CAPE values during all four seasons of the year

[STurner]

I have a few questions about CAPE and how it is variable for a tornado outbreak during different times of the year. I know what CAPE stands for and what it means but why you may need some during the fall/winter but a lot during late spring or summer. This is also if you have all the other ingredients that come into play for a tornado outbreak. For instance I believe the Super Tuesday outbreak had CAPE values around 1000-1500J/kg but during the later spring CAPE values ranged from 2000-3000J/kg+. I was wondering if it may be during the winter that the atmosphere is not adapted to warm temperatures, 60 degree dewpoints and moderate instability. On the other hand during the late spring you would want higher values. Just want to know for I am still learning about ingredients needed for a tornado outbreak. Also good luck to those of you who will chase on the 26th. I hope it pans out for I like to watch tornado outbreaks online or TWC.

 

[Andrew Ryan]

I think I can answer your question, but I'm not quite sure as to what you're asking and if I'm qualified to answer it.

First of all, there is no "required" value of CAPE in order for tornadic supercells to form. It just happens in the spring that you have more low level moisture and higher temperatures which lends to higher CAPE values. Although you can have supercells form in low CAPE environments, >1500 J/Kg of CAPE is ideal.

CAPE values are lower in winter/fall setups because there's less energy available in the atmosphere. Temperatures are cooler at the surface and thus temperatures in the upper parts of the atmosphere have to be cooler in order to have higher CAPE values.

I hope this answers your question(s).

BTW, I wouldn't hold your breath on a tornado outbreak on the 26th. I haven't look at the models and such but from what I've read, it's not going to be a huge tornado event.

 

[Matt Sellers]

I was wondering if it may be during the winter that the atmosphere is not adapted to warm temperatures, 60 degree dewpoints and moderate instability. On the other hand during the late spring you would want higher values.

Such conditions do occur during the winter months - mainly in the Gulf region. However, strong synoptic-scale troughs can advect richer boundary-layer moisture farther north and northeast, but at a "price" - the moisture advects through a deep layer of the atmosphere (not just in the lowest 1-2km) resulting in widespread cloud cover and showery precipitation. Even in areas where the clouds break, allowing for insolation, the effect on the boundary layer is diminished due to the lower sun angle at that time of year; an angle that gets lower as one moves farther north.

Such instances may only generate about 500-1000 j/kg of CAPE, yet shear is usually not a problem, the moist boundary layer results in low LCL heights, and any isolated strong cells that develop almost immediately take on supercell characteristics in a WAA-dominated environment. Due to strong winds aloft, these storms move at rocket-up-your-arse speeds, but have just as high a chance (if not higher) of producing tornadoes than a mid June 7K-CAPE Nebraska supercell.

 

[Dean Baron]

the March 12th, 2006 outbreak in Missouri was a good example of how you dont need a ton of instablity for tornadoes. Correct me if i'm wrong but I remember people talking about tornadic supercells where the surface temp was in the upper 30's or low 40's. Cold core events also are good examples too. There are many things that impact tornado development, instability is just one of them.

 

[Andrew Ryan]

the March 12th, 2006 outbreak in Missouri was a good example of how you dont need a ton of instablity for tornadoes. Correct me if i'm wrong but I remember people talking about tornadic supercells where the surface temp was in the upper 30's or low 40's. Cold core events also are good examples too. There are many things that impact tornado development, instability is just one of them.

I don't think there have been any tornadic storms witht temps that low, but I could be wrong. There was a pretty exceptional cold core storm this November 10 that produced a brief tornado. The air temperature was around 50F. But the 700mb temp was around -10F, which allowed for reasoable instability.

 

[Rob Wadsworth]

I'll bite. I will have to agree with Matt S concerning the dynamics that are most favorable for tornadic conditions. But if one looks at the records for this year, one would have to say 'oh yeah; I must have forgot about the Jan/Feb tornadoes this year'. We even had them well into Dec of '07. Regardless if it because of warming or not, the NWS predicts that tornadic occurences will be increasing over the next ten years - if you can accept their methods that they base this long -range forcast on - 'GW'. If CAPE is going to be in the right place at the right time - my money is on a show!

So, to say that the conditions of the upcoming !2/26 setup aren't going to produce, I wouldn't reserve my bet and watch to see if things don't come around. At this rate, someone is making book in Las Vegas and the odds are 50/50. Knowing the record for this year and last.

Quote:"...Due to strong winds aloft, these storms move at rocket-up-your-arse speeds, but have just as high a chance (if not higher) of producing tornadoes than a mid June 7K-CAPE Nebraska supercell." -Matt S
Yup.

Good Hunting for those who are going to give it a try. If nothing else, it will sharpen the skills. My guess is - considering the past year or so - that they won't be going home empty-handed...

 

[Bob Hartig]

the March 12th, 2006 outbreak in Missouri was a good example of how you dont need a ton of instablity for tornadoes. Correct me if i'm wrong but I remember people talking about tornadic supercells where the surface temp was in the upper 30's or low 40's.

Temps were in the mid 60s, dewpoints around 60, and SBCAPE was over 2,500, at least according to the 00z RUC for that evening.

I'm planning on heading out Friday, but I confess, I'm not sure what to make of a CAPE-less scenario. I see a lot of other elements coming into play, but not CAPE, not for where I can get to within a reasonable day's drive. The Super Tuesday Outbreak offered better CAPE closer to home.

 

[Chad Cowan]

the March 12th, 2006 outbreak in Missouri was a good example of how you dont need a ton of instablity for tornadoes. Correct me if i'm wrong but I remember people talking about tornadic supercells where the surface temp was in the upper 30's or low 40's.

I think I remember what you're talking about... there was an early start with the supercells that day near Lawrence, KS around 9am and I believe there were sfc obs in the near-storm environment that showed 40-50F temps in the 'warm sector'.

To help answer the initial question, I will take big CAPE over big shear any day, but all you need is a delicate balance between the two for tornadoes. High CAPE will compensate for low shear and vice-versa. This is why I'm a big fan of the Energy Helicity Index (EHI) for tornado forecasting.

 

[Paul Knightley]

Tornadoes have occurred in the UK with temps below 10C (50F). The parent storms are located within deeply unstabled polar maritime airmasses, and almost exclusively associated with strong low-level shear. Perhaps just 100 J/Kg of CAPE may be present in these cases, but remember that any amount of CAPE can allow a tornado develop, if the shear is right. Indeed, the shear is much more important than CAPE.

 

[R R Smith]

In California, tornadoes occur primarily during winter and early spring. Typically during these events, CAPE values have been below 500 J/Kg. There's a couple of papers by Chuck Doswell and John Monteverdi that analyze these events. Strong low level shear plays a huge role in the development of these storms. Here's the link from Monteverdi's chase site that lists some of his research on cool season tornadoes:
http://tornado.sfsu.edu/geosciences/tornadochase/Research.html

 

[Jeff Duda]

I agree with Chad in both points: I would rather watch storms go up in 4500 J/kg CAPE situations with less shear than in 500 J/kg CAPE with gobs of shear. You're more likely to see not only tornadoes, but also structure, in the springtime/summertime high-CAPE situations than in the winter situations. LCLs are very low, and high PBL RH's usually cause reduced visibility. Almost all supercells that form in wintertime storms are HP, which is a vision killer right there. I also like the EHI due to its determining the balance between instability and shear. I refer readers to a classic article, Thompson et al. (2003), which can be found at this link:

http://ams.allenpress.com/perlserv/...1243:CPSWSE>2.0.CO;2#I1520-0434-18-6-1243-F02

PDF form: http://ams.allenpress.com/archive/1520-0434/18/6/pdf/i1520-0434-18-6-1243.pdf

It discusses critical values of some parameters that separate non-tor, weak-tor, and sig-tor supercells. Granted this is based mostly on springtime scenarios, but maybe it also applies to wintertime situations.

 

[Doug Mitchell]

http://bangladeshtornadoes.org/UScases70to84.html

February 22nd, 1975. Tornadoes formed in SW OK with surface temps ~40 deg F

 

[Bob Hartig]

This is a great topic, and one that has mystified me. Obviously there's some kind of a balance required. My philosophy last year was that big helicity would compensate for small CAPE, but after getting skunked by a few such scenarios, I'm not so convinced of that anymore, at least when it comes to something chaseable. Yesterday's bout of severe Wx reinforces that impression. Without adequate energy to sustain localized updrafts, it seems to me that lift is going to come in the form of a cold front with embedded supercells. I just haven't had much luck with those. They tend to produce far more Doppler-warned tornadoes than actual touchdowns, and the helicity-driven ones that do occur are often brief.

My concern is to figure out how this stuff works in actual practice. For now, at least, I'll settle for a little more convective energy and a little less spin. That for me is what will warrant a drive out West. Closer to home, though, I'm not so selective. Slim chances out in the field are better than no chance at all sitting at home.

 

[rdale]

I think extreme helicity CAN compensate for small CAPE, but as you noted it's primarily brief spin-ups in the kinds of events like yesterday. And rarely chaseable. More frequent is the doppler tor warning, as they issue one, spotters "see" a funnel cloud and the doppler process continues until finally the lack of tornado reports give mets enough confidence to stop TOR'ing and just SVR'ing.

 

[Bob Hartig]

Agreed. I'm not discounting big helicity, just considering the quality of chase I'm likely to get from different ends of the CAPE/helicity spectrum. Yesterday, it seemed to me that some pretty huge helicities (in places, 1 km helicities over 1,000) in the warm sector went unrealized due to lack of CAPE. A small army of teeny popcorn convection was getting kicked up ahead of the squall line, but it never had a chance to develop. Better CAPE could have turned some of that stuff into pre-frontal supercells (as happened with the Super Tuesday Outbreak), and things would have gotten a lot more interesting.