Structure Chasing Parameters

[David Williams]

Hi all,
I have been predominately a tornado chaser in the past and all my attention and energy was poured into that. However, this last season, our tornado drought helped turn my attention to storm structure. I was impressed with a lot of the structure shots I saw from chasers and my interest in chasing structure eventually piqued. So, what are the environmental and thermodynamic parameters differences between tornado days and structure days?

 

[JamesCaruso]

It is a question of storm mode. You've got to have isolated/discrete supercells, as opposed to embedded supercells. LP and classic supercells generally have the most photogenic structure. Discrete HP storms can also look impressive but typically you will need to be further out for a structure view, whereas LP and classic supercell structure can be visible even very close in. Ideally you want a day where there are not too many low clouds obscuring the view, which can happen with high humidities (low temp/dewpoint spreads) and minimal capping - although typically a strong supercell, as it matures, is going to dominate the surrounding environment and clear out such cloudiness. Again, this assumes discrete supercells as opposed to embedded supercells.

There is a thread that I started on July 5, 2014 entitled "Storm Mode"; you may find it helpful to read the responses to my question there.

 

[Skip Talbot]

Great question and answers.

I've found that some of the best tornado chase days are also the best structure days. That's not necessarily the days with the highest tornado probabilities or outbreak days, but the days that feature those slight risk, cyclical supercells that put down strings of tornadoes. They often have amazing structure, and I think that's probably due to the ample directional shear those days often have, along with the preferred storm mode: discrete classic.

What about some parameters that work against tornadoes but for structure? One of the best is a fairly stout cap, and mainly a cap that's filling back in after your supercell matures. Air forced upwards through a stable layer often has clouds with a smooth, striated look to them. You get beautifully sculpted supercells at dusk and after dark when the boundary layer cools off and the bottom part of the storm is in that cooler, stable air while the updraft remains strong. So a fair amount of CINH, but after your storm is established. You don't want to choke off the updraft completely before the storm can go up. Once you see those photogenic striations, the tornado show is usually over.

Another that's contrary to what we look for when chasing tornadoes and was also mentioned above is the temperature/dewpoint spread. A low spread often favors tornadoes, but also means low and hazy bases. A higher spread might mean it's too dry for tornadoes, but those high bases are often much easier to shoot from a distance and are higher contrast.

Another parameter I look for when after structure: steep lapse rates especially in the mid levels (greater than 8 or 9). Those often give you that crisp, explosive convection, whereas more meager lapse rates (7 or 6 or less) often result in weak, watery updrafts (even if you still have good CAPE values). You can get really steep midlevel lapse rates when there is a lot of cold air aloft or a deep trough moving through.

The big one is location though. Where do we often have steep lapse rates, fairly high T/Td spreads, the EML is a big player with stout capping, and skies free of haze. The high plains. Colorado, Wyoming, New Mexico often have gorgeous storms for all of these reasons. You might have setups in Colorado and Arkansas both with 3000 CAPE and 45 knots of deep layer shear, but with radically different quality of storm structure. Setups down in the southeast are often plagued by weak lapse rates, a lack of capping that results in the entire warm sector going up in clusters weaker updrafts, and low, hazy bases. Extreme shear can compensate for that sometimes though. There was some amazing structure coming out of MS and AL during the 2011 super outbreak when there was 100+ knots of deep layer shear and helicity values well over 500.

 

[JamesCaruso]

Wow, I learned a lot from that post Skip, many factors that I hadn't contemplated in my own (comparatively weak) response! Great information, thanks as always for contributing.

 

[David Williams]

Great question and answers.

I've found that some of the best tornado chase days are also the best structure days. That's not necessarily the days with the highest tornado probabilities or outbreak days, but the days that feature those slight risk, cyclical supercells that put down strings of tornadoes. They often have amazing structure, and I think that's probably due to the ample directional shear those days often have, along with the preferred storm mode: discrete classic.

What about some parameters that work against tornadoes but for structure? One of the best is a fairly stout cap, and mainly a cap that's filling back in after your supercell matures. Air forced upwards through a stable layer often has clouds with a smooth, striated look to them. You get beautifully sculpted supercells at dusk and after dark when the boundary layer cools off and the bottom part of the storm is in that cooler, stable air while the updraft remains strong. So a fair amount of CINH, but after your storm is established. You don't want to choke off the updraft completely before the storm can go up. Once you see those photogenic striations, the tornado show is usually over.

Another that's contrary to what we look for when chasing tornadoes and was also mentioned above is the temperature/dewpoint spread. A low spread often favors tornadoes, but also means low and hazy bases. A higher spread might mean it's too dry for tornadoes, but those high bases are often much easier to shoot from a distance and are higher contrast.

Another parameter I look for when after structure: steep lapse rates especially in the mid levels (greater than 8 or 9). Those often give you that crisp, explosive convection, whereas more meager lapse rates (7 or 6 or less) often result in weak, watery updrafts (even if you still have good CAPE values). You can get really steep midlevel lapse rates when there is a lot of cold air aloft or a deep trough moving through.

The big one is location though. Where do we often have steep lapse rates, fairly high T/Td spreads, the EML is a big player with stout capping, and skies free of haze. The high plains. Colorado, Wyoming, New Mexico often have gorgeous storms for all of these reasons. You might have setups in Colorado and Arkansas both with 3000 CAPE and 45 knots of deep layer shear, but with radically different quality of storm structure. Setups down in the southeast are often plagued by weak lapse rates, a lack of capping that results in the entire warm sector going up in clusters weaker updrafts, and low, hazy bases. Extreme shear can compensate for that sometimes though. There was some amazing structure coming out of MS and AL during the 2011 super outbreak when there was 100+ knots of deep layer shear and helicity values well over 500.

Hey Skip, thanks for the post. I had a noob question about the lapse rates you mentioned. Is that 8C/km - 9C/km or 8F/1000ft - 9F/1000ft? Also, do you check those lapse rates on forecast soundings by checking the temp (and dews) between the EML (which I think is 850 mb, right) and about 500mb?

Thanks

 

[Skip Talbot]

Sorry, my mistake for leaving the units off. Yes, C/km. There's a few places you can get them:
Jeff Duda's page:
http://www.meteor.iastate.edu/~jdduda/forecast/forecast.html

Earl Barker has some:
http://wxcaster.com/smallfiles_central_svr.htm

SPC SREF:
http://www.spc.noaa.gov/exper/sref/sref.php
Mesoanalysis:
http://www.spc.noaa.gov/exper/mesoanalysis/

You can eyeball the forecast soundings too.

Those parameter descriptions on the Mesoanalysis page are great. For those curious:

Lapse Rates (C/km)

A lapse rate is the rate of temperature change with height. The faster the temperature decreases with height, the "steeper" the lapse rate and the more "unstable" the atmosphere becomes. Lapse rates are typically displayed in ranges from 850-500-mb (4,500-18,000-ft above sea level) and 700-500-mb (10,000-18,000-ft above sea level).
Lapse rates are shown in terms of degrees Celcius change per kilometer in height. Values less than 5.5-6.0 degrees C/km ("moist" adiabatic) represent "stable" conditions, while values near 9.5 degrees C/km ("dry" adiabatic) are considered "absolutely unstable." In between these two values, lapse rates are considered "conditionally unstable." Conditional instability means that if enough moisture is present, lifted air parcels could have a negative LI (lifted index) or positive CAPE.

 

[David Williams]

Awesome! Thanks Skip

 

[Mark Blue]

Also, do you check those lapse rates on forecast soundings by checking the temp (and dews) between the EML (which I think is 850 mb, right) and about 500mb?

Thanks

On the SPC Mesoanalysis page, after selecting a geographic sector to view, go to the "Thermodynamic" column where you will find charts for low and mid level lapse rates with the values plotted all by themselves. This helps identify the areas with the most unstable conditions rather readily. I believe the EML can generally be found between roughly 850 - 700 mb if you're looking at a sounding, particularly if it was created from WAA in middle layers of the troposphere.

 

[David Williams]

Thank you for that clarification Mark. That's probably better than eyeballing a sounding.