Minimum dewpoints for a massive tornado outbreak

[STurner]

I believe some tornadoes have been able to form in dewpoints as little as the lower 40s. But if you want a massive tornado outbreak with at least 50+ tornadoes and several of them to be violent(EF4 or EF5) what would be the minimum dewpoints needed. Myself was thinking in the order if you have cape above 2000J/kg, helicity values around 300-500m2/s2, insane wind shear, LLC's below 500m etc. dewpoints around 55-56 degrees might be able to do it. I am sure there are several other things I am missing here but wondered what would be the minimum dewpoints for multiple long-lived, long-tracked tornadoes.

 

[afischer]

A couple tornado outbreaks that come to mind with regard to "relatively" low dewpoints are 03/28/07 and 03/13/90.

03/28/07 had several cyclic tornadic supercells in western Nebraska, extreme western KS, and extreme eastern CO (along a line from Merriman NE to Holly CO) with surface dewpoints of 54-58F. The higher terrain played a role that evening... all else being equal, had the setup taken place 300 miles farther east, the instability would have probably been substantially weaker (e.g. over the lower terrain of the eastern Great Plains) and not supportive of such an outbreak given those dewpoints.

03/13/90 was relatively ridiculous especially on the northern end of the warm sector, with several very long-track violent tornadoes occurring in southcentral/eastern Nebraska with pretty similar moisture (surface dewpoints 55-58F).

Cold core tornadoes are usually pretty localized and don't cause a lot of damage so may not really meet "outbreak" status, but highly productive cold core tornado events happen once and a while with dewpoints in the mid 50s. (e.g. 05/17/2000, 10/26/2006)

 

[Bobby Prentice]

"Minimum dewpoints" is a relative term. Let's get back to basics.

The three ingredients necessary for a thunderstorm are:

1) Sufficient water vapor (i.e., "dewpoints") to fulfill ingredient #2
2) Unstable air (almost always condition instability)
3) A source of lift (front, dryline, upslope flow, etc.) to release the condition instability

Thus, whatever the minimum dewpoint is necessary to create unstable air would be the "minimum dewpoint" required.

As long as those three ingredients exist, a thunderstorm can continue to generate new cells to keep itself alive.

For supercells, a balance between CAPE and shear is necessary. A big limiting factor for cold/cool season outbreaks is insufficient CAPE for the high-shear environment.

The Bulk Richardson Number (BRN) is a dimensionless number in meteorology relating vertical stability and vertical shear (generally, stability divided by shear). High values indicate unstable and/or weakly-sheared environments; low values indicate weak instability and/or strong vertical shear. Generally, values in the range of around 10 to 45 suggest environmental conditions favorable for supercelldevelopment. If the BRN is too low, the updraft gets strongly tilted with height and is disrupted.

Now, what are optimal dewpoints for a "massive tornado outbreak." I assume by tornado outbreak, you mean an outbreak in which multiple supercells produce multiple significant (EF2+) tornadoes across multiple states. Something that would rank in the top 20 tornado outbreaks of all time.

A tornado outbreak on that scale would typically require a large breeding ground (over ~250,000 square miles) with a tropical maritime airmass. Tropical maritime airmasses (mTw) then enter the U.S. originate over the topical Atlantic Ocean and Carribean Sea. They are characterized by a 1,500 to 3,000 meter (5,000 to 10,000 ft) deep, well-mixed moist layer. Surface dewpoints are typically in the low-mid 60s in cold/cool season (i.e, October-April) tornado outbreaks and upper 60s-mid 70's in warm season (i.e., May-September) tornado outbreaks.

 

[Perry Williams]

I believe some tornadoes have been able to form in dewpoints as little as the lower 40s. But if you want a massive tornado outbreak with at least 50+ tornadoes and several of them to be violent(EF4 or EF5) what would be the minimum dewpoints needed. Myself was thinking in the order if you have cape above 2000J/kg, helicity values around 300-500m2/s2, insane wind shear, LLC's below 500m etc. dewpoints around 55-56 degrees might be able to do it. I am sure there are several other things I am missing here but wondered what would be the minimum dewpoints for multiple long-lived, long-tracked tornadoes.

I've conducted quite a bit of research into historical tornado outbreaks which affected the southeast and lower Mississippi valley. In virtually all cases when a major tornado outbreak took place, the surface dewpoints were at least low to mid 60's....and usually 65 or higher. I've seen isolated violent tornadoes occur with dewpoints in the upper 50's to around 60 (last year in Polk and Floyd county, Georgia; March 20, 1998 in Hall county, Georgia), but these were isolated events. Most large outbreaks occur with large warm, moist and unstable sectors.

Here's a great website resource guide for anyone interested in learning what type of atmospheric parameters caused historical tornado outbreaks of the past:
http://bangladeshtornadoes.org/UScases.html

 

[Taylor Campbell]

I've found that large lapse rates or temperature differential between 500/850mb will overcome a low dewpoint environment to produce instability. I'm still researching the values needed etc. in determination of how much instability will be available w/ a set value.

 

[Jeff Passner]

I believe some tornadoes have been able to form in dewpoints as little as the lower 40s. But if you want a massive tornado outbreak with at least 50+ tornadoes and several of them to be violent(EF4 or EF5) what would be the minimum dewpoints needed.

I'm not flaming here, but days with 50+ tornadoes with "several" being EF4 or EF5 are rare and I mean rare; like one day a year or less. There have been three or so EF5s in the last 10 years. About 1 percent of all tornadoes are
EF4 or EF5.. However, talking about major outbreak days, I would think a dewpoint of 60 degrees is a benchmark. Typically, most of the major days feature surface-based dewpoints over 60 degrees F. As others have said above, it's not that simple and storms do not obey any rules or numbers. You are correct, tornadoes can form with dewpoints in the 40s and tornadoes can occur with a dew point of 80F (very rare)... Actually, that would be an interesting study tornadoes in high dewpoints... Man, those would be HP!!!

Still, I think mid to upper 60s would be the optimal dewpoint for the events you are discussing.

 

[Mike Johnston]

To get an idea of some typical values, as well as extreme values, of various parameters for tornado events, you might want to read this paper:

Close proximity soundings within supercell environments obtained from the Rapid Update Cycle. Wea. Forecasting, 18, 1243-1261.

Notice it includes "box and whisker" diagrams for various severe parameters over a pretty large sample size of tornado events.

Related to the dewpoint, for example, you can see that significant tornado events seem to have a minimum threshhold for ML CAPE of ~1,000 j/kg. Also, remember that relatively low LCL's (which correspond to higher dewpoints relative to the surface temperature) are more favorable. Notice that this study suggests ~1,300 meters ML LCL height seems to be the maximum for significant tornadoes.

 

[MatthewCarman]

I have seen dew points in the 80's to low 90's before if my memory serves me right. (I could be wrong) How would that affect tornadoes or tornado outbreaks?

 

[STurner]

A couple tornado outbreaks that come to mind with regard to "relatively" low dewpoints are 03/28/07 and 03/13/90.

03/28/07 had several cyclic tornadic supercells in western Nebraska, extreme western KS, and extreme eastern CO (along a line from Merriman NE to Holly CO) with surface dewpoints of 54-58F. The higher terrain played a role that evening... all else being equal, had the setup taken place 300 miles farther east, the instability would have probably been substantially weaker (e.g. over the lower terrain of the eastern Great Plains) and not supportive of such an outbreak given those dewpoints.

03/13/90 was relatively ridiculous especially on the northern end of the warm sector, with several very long-track violent tornadoes occurring in southcentral/eastern Nebraska with pretty similar moisture (surface dewpoints 55-58F).

Cold core tornadoes are usually pretty localized and don't cause a lot of damage so may not really meet "outbreak" status, but highly productive cold core tornado events happen once and a while with dewpoints in the mid 50s. (e.g. 05/17/2000, 10/26/2006)

Yeah, March 13, 1990 was an interesting day to say the least. I was only nine-years old but remember it quite well. My dad was watching the news and told me one of those tornadoes was on the ground for 2 and a half hours and traveled over 100+ miles. I believe the Hesston F5 tornado happened when the temperature was around 73-74F and a dewpoint around 61-62F. You probably are right about the dewpoints not being extremely high. That tornado I mentioned above was rated an F4 on the fujita scale and crossed six counties in NB and temperatures may have only been 67-68F and a dewpoint around 56-58F along the KS/NB border. Two F4's and two F5's happened on that day and there was around 60 tornadoes. I tend to think these temperatures and dewpoints may be enough for a late fall to early spring outbreak but probably not enough for a late spring/summer outbreak.