News•March 21, 2014
Where are the Tornadoes? Slow Start, but No Guarantees
By Brian Kahn
In terms of sheer numbers, the past two years have been one of the quietest periods for tornadoes in the U.S. since the late 1980s, that despite some terribly destructive storms.
March represents a major weather transitionperiod. As spring spreads from north to south, snow melts, leaves unfurl, and heat returns to much of the U.S. The odds for tornadoes also increase as an influx of warm, moist air generally starts to push up from the Gulf of Mexico while cold, dry air drops down from the northwest. Those differences in winds and air temperatures are some of the large-scale factors that can give rise to severe storms that produce rain, hail and tornadoes.
An animation showing every tornado recorded from 1950-2012 by month. Data via NOAA Storm Prediction Center.
While tornadoes can happen any time of year and in any state, the Southeast is historically the first bullseye with most March tornadoes occurring in that region. From there, tornado-friendly conditions become widespread in the Southern Plains in April through June and in the Northern Plains in June and July.
This March hasstarted relatively quiet. On average, there are 80 tornadoes reported around the country during March, but so far only four tornadoes have been reported this month as a winter chill has lingered longer than usual over the eastern half of the country and cold weather tends to dampen the conditions tornadoes need to form. Average temperatures have been 3°-6°F below normal since the start of March for Mississippi, Alabama, and Georgia, the historical epicenter for March tornado activity.RELATEDOklahoma Tornado Shows Progress in Weather Warnings
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Cold weather played a role in suppressing the number of tornadoes in 2013. Persistent cool temperatures in March, followed by a finicky jet stream, helped keep tornado activity low in March and April. The year ended with 908 reported tornadoes, about 25 percent below the annual average of 1,200.
Chilly temperatures aren’t the only deterrent to tornadoes. Drought can also play a major role in squelching activity.
That was evident in 2012, when intense drought gripped the U.S. in May and didn’t let go all summer. In addition to crops and cattle, the parched ground left tornadoes with no moisture from which to draw. The year ended with 939 tornadoes reported.
Currentlydrought conditions are locked in across more than 60 percent of Oklahoma and Kansas, where tornado season typically shifts to in April. But even with the early season cold, and the ever-present drought conditions, how the 2014 tornado season plays out remains to be determined.
It's been 294 days since we issued a tornado warning. That's a new record for the longest tornado warning-free stretch at our office!
— NWS Norman (@NWSNorman) March 21, 2014
“Once we get to the point where we’re getting winds out of the south (from the Gulf of Mexico) at low levels, the atmosphere can recover pretty quickly,” said Harold Brooks, a preeminent tornado researcher at the National Severe Storm Laboratory.
Brooks said that winter conditions don’t really influence tornado season as a whole, citing 2012 as an example. Warm, wet weather in March helped fuel 154 tornadoes, more than double the average from 1981-2010. But the immediacy of the summer drought ultimately brought a quick end to tornado season.
While local factors like temperature, humidity, and soil conditions in a specific place on a specific day ultimately play a large role in determining if tornadoes form, scientists have been looking at the bigger climate picture for clues for seasonal tornado predictions. Those predictions would paint a broad outline of a given tornado season, which could be crucial information for disaster managers trying to decide how much money and resources to allocate for emergency response. One recent effort indicates ocean temperatures thousands of miles from Tornado Alley could provide that rough sketch.
That research, performed by Jim Elsner, a climate researcher at Florida State, looked at temperature anomalies in February in the western Caribbean Sea and the Gulf of Alaska. Though separated by nearly 4,000 miles, he said the sea surface temperatures in each region can influence tornadoes.
“If you think about what might portend activity a few months in advance, you have to look at ocean temperatures because ocean has memory and atmosphere responds to changes in ocean temperatures,” Elsner said in an interview.
Damage in Moore, Okla., a week after a powerful tornado ripped across the town in May 2014.
The long “memory” of the ocean is due to the fact that ocean temperatures change much more slowly than those over land because the ocean is a huge, moving mass and a lot of heat must be added to or removed from a body of water before it changes temperature (that is why coastal areas have such balmy climates). Patterns in the atmosphere such as the jet stream and storm tracks are, in part, dictated by those slow changes.
Elsner found that for every 1.8°F increase in ocean temperatures in the western Caribbean, 51 percent more tornadoes are reported from April-June in the Great Plains. In the Gulf of Alaska, each 1.8°F above normal corresponded with a 15 percent decrease in tornado reports.
That makes sense based on the dynamics needed to form tornadoes. Warm waters in the Gulf of Alaska tend to fuel blocking ridges and reduce the amount of wind shear severe storms need to ramp up. Meanwhile, warm Gulf of Mexico waters can steer more moisture to the Great Plains.
This past February, sea surface temperatures were near normal in the Gulf of Mexico and 1.8° to 3.6°F above normal in the Gulf of Alaska. That means the odds might be in favor of less tornado activity, though the outcome is far from certain. Elsner said ocean temperatures in the Gulf of Alaska were one of the factors that could affect a tornado season, but they’re far from the only one, explaining only a fraction how many tornadoes form in any year.
Even in a quiet year, major tornadoes can still drive up damages and loss of life. In 2013, the incredibly destructive Moore and El Reno tornadoes in May as well as a rare November outbreak in the Ohio River Valley did just that.
The Moore tornado was an EF-5 tornado—the highest rating—that killed 25 and caused $2 billion in damage in and around Moore, Okla. Just 11 days later, the El Reno tornado formed in central Oklahoma and quickly expanded to 2.6 miles across, making it the widest tornado ever recorded. It killed 8, including 3 experienced storm chasers who were caught off-guard by its rapid expansion and sudden change of direction.
And we’re only three years removed from the incredibly destructive 2011 tornado season. That year saw 1,679 tornadoes reported including a record-breaking 758 tornadoes in April, the most ever recorded in a single month by a wide margin.
This year forecasts from the Climate Prediction Center indicate by late May there could be some improvement to the drought in the Southern Plains, which could increase the chances for tornadoes in the region. In a spring outlook issued on Thursday, forecasters with the National Oceanic and Atmospheric Administration said there are no clear indications that this year will mimic last year in terms of tornado activity, but they also refused to speculate on if the season would become more active.
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