AccuWeather long-range forecasters are predicting an active severe storm season during the mid-spring and early summer of 2013, despite a slow March this year, compared to last year. In short, atmospheric conditions that have kept a lid of severe weather thus far will soon change.
"People can't let their guard down," AccuWeather Expert Senior Meteorologist Dan Kottlowski said. "It looks like everybody is going to be vulnerable to severe weather this year from the Gulf of Mexico in early April up to the Midwest by late in the spring and early summer."
Storm Threats and Areas of Concern
AccuWeather Lead Long-Range Forecaster Paul Pastelok is forecasting an average ramp-up of severe storm events with damaging wind and hail moving forward.
"The Deep South is going to be under the gun during during April," Kottlowski said.
According to Severe Weather Expert Henry Margusity, "The blocking pattern responsible for sending cold, dry air masses over the South and Gulf of Mexico should wind down during April."
The pattern has driven the jet stream well to the south, a necessary ingredient for providing energy for severe thunderstorm and tornado development.
"Weighing in a slightly later start to the severe weather season, the atmosphere will be hard-pressed to produce an above-normal amount of tornadoes this season, but we are likely to see the counts of tornadoes increase as we normally would moving forward from April onward through the spring," Margusity added.
Water temperatures have trended to near normal in the Gulf of Mexico after running below normal during the late winter.
That means low-level moisture supply (higher dew point temperatures) for the Deep South is poised to return when winds swing in off the Gulf.
Moisture-rich (high dew point) air is essential in the development of severe storms. However, Pastelok said that when a storm system is strong enough, it can compensate for the lack of moisture.
"We have seen a couple events already, and I think there are a couple more events coming in the next few weeks," Pastelok said.
The ingredients may come together for more violent outbreaks of severe storms and tornadoes during the second half of April and May. As water temperatures increase in the Gulf of Mexico and more humid air reaches the South, the northward shift of the jet stream will also be coming together.
Rotating severe storms, which parent tornadoes, require rapidly increasing wind speed or a change in direction with increasing height to help create a twisting motion in the atmosphere. This is known to meteorologists as wind shear.
The proximity of the jet stream to a potential severe thunderstorm formation area often increases wind shear and exploits other atmospheric conditions, tipping the scale toward development.
The transition time where the jet is slowly shifting northward during late April and May can set the stage for the worst outbreaks of the year.
"The quickness at which severe weather parameters come together is what usually determines how bad a severe weather outbreak is," Kottlowski said.
During the heart of the severe season, when the threat of tornadoes should be highest, an area to watch will be the lower Mississippi, Ohio and Tennessee valleys. Little Rock, Memphis and Nashville are among the cities that lie in the zone of greatest risk this year.
Overall, the number of tornadoes is expected to be near to slightly below average in 2013. The 10-year estimated average number of tornadoes to hit the U.S. annually is 1,300. However, the actual annual average is not known due to observing and reporting methods that have changed drastically over time.
Comparison to Last Year
This year is expected to be a more active severe weather year than 2012 when only 939 tornadoes occurred, according to the Storm Prediction Center. February and March only account for a small percentage of tornadoes, on average for the season.
The beginning of 2012 was very active with severe storm outbreaks and numerous tornadoes during January and February. During the typically active months of April and May, severe weather decreased in frequency in 2012.
The AccuWeather.com Severe Weather Team expects violent weather outbreaks to significantly ramp up during the second half of April and May of 2013, all things considered.