When is Tornado Season?

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This story:

10 tornadoes confirmed in Ga., including one with winds topping 160 mph

Ten tornadoes, one packing winds of more than 160 mph, touched down in parts of Georgia on Wednesday, the National Weather Service said Friday.

The storms caused an estimated $25 million in insured losses, said John W. Oxendine, the state’s insurance commissioner.

“I spent some time surveying damage and talking to residents in Jasper, Putnam and Hancock Counties” on Friday, Oxendine said in statement. “I believe claims will easily reach $25 million. Actual losses are much higher when you consider things like infrastructure damage and uninsured losses.”

Reminds us that Tornado season is coming. Maybe it is already here in parts of the country, or maybe it is a bit early this year in the south. It is important to keep tornadoes in perspective. It would appear that for the last half century, the frequency of tornadoes in the US is rising, though this could be totally or in part because of increases in reporting. Warming climate should result in more tornadoes in areas where tornadoes already occur, or at least that is a reasonable assumption unless countervailing effects can be demonstrated.

So when is tornado season exactly?

Its complicated. The best way to answer this question is locally. Here in Minnesota the frequency of tornadoes starts to pick up in April, with the peak coming in June. In Georgia, where these reported tornadoes were, there is almost a bimodal season. They really can happen any time of the year in Georgia, but there is a major peak around mid february through about May, and another peak late in the year, around November, with this second peak being fairly weak.

The following is an animated GIF showing tornado activity from 1985 through 1989 over the course of a year.

(If this starts to annoy you, just hit the “escape” key. That will stop any animated GIF in most browser windows. File under “computer tricks.”)

This is a fairly complicated pattern, but it is a pattern. Throughout the year, tornado activity spreads from south to north, as does thunderstorm activity. This sort of storm is most common in the middle, flatter parts of the country, but also, the energy that feeds these storms comes mainly from the Gulf of Mexico. This is the same sea-surface temperature (and over-sea moisture) effect that results in hurricanes which strike land in the US having a similar pattern. And, this is why it is logical to expect a global warming – tornado link.

NOAA has a great web site with the data referred to here, and more cool graphics, here.

And if you live in any of the lit up areas of this GIF, make sure youre Weather Radio is working!!!

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0 thoughts on “When is Tornado Season?

  1. So tornadoes generally begin near the outflow of the freshwater muddy Mississippi into the warm saltwater Gulf? Does that pattern hold for other big rivers dumping into warm seas? How about the Congo, Amazon, Yellow, Nile, Ganghes?
    I always thought twisters started in Kansas and Oklahoma.

  2. No, they don’t actually begin there, but the storms that spawn them are often initiated as waves coming up from the gulf. The river does not have much to do with it.

  3. It’s not obvious that tornadoes will increase under global warming. One of the big things that separates tornadic from non-tornadic environments is the vertical wind shear. Mid-latitude wind shear is projected to decrease, which would be unfavorable for an increase in tornadoes. Another question is related to boundary layer relative humidity. I don’t believe we have much confidence in how it will change.

    Of all of the severe thunderstorm phenomena (tornado, wind, hail), non-tornadic winds are the most likely to increase since they tend to occur more often in low shear environments.

  4. Harold: We simply can’t say if tornadoes have or have not increased in frequency and we can’ at this time make the link at all. We can’t even say if the issues you cite are relevant.

    I’m guessing there will be an increase in tornado frequency associated with global warming in the region north of the Gulf for the very simple reason that tornadoes are common components of the severe storms typical of the region, these storms are energetically born of warm gulf waters, and gulf waters are warmer with global warming.

    However, there are a lot of other ways this could go. There may be a decrease in tornadoes and an increase in severity of straight line winds or just an increase in the number of tornadoes. Or this energy shift could simply all end up as extra hurricanes.

    This graph in this post shows an increase in tornado frequency over the same time that we see an increase in various global temperature measures, at the decadal scale. But although the trends match, there is no short term positive correlation (there may even be a negative correlation).

    Not surprisingly, this is complex and will be difficult to suss out.

  5. Greg: Several points

    1. The fraction of a thunderstorm’s energy in the wind component (tornadic or non-tornadic) is small. In the energy budget of the thunderstorm, the change of phase of water is ~2 orders of magnitude larger.

    2. The increase in tornado reports shown in the other graph is almost entirely in the F0 tornadoes. See this paper.

    3. We know that, historically, the probability of a tornado occurring given that a storm is severe is almost entirely a function of the wind shear. I can’t get the image uploaded right now due to computer difficulties, but I’d be happy to e-mail it to you.

    4. We know that the the active tornado years of the early 1970s had high shear and low Convective Available Potential Energy (CAPE), the simplest good measure of the thermodynamics, and that the inactive tornado years of the late 1980s had low shear and high CAPE.

  6. Luckily, these storms passed just to the south of us in Lawrenceville, GA.

    My suggestion would be to get a weather radio where you enter your home county and only receive the audible warnings when that county is included in a warning. (The NWS encodes the effected counties into every warning bulletin. County codes can be found here: http://www.nws.noaa.gov/nwr/indexnw.htm) That way you will get the general Tornado/Severe Thunderstorm Watch alert but not Tornado Warnings for counties 50 miles to the north or south. Cuts down on the “false alarms” for storms that have no chance of coming your way.

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