colostate.edu/pdfs/Climatology_of_Colorado_Tornadoes.pdf).

I would like to state again that much of this info is purely my opinion and that I am by no means any kind of an authority on the matter. ST has a number of very experienced members from Colorado and maybe they'll have more accurate info on these things.

 

[John Farley]

Harlan covered it quite well. One thing I would add regarding terrain effects is that you can get similar effects to those of the Palmer Divide farther south with the Raton Mesa, along the CO-NM state line east of Trinidad and Raton.

 

[JamesCaruso]

For some context on why Colorado can get it done with a lower dewpoint compared to areas to the east - Dewpoint is not a completely objective measure of moisture in the atmosphere; it is also affected by pressure. If the same amount of moisture exists in Oklahoma City and in La Junta CO, it will result in a lower dewpoint in La Junta because of its elevation and associated lower air pressure. So that’s why the lower dewpoint is still “enough” in CO: the moisture is there, it’s just the impact of pressure on the dewpoint calculation itself.

(The more objective measure of moisture, which is independent of pressure, is the mixing ratio.)

One thing I wonder about and don’t know enough to answer - why, then, is a sub-20 degree temperature/dewpoint spread still needed for tornados in CO? The dewpoint is pressure-dependent, so a lower dewpoint in CO still represents sufficient moisture content. But the temperature is not pressure-dependent. So one would think a higher temp-dewpoint spread can still work in CO, but I don’t think it does…

As a related aside about moisture metrics, it always frustrates me that “relative humidity” is still used so often in mainstream forecasts and by non-weather enthusiasts. Temperature is also a variable in RH. A 90-degree day with a 65 degree dewpoint is clearly more uncomfortable than a 75 degree day with the same dewpoint, yet the hotter day will have the lower RH. I wish more people were indexed to the dewpoint! (I got my father-in-law to pay attention to it ) Dewpoint is a more objective measure of moisture in the atmosphere than RH. Similarly, mixing ratio is even more objective than dewpoint because it takes pressure out of the equation.

 

[Mikayla Norris]

Thanks for the great responses everyone! To the point about dewpoint, that was probably the metric throwing me off the most. Like I was sitting there looking at a sculpted tornado warned supercell with my own eyes, but temps were only in the 50s with a dewpoint barely in the 40s (sub-20 spread now that I think about it). Inflow was cranking, but it was chilly. I've never been on a chase standing outside my car with my camera shivering before. I was baffled to say the least

 

[Boris Konon]

Also note that decent supercells can occur *behind* a weak cold front as the E and NE low-level winds promote upslope flow and can advect higher dew point from the lower elevations to the E.

With the lee-side low pressure the develops E of the Front Range, directional shear is often very good, even if speed shear is not, and you can get supercells with what appears to be not-so-favorable at first glance. For instance, 20 kt SE winds at the surface and 20 kt NW winds at 500 mb, at 180 deg difference, that's 40 kt of deep-layer shear, enough to support supercells.