Severe weather season and tornadoes 

Severe Weather Awareness Week for the state of Georgia is February 6-10, 2012. This year, the topics to be covered at this link http://www.srh.noaa.gov/ffc/?n=swaw12_main include:

• Family Preparedness
• Thunderstorm Safety
• Tornado Safety ( Statewide Tornado Drill)
• Lightning Safety
• Flooding

In addition, the statewide tornado drill will be held on Wednesday, February 8 at 9am.

A La Nina spring like this one in the past have been deadly in the nation. The winter headlines in Europe have in the past been a precursor to such cold and snow in the Eastern Third of the US so we have to stay alert that we don't go from mild to wild and from shorts to shivers. Some models suggest this in the Feb. 11-Feb 20 period but others do not.

The preliminary count for tornadoes in the US last month was 97 including three here in Georgia. Nationally it’s at least the third most active start to the season since 1950. Last year was the deadliest tornado season since 1925. And the final count of 1,700 twisters ranked in second place for all-time yearly numbers. Last April 758 tornadoes touched down which is an all-time monthly record.

The climate pattern known as La Nina was present last year and a weaker version is again with us. This raises concern for another active spring severe weather season. The lack of cold so far this winter has allowed the water in the Gulf of Mexico to be unusually warm. So now any time the wind turns out of the south, warm, humid air is easily drawn across the Deep South. High humidity sets the stage for thunderstorms violent enough to produce tornadoes. The gulf waters provide the moisture (dew point/humidity) are the fuel source for storms and if the waters are warmer you get more evaporation and warm air to feed the storms and increase the temperature contrast between southern air and cold air trying to invade from the north.

An above-normal number of tornadoes are anticipated in areas from Texas and Louisiana to Missouri and Arkansas to Tennessee and as far east as western Georgia.

Research indicates that La Nina winters and springs have a tendency to produce stronger and more destructive tornadoes. Studies show they also tend to produce more tornado outbreaks or what are termed “families” or “tornado swarms” where a high number of tornadoes occur from a single storm system often in just one or two days.  Last April 27^th set the all time record of 200 twisters on a single day.

These type seasons also often produce more long-lived long duration long-track tornadoes that may be EF-3 to EF-5 (winds 136 to over 200 mph) sometimes traveling hundreds of miles on the ground covering multiple states.

So the concern again for February to May is for a greater than average number of larger and more destructive tornado outbreaks. During La Nina’s the jet stream storm track and its energy is increased as the cold air to warm air contrast is often greater than normal increasing the strength of the baroclinic zone making for stronger jet stream winds.  Those more powerful winds add dynamic energy and spin to aid in storm rotation and faster than usual forward movement of parent thunderstorms and their offspring tornadoes.

Last year a local weather pattern nick-named “the wedge” kept the worst severe weather north and West of Atlanta and for the most part away from Georgia.

We can hope this happens again but there are no guarantees.

In general, if we can have a coolish end of winter and spring season the threat of severe weather and damaging storms will be lessened. Warm humid air is more unstable than cool dry air. So let’s hope we do not have too warm an early spring, or that if it is warm that it is also dry. We don’t want drought but you can’t have tornadoes without thunderstorms.  The lakes in North Georgia are dong well as the typical dryness associated with La Nina has been more in Central and South Georgia so far.

If we are lucky the jet stream will stay far enough North to keep the worst of the severe weather away from us, or maybe it will turn cool long enough to keep it at bay, or perhaps we just get lucky and have our protective friend the wedge kill off storms as they move toward us. The colder more stable air of the wedge pattern can act like a protective shield when it is in place prior to a squall line approaching. But if we don’t have a wedge in place and we get an unseasonably warm humid air mass in place ahead of a strong jet stream cold front, look out and be prepared. Fingers crossed for another benign storm season.