Is a busy tornado season ahead ?

Alexandre Aguiar

The posibility of La Niña event in the next fews months grows each day. The most recent climatic models indicate El Niño is collapsing and that the Pacific would cool very rapidly between March and April. As we know there are many papers claiming La Niña episodes prompt more tornadoes in the United States, the question is: Is a busy tornado season ahead ?

Source: Purdue University
Date: March 1, 1999

La Nina Cycle Puts New Twist On Tornado Season

Science Daily — WEST LAFAYETTE, Ind. -- Last year's El Nino brought a variety of weather-related woes, but this year's La Nina may stir up double trouble during the tornado season for Indiana, Arkansas and Mississippi and the western parts of Kentucky and Tennessee.

Purdue University Professor Ernest Agee says those areas may expect twice as many tornadoes as last year, based on the findings of his graduate student, Suzanne Zurn-Birkhimer, who compared the geographical variation in tornadoes for El Nino years vs. La Nina years during an 81-year-period.

"Though the study provides little reason to expect more or fewer tornadoes overall, the findings show clear evidence of geographical shifts in tornado activity within the United States when comparing strong El Nino years to La Nina years," says Agee, professor of atmospheric sciences at Purdue who has studied tornadoes for more than 30 years.

Zurn-Birkhimer compared tornado activity during El Nino and La Nina events by calculating a ratio of tornadoes on a state-by-state basis. Her findings show more tornadoes in the central and southern plains and the Gulf Coast during strong El Nino years, with a shift to more tornadoes in the lower Midwest, the Ohio and Tennessee valleys, and the mid-Atlantic region during La Nina years.

"That means that this year's La Nina event is likely to increase tornado activity in Mississippi, Arkansas, western Tennessee, western Kentucky and Indiana," Agee says. "In fact, these areas might expect twice as many tornadoes as last year, when they were at a lower risk of tornado activity due to the strong El Nino."

La Nina, which means "the little girl," is characterized by unusually cold ocean temperatures in the eastern tropical Pacific, as compared to El Nino, which is characterized by unusually warm temperatures in the same waters.

The changes in ocean temperatures also cause a shift in the jet streams patterns, Zurn-Birkhimer says. In her study, she calculated the positions and strengths of the polar and subtropical jet streams during El Nino and La Nina events from 1916 to 1996 to study the effect on the distribution and strength of tornadoes in the tornado alley region of the United States.

"During an El Nino event, the polar jet stream -- which carries cold, dry air from the north -- shifts south, bringing cooler air to the Midwest and Southeastern regions of the country," she says. "This cooling effect might also serve to suppress tornado activity in those areas."

By contrast, during a La Nina event, the subtropical jet -- the jet stream that brings warm moist air from the south -- shifts to the far north, bringing an influx of warmth and moisture to these regions, and increasing the odds for tornadoes, Zurn-Birkhimer says.

"There has to be a threshold of heat and moisture to build severe thunderstorms," she says. "If the atmosphere's too dry or too cold, you just can't get large dynamical cloud systems like the super-cell storms. When the jet stream is farther north, as it is in a La Nina event, you have a better chance of achieving these kinds of temperatures and dew-points."

Despite the popularity of blaming El Nino for all of last year's weather woes, Zurn-Birkhimer says her study shows there is little evidence that El Ninos are associated with more or less tornado activity.

"La Nina events, however, seem to favor an above-average annual number of tornadoes in select geographical regions," she says.

Interestingly, she adds, the years with neither an El Nino or La Nina event tend to favor a below-average number of tornadoes by more than 2-to-1.

Agee notes that the 1999 tornado season already is off to a record-breaking start, with two major tornado outbreaks in January.

"Preliminary reports from the National Weather Service show that there were 169 tornadoes in January, and 19 tornado-related deaths," he says.

Zurn-Birkhimer presented her study last fall at the 19th annual meeting of the National Severe Storms Conference in Minneapolis.

Also a very interesting research from FSU:

http://www.coaps.fsu.edu/papers/impacts_enso_tornadic_activity/
 
Though it is not proven La Nina years do seem to produce more violent and large tornado outbreaks. Some famous La Nina tornado outbreaks include April 3-4,1974; Palm Sunday Outbreak from 1965; and May 3,1999.

So if this verifies we could see a fairly active tornado season. Maybe another 1999 type of season?

This will be very interesting to see how the season pans out especially with the much improved conditions across the southern plains and the abundant moisture in the Gulf.

Bring on Spring!!!!:)
 
I think 2007 is going to SUCK. The season's "greatness" is directly proportional to the amount of threads started wondering how great it will be.
 
Personally, I can't wait 'til Spring so all of these speculative threads will stop. There are cases both supporting / not supporting these theories of increased tornadoes during El Nino or La Nina years.

I'm just a traditionalist I suppose, but I say it's up to higher powers and we can speculate all we want, it's not going to make one difference in whether we have a record year, a normal year, or a slow year.

As far as what I "believe," I think it will be another pathetic year for the southern plains, with more drought and heat waves, while the majority of severe activity will, once again, be focused on Kansas, Nebraska, Missouri, Iowa and the Dakotas. Again, pure speculation.
 
Billy

Sorry for posting this "speculative thread" that stress you. I still belive discussion on long term conditions are useful. To the contrary, why hurricane forecasts are released six months ahead of the season ?
 
Personally, I can't wait 'til Spring so all of these speculative threads will stop. There are cases both supporting / not supporting these theories of increased tornadoes during El Nino or La Nina years.

I'm just a traditionalist I suppose, but I say it's up to higher powers and we can speculate all we want, it's not going to make one difference in whether we have a record year, a normal year, or a slow year.

As far as what I "believe," I think it will be another pathetic year for the southern plains, with more drought and heat waves, while the majority of severe activity will, once again, be focused on Kansas, Nebraska, Missouri, Iowa and the Dakotas. Again, pure speculation.
What's wrong with speculating? I think it's interesting to examine conditions that allow for active vs. inactive tornado seasons. Does anybody do seasonal tornado forecasts (you know, like they do for hurricanes)?
 
The only model I'm seeing that shows a possibility of a La Nina is the NASA GAMO and ECHAM. You are correct that the models have certainly gone on the downward SST anomaly trend and towards ENSO netural conditions by the time May comes around.

Persistence of El Niño conditions remains the most likely outcome through the next few months of 2007. For the JFM season, there is an estimated 85% likelihood for El Niño, approximately 1% probability for La Niña conditions to return, leaving less than 15% probability for ENSO-neutral conditions. By mid-2007, a return to ENSO-neutral conditions becomes the most likely outcome.
Keeping in mind that anything can happen and speculation at this point is very speculative, Knowles and Pielke did show some evidence that outbreaks and strong tornadoes were less likely during a warm phase. Mark C. Bove also supported these findings that El Nino years typically see a less favorable configuration with the larger scale factors that can support supercell development. Though Schaffer did not present as strong a case for ENSO linked potiental, it did also support the primary conclusion.
The current ensemble ENSO forecasts also look similar to several other El Nino years I looked at recently, namely that they were present prior to the on-set of Winter and Nino region anomalies decreased to less then .5C by April. There were four specific years like that and each one had a similar or higher number of tornadoes for MN,SD,ND then the usual average.

The season could be terrible but it's still interesting to look at the little larger scale clues that may play a factor in location of larger scale features.
 
Scott

As you know CFS and POAMA are uptaded daily. Each day the cooling is faster:

CFS Niño 1+2 that is off the coast of South America and was already mentioned as major forcing to pattern changes in the Americas:

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/images/nino12SSTMon.gif

CFS Niño 3+4 (main Niño area)

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/images/nino34SSTMon.gif

BOM's POAMA (the first 15 casts are pushing the line higher or warmer)
ftp://ftp.bom.gov.au/anon/home/ncc/...ges/plumes/sst_indices_30_days_20070125_3.gif

FSU

http://ocean.fsu.edu/faculty/clarke/clarkeelnino.html

If it is valid to speculate on the date of the "first high risk of the 2007" I believe it is interesting to discuss how could the tornado season could develop in the next few months. At least, we have some papers, some brilliantly indicated by you Scott, suggesting some patterns.

Alexandre
 
The only model I'm seeing that shows a possibility of a La Nina is the NASA GAMO and ECHAM. You are correct that the models have certainly gone on the downward SST anomaly trend and towards ENSO netural conditions by the time May comes around.

Keeping in mind that anything can happen and speculation at this point is very speculative, Knowles and Pielke did show some evidence that outbreaks and strong tornadoes were less likely during a warm phase. Mark C. Bove also supported these findings that El Nino years typically see a less favorable configuration with the larger scale factors that can support supercell development. Though Schaffer did not present as strong a case for ENSO linked potiental, it did also support the primary conclusion.
The current ensemble ENSO forecasts also look similar to several other El Nino years I looked at recently, namely that they were present prior to the on-set of Winter and Nino region anomalies decreased to less then .5C by April. There were four specific years like that and each one had a similar or higher number of tornadoes for MN,SD,ND then the usual average.

The season could be terrible but it's still interesting to look at the little larger scale clues that may play a factor in location of larger scale features.

An El Nino favors ridging across the west, whereas a La Nina favors troughing. As for results in the tornado department, I've done some studying, and the fact is I have seen no real strong correlation, although, generally, Ninas and neutral conditions favor tornadoes more than Ninos, undoubtedly due to the aforementioned reason.
 
It's true that La Nina does appear to show a tendency for a deeper trough in the west . Here is a quote from Forbes paper on meteorological conditions of some of the largest outbreaks, it does differ a bit in the outbreak vs. ENSO analysis that Bove did.

Of the 19 largest and highestimpact
tornado outbreaks since 1950, 10 occurred
during an El Nino, 4 during La Nina, and 5 during
neutral conditions.

El Nino 500mb Height:
ElNino_heights.gif


La Nina:

LaNina_heights.gif
 
Last edited by a moderator:
Dear Scott

One benefit of this discussion. I always heard that a cool Pacific favors tornado activity in the United States. As you indicated by several papers there is no consensus on the matter. Some say it increases, other say there is not a relationship between ENSO and tornado activity. It looks like the global warming question with different studies favoring diferent conclusions. In the last two decades here in South America, that you all know is very subject to tornadoes, the most important events ocurred during a cool Pacific. As we have members from Australia it would be interesting to know if a link can be established there.

Alexandre
 
I should have noted, the Forbes paper is a bit of an outlier and takes into account things like width, fatalities and damage in deciding which outbreaks get included. Plus it doesn't differntiate how strong the ENSO phase is. The Pielke-Knowles findings just looked at 40+ tornado outbreaks since they only look at strong phases. Still, even with some difference of methodology the number of outbreaks and strong/violent tornadoes is significantly higher during La Nina. The question then becomes whether the cooling takes place fast enough to have a thermal response in the troposphere which usually takes several months. I'm encouraged by some of the very recent data which shows a strong swing into negative territory and a good snowpack in the west. There's another factor that should be considered (brought up by Hollingshead) The CAI (Chase Anticipation Index) is directly proporational to the number of tornado outbreaks as well.

Alexandre, are you aware of any similar studies done on tornadic activity vs. ENSO phase in any South American countries?
 
Alexandre, are you aware of any similar studies done on tornadic activity vs. ENSO phase in any South American countries?

Dear Scott

It is almost impossible to answer your question. There is a tornado alley in South America (central and northern Argentina, Uruguay, Southern Brazil and Paraguay). Our Midwest/Central Plains. It is one of the most active regions regarding tornadic activity around the world. The following graphic was published by Scientific American mnagazine:

0001mapamundidostornadodt5.jpg


Do we have a ntework of radars in this region ? No !! Do we have and EAS system ? No !! Do we trained spotters ? No !! Do we have storm chasers ?? No !!! Government weather centers issue tornado watches and warnings ? No !!! In Argentina, there are only two weather radars and they are located around the capital Buenos Aires. Uruguay does not have a radar. The same in Paraguay. Southern Brazil has 4 weather radars. Just one is useful and it is located towards the north of the region. The result ? I am sure 95% of the tornadoes are not counted. Besides, the vast majority of the area is rural. So, we only find a tornado has touched down when it strikes an urban zone. So, any correlation of tornadic activity and ENSO is quite difficult here. But, there is always a "but", the trend is very clear here that in urban areas the most important events took place during a cold phase of the Pacifc Ocean. Let's see:

- May 2005 F3 Multivortex Tornado in Sao Paulo (see video in the Severe Weather in South america thread) ocurred when the Niño 1+2 region was cool.
- The Sao Francisco de Paula tornado in July 2003 was registered under the same conditions (cool SST in the Niño region of Peru and Ecuador).
- Bom Jesus 2001 tornado was registered under a near neutral/cool phase of the Pacific.
- And the most important event in Southern Brazil (a real outbreak of several tornadoes in one night with a F3) on October 11, 2000, took place during a cool phase of the Pacific.

It can be merely a coincidence, I have to stress that, but there are too many coincidences to my taste. See more on this events mentioned above (including photos and video) here:

http://www.stormtrack.org/forum/showthread.php?t=10771

For that reason we fear some important events could be on the way to us. There is statical evidence based on the most important tornadic events of the last decade that in Southern Brazil important tornadoes coincide with a cool or near neutral Pacific, specially the Niño 1+2 Region. By the way, the speed the Australian POAMA model (very reliable) is cooling the Pacific in the next few weeks is astonishing. Just check the red line (first 15 daily runs) and the blue lines (last 15 daily runs):

ftp://ftp.bom.gov.au/anon/home/ncc/...ges/plumes/sst_indices_30_days_20070125_3.gif

Time may prove I am wrong, but at least in my region I believe 2007 may reserve a tornadic activity higher than the average.

Alexandre
 
Last edited by a moderator:
Back
Top