Dry autumns and winters may lead to fewer tornadoes in the spring, according to new analysis of long-term data

Global warming will likely mean more unpredictable weather, scientists say, and a new study by researchers at the University of Georgia pins down, possibly for the first time, how drought conditions in an area’s fall and winter may effect tornado activity the following spring.
The study, published today in the journal Environmental Research Letters, is specific to Georgia and the Southeast, but further study could reveal patterns that might make this more general—including the already tornado-prone Great Plains.

“Our results suggest that there is a statistically significant reduction in tornado activity during a tornado season following drought the preceding fall and winter,” said Marshall Shepherd, a meteorologist and lead author of the study. On the other hand, wet autumns and winters examined in the study had nearly twice as many spring tornado days as drought years did.

The research gives hope that one day meteorologists and climatologists may be able to predict the severity of a spring tornado season the way they now do for hurricanes. Other authors of the paper were Thomas Mote, also of the University of Georgia, and Dev Niyogi of Purdue University. Shepherd and Mote are in department of geography in the UGA Franklin College of Arts and Sciences.

The genesis for the research was the severe Atlanta tornado in March 2008, and Shepherd’s interest in how tornadoes form during severe drought years.

While such tools as Doppler radar have increased our ability to “see” tornadoes as they form, predicting a tornado season’s potential severity has remained elusive. The Intergovernmental Panel on Climate Change projected in 2007 that the frequency and severity of droughts may increase over time, but very little is known about drought conditions affect the frequency or intensity of severe weather hazards such as tornadoes.

To help understand how fall and winter weather might affect spring tornado seasons, the research team acquired the historical database of severe thunderstorms and tornado occurrences from 1951-2006 from the Storm Prediction Center of the National Oceanic and Atmospheric Administration. They also analyzed storm data reports from the National Climactic Data Center and meteorological drought conditions using historical rain gauge and Tropical Rainfall Measuring Mission (TRMM) satellite data from the National Aeronautics and Space Administration (NASA).

Using a number of tools of scientific analysis, the team primarily focused on tornado activity from March-June in Georgia and the Southeast. What they found was shocking, Shepherd said, yet plausible.

On average, wet autumns and winters presaged nearly twice as many spring tornado days in the study area as prior drought seasons. Springs following wet winters and falls were also five to six times more likely to have multiple tornado days than antecedent drought years.

“We do not suggest that soil moisture or precipitation the previous fall and winter exert a direct control on which individual storms will spawn tornadoes,” said Shepherd. “But these long-term seasonal relationships in the study area are striking.”

Correlating historical records and tornado activity has been difficult at best for scientists over the years. For one thing, the National Weather Service did not implement its watch and warning system until the mid-1950s, and only with advent of advanced radar techniques and ground examination of storm sites have researchers been able to say categorically that a certain storm even was a tornado. Also, studies linking tornadic activity with the El Niño cycle have been contradictory.

While it clearly seems that wet falls and winters lead to more severe spring tornado seasons, antecedent seasonal drought scenarios in north Georgia were almost never associated with above-normal tornadic activity the following spring over the 50-years period of the study.

The results for north Georgia were essentially replicated for the larger region encompassing Tennessee, Georgia, Alabama and Mississippi. For this entire region, a stunning 75 percent of years characterized by meteorological drought in falls and winters had below-normal tornado seasons in the spring.

While the new study, which was supported by grants from NASA, offers strong clues about how spring tornado seasons form, the authors urge caution in interpreting the findings until the analysis is repeated for other locations.

Just how the connection works between fall-winter rainfall and spring tornado seasons remains unclear. One possibility is that the atmosphere uses soil moisture “memory” from the fall and winter to modify conditions suitable for severe weather. A related hypothesis is related to “soil moisture” pockets and storm initiation.

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Spring’s best time to find invasive cogongrass

Cogongrass can completely take over a forested area, smothering out all other vegetation and changing the ecosystem. It’s a widespread problem in several Southern states. But not in Georgia -- yet, says a University of Georgia forestry expert.

From now until early summer, its fluffy, silvery seed heads will wave like flags marking spots in the forest, along roadways or other places in Georgia where cogongrass has taken a hold, said Dave Moorhead, co-director of the Center for Invasive Species & Ecosystem Health on the UGA Tifton, Ga., campus.

“Right now, no other grass in Georgia has that kind of seed head,” he said. “It’s the best time of year to find it and start measures to control it.”

Moorhead, along with 22 public and private partners in the Georgia Cogongrass Task Force, has spearheaded state-wide trainings to help UGA Cooperative Extension agents and county road crews identify cogongrass.

“County road crews out on equipment are more likely to see infestations, especially this time of year,” Moorhead said.

Since arriving as packing material aboard cargo ships landing in Alabama a century ago, the invasive grass has moved throughout that state, Mississippi and Florida, where it covers more than a million acres. But its spread in Georgia has been limited and slow, he said.

The likely reason is that cogongrass was planted as livestock forage for several years in the states that now have problems with it, but not in Georgia. And it’s best to keep it that way, he said.

The grass can grow any place that isn’t permanently wet. It’s even saltwater tolerant. Once it has a foothold in an area, the grass can be very difficult to kill, he said.

“You can’t control it with a single herbicide treatment. You can’t treat once and just walk away,” he said. “It’s an ongoing treatment for many years to eliminate it from an area.”

To date, cogongrass has been identified on 220 sites in 28 Georgia counties. The largest site is 10 acres. “But we know there are more sites out there,” Moorhead said.

To learn more about the grass and how to identify it, go to the Web site cogongrass.org. If you see a suspect site, contact your local UGA Extension office by calling 1-800-ASK- UGA1.

By Brad Haire
University of Georgia


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Georgia’s Spring Climate Outlook

Spring will likely be drier than normal for most of Georgia. March will have wide swings in temperature. A late freeze or even a snow or an ice storm isn’t out of the question. Severe thunderstorms and tornadoes are common this time of year.

Through most of this winter, Georgia’s climate was primarily a response to a neutral climate pattern. The neutral pattern is one of three climate patterns that have major influences on Georgia’s climate. The other two and better-known patterns are El Niño and La Niña.

This past winter, Georgians have experienced a classic neutral winter with periods of very cold and very warm weather. Rainfall in neutral winters can be very dry, near normal or very wet. This winter has been a very dry one.

Since the beginning of the year, the climate pattern has gradually shifted to a weak La Niña. This change is expected to have a major influence on the state’s spring climate.

It is important to realize that knowing which climate pattern we are in gives us only the probabilities of what to expect. It can tell us how we might want to hedge our bets, so to speak. It isn’t a guarantee.

Because the climate pattern is now in a weak La Niña, there is a very high chance that the coastal plain will experience a very dry spring. The chances of a very dry spring decrease into the northern piedmont. In the piedmont north of a Carrollton-to-Elberton line, near normal rainfall is the most likely outcome.

In the mountains of north Georgia, there are no clear indications of what to expect rainfall wise over the next three months. If a consistent storm track occurs over the mountains, then the spring may be wet. However, if the storm track is just 50 to 100 miles north of Georgia, then the mountains will experience a dry spring. The good news is that typically under a weak La Niña – like we have now – the storm track has a tendency to be further south, which means the mountains might receive some beneficial rains.

As far as temperatures are concerned, we can expect a continuation of a wide range, especially through the middle of April.

The date of the last killing freeze, or 28 F or below, or the last frost has no relationship with the climate pattern. Knowing that Georgia is currently under the influence of a weak La Niña tells us nothing about when the last freeze will occur.

Just as important, a warm March does not tell us anything about the likelihood of a late freeze. The 2007 Easter freeze is a prime example. March 2007 had been very warm and most plants had broken their dormancy. Then a devastating freeze hit in early April.

Snow and ice storms are not unusual in Georgia during March. Additionally, March through May is a time when severe thunderstorms and tornadoes are common. Because tornadoes can occur at anytime, day or night, all Georgians are encouraged to have a National Oceanic and Atmospheric Administration, or NOAA, weather radio at home. NOAA weather radios can be purchased at most electronic stores, large discount stores and many grocery stores.

By David Stooksbury
University of Georgia

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When Does Spring Begin?

When does spring begin? Well, it depends on whom you ask.

Most calendar makers list the first day of spring as the day of the vernal equinox, which occurs around March 20. This is referred to as astronomical spring.

Most atmospheric scientists, meteorologists and climatologists say it begins March 1. The National Weather Service uses March 1 as the beginning of spring for climate summary purposes. Dates for the beginning of the climatological seasons are March 1 for spring, June 1 for summer, September 1 for fall and December 1 for winter.

The dates for the beginning of the astronomical seasons are the vernal equinox (around March 20) for spring, the summer solstice (around June 21) for summer, the autumnal equinox (around September 22) for fall, and the winter solstice (around December 21) for winter.

Other ways of defining the seasons have been proposed. The one that is the most logical is to define the summer solstice as the midpoint of summer since the daylight is greatest then. The midpoint of winter is defined as the winter solstice since this is when the daylight is least. By this method, summer would be the six and half weeks before and after the summer solstice. Winter would be the six and half weeks before and after the winter solstice. The midpoint of spring would be the vernal equinox. For fall, it would be the autumnal equinox.

Other designations of spring include Good Friday or the start of the baseball season. My personal favorite is the Masters Tournament week in Augusta. Once the Masters is over, spring is usually here to stay in Georgia. Of course in Georgia, we can still have a dogwood or blackberry winter cold snap in late April or early May.

Regardless of which definition of the seasons one uses, spring in the Southeast is a season of transition. Early spring is characterized by wide swings in temperature with periods of very cold and very warm weather.

Freezes are common throughout Georgia during March. Freezes in late April do happen at times, especially in the northern half of Georgia. Snow and ice storms can happen in March, too, which has included a major blizzard in 1993 and an ice storm in the early 1970s.

By David Stooksbury
University of Georgia

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