Severe Weather Season off to fast, active start
This severe weather season is off to a fast and active start just as forecast, similar to past La Nina springs. We are ahead of the three-year average. January started with a bang and now so has March.
We are well above the March average. Last April was particularly bad at a record level and May of 2008 was a record for tornadoes and February 2008 was very active (Super Tuesday outbreak) all during La Nina. So we have a record May 2008, record April 2011, and now maybe a record March 2012. The current La Nina is weak and weakening fast with neutral to El Nino conditions expected by next winter.
Research by Cook and Schaefer has shown that compared to neutral El Nino La Nina (ENSO) springs significant differences are season based on the ENSO phase. In the cold phase or La Nina outbreaks typically occur in a zone stretching from SE Texas Northeast to IL, IN, and MI. During the warm phase or El Nino activity is mainly limited to the Gulf Coast states and FL.
For the Mid-South and Southeast states more tornadoes occur during La Nina months and the number of strong and violent tornadoes goes up. Georgia is not in the top ten states for tornado occurrence, deaths or damage but Tennessee and Alabama are.
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April 1965 had a swarm of killer tornadoes and it, too, was a La Nina.
The Friday-Saturday system produced 303 tornado warnings and over 389 severe weather warnings giving the young year over 237 reported tornadoes.
While a more active than normal season is still expected in the U.S. it does not look as bad a tornado season as last year for the country as a whole. This is because the baroclinic energy index or measure of horizontal temperature differential north to south is expected to be less extreme this spring than last year, in part due to a lack of snow cover up north. This means a weaker jet stream (lesser dynamics) and an earlier shift pole ward of the mean jet stream storm track.
It also looks like the big out of the gates start to March is not a sign of a dangerous active month. IF I am right about following model evidence of more March warmth and high pressure ridging aloft the next 30 days, then most of March after the 12th will be on the quiet side for tornadoes especially in the South and East US.
However, severe weather season may ramp up again in April and May as the ridging relaxes allowing more temperature clashes as the jet stream intensifies, as well as a tendency in the modeling and analog sets to show “bowling balls” i.e. closed lows aloft. So we still expect a worse than normal tornado season but not as bad as last year. It is typically the Mid-Mississippi River Valley, Tennesee Valley and Ohio River Valley that are most under the gun in a La Nina spring like this one.
Strong jet stream-level winds set the stage for Friday’s deadly weather
Powerhouse 160+ mph jet stream-level winds (between 18,000 and 39,000 ft.) were in place across the United States Friday. The product of a huge north to south spread in temperatures, they played a pivotal role in turning swarms of thunderstorms severe.
The shift in wind speed and direction that occurs with height in such an environment, twists air as it ascends through the atmosphere. It’s a process that can literally rotate entire thunderstorms, helping to generate tornadoes.
2012 severe season may rank among the busiest 10 to 20-percent to date in the wake of Friday’s tragic twister outbreak
Depending on final tallies this latest severe weather outbreak could be one of the six largest in the past 60 years.
It’s exceedingly difficult to rank tornado outbreaks right after they occur. Many try only to later find that the tornado counts released right after a storm outbreak include multiple reports of some of the tornadoes, inadvertently inflating their number. Experience has shown 15-40 percent of early tornado reports fall into this category.
Storm Prediction Center (SPC) meteorologists perform post-mortems on the country’s tornado outbreaks; efforts, which cull, duplicate reports from the database yielding more accurate final tallies.
Be that as it may, what happened across sections of 15 states Friday to Saturday was a tragedy of the first order and appears to be the biggest March tornado outbreak in at least 4 years. Greg Carbin, Warning Coordination Meteorologist at SPC, says the horrific March 1, 1997 outbreak, which included 35 EF2 or stronger tornadoes and left 27 dead, might offer an early benchmark for gauging the relative severity of this 2012 meteorological horror.
Carbin says preliminary data leads him to believe the 2012 U.S. severe weather tally from January and February into the opening days of March may rank among the busiest 10 or 20 percent of all the severe seasons since SPC records began in the 1950s.
As far as March is concerned this outbreak could go down as the largest in March on record! March 11-13 2006 holds the record now with 74 confirmed tornadoes.
Though the 36 tornadoes that occurred during the February 28 – 29 Leap Day outbreak were part of a separate storm system, the 5-day tornado total from February 28 – March 3, 2012 may eclipse the late January 1999 tornado outbreak as the most prolific 5-day period of tornado activity on record for so early in the year.
At one point, 31 separate tornado warnings were in effect simultaneously during the outbreak. An area larger than Nebraska–81,000 square miles–received tornado warnings, and tornado watches and severe thunderstorm warnings were posted for almost 400,000 square miles–an area larger than Texas. Some of the storms were moving so fast 60-85 mph it reduced warning lead time and the time needed to take cover and the twisters so large (wedge tornadoes) it was impossible to get away. The tornado that hit Henryville, IND for example was a mile wide, the town is only two miles wide. To add insult to injury many badly hit towns will now get some snow.
The speed with which some of the storms moved was truly exceptional. A number of the tornadoes ripped through Kentucky with forward speeds of 70 mph, and two tornado warnings in Central Kentucky were issued for parent thunderstorms that moved at 85 mph. If damage surveys reveal that these thunderstorms did indeed spawn tornadoes, they will set the record for fastest-moving tornadoes in recorded history. The record for the fastest moving tornado is 73 mph, set in 1925 for the great Tri-State Tornado, the deadliest U.S. tornado of all-time.
Could it have been worse in Georgia?
You bet. Look no further than Alabama for proof of that. Remember my forecast information on the radio Friday morning during “Atlanta’s Morning News” pointing out a couple factors that would impact how it would play out for Atlanta. I felt that if we got a lot of sunshine during the day that would make things worse. Fortunately sun was limited. I also felt that if the front accelerated and the storms came in earlier than expected it would be worse but if the squall line slowed down the threat would also go down.
As it happened we kind of split the difference with the first storms coming at the start of the ‘window of worry’ but most of them holding off until after midnight. This gave the atmosphere time to cool and stabilize somewhat and allowed the strongest wind shear to lift northeast away from Atlanta while the greater moist unstable air settled over central and south Georgia. Another factor that I think may have helped prevent the EF-3 tornado to grow even stronger and stay on the ground longer is one I did NOT anticipate. I think it also helped prevent additional tornadoes.
A thin layer of dry air around 4,000 feet came into the region. Gusty winds during the day mixed some of that dry air down to earth reducing dew points and helping to disrupt storm development and the connection between upper level wind shear rotation and the surface. This meant the spin had trouble remaining rooted to the surface boundary layer. Elevated instability in the atmosphere is less dangerous. I really fear that if the front had been faster and that dry air had not moved in we would have had a destructive and deadly EF-3 or EF-4 tornado on the ground from Paulding across Cobb, far north Fulton, northwest Gwinnett and into Hall where doppler tracked the supercell storm.