By Joe D'Aleo
Monday, June 2, 2008
In an earlier story in early April, we showed how La Ninas produce more extremes of weather and are more costly than El ninos. La Ninas typically bring more winter snows across the west and north (see that story here), more spring flooding, more spring tornadoes, dryness with more brush fires in Florida, more summer heat waves and drought, and more hurricanes. Last week we showed how the Gulf of Mexico and the Atlantic coast are more at risk of landfall in La Nina years. This week we will focus on tornadoes, in the news a lot this winter and spring. Active tornado seasons have been statistically shown to be a characteristic of La Ninas.
This paper by Bove in 1999 supported this La Nina to severe weather outbreak connection. An unpublished manuscript by Knowles and Pielke (1993) observed that tornadoes during ENSO cold phase (La Nina) are stronger and remain on the ground longer than their warm phase (El Nino) counterparts. They further showed that there is an increased chance of large tornado outbreaks (40 or more tornadoes associated with a single synoptic system) during ENSO cold phase (La Ninas).
Major tornado outbreaks occurred in January 1999, a recent La Nina year, in Arkansas and Tennessee and in May in Oklahoma and Kansas with $2.3 billion in damages.
The biggest outbreaks were decades ago in earlier La Nina seasons. The super outbreak of April 1974 with its 148 tornadoes that left 315 dead and 500 injured occurred during a very strong La Nina.
There were many F3-F5 storms in that 1974 Superoutbreak event (64).
Read more about that historic tragic day here and here.
In a prior La Nina event in 1965, the so-called Palm Sunday Outbreak had 78 tornadoes, 271 deaths and 1500 injuries. There were 38 F3-F5 tornadoes in that event in 1965, the second most active day on record. The outbreak is described in this two part blog by the Weather Doctor (Part 1, Part 2)
Massive double-funnel tornado near Dunlap, Indiana between Goshen and Elkhart. Photo courtesy of Paul Huffman of the Elkhart Truth.
This tornado formed just south of South Bend and tracked to just south of Elkhart. This tornado killed 45 in the town of Dunlap, 33 of those killed in a trailer park.
THE GREAT PACIFIC CLIMATE SHIFT AND DIMINISHED TORNADO FREQUENCY
The frequency of F3-F5 tornadoes dropped off after the Great Pacific Climate Shift in 1977. In the warm Pacific PDO, La Ninas became much less frequent and weaker and El Ninos dominated. In the cool Pacific PDO mode that preceded it, more La Ninas had been favored and occurred.
With the return of the negative PDO and La Ninas now, severe spring storms are increasing again. We have had over 1258 tornadoes (a preliminary number that will drop as a number of tornadoes are counted more than once) to date for the United States since January 1. They have caused 111 deaths. This is the most in a decade but not near the totals during the superoutbreak years or the years prior to when severe weather forecasts were issued and these storms took more people by surprise. In the late 1920s, as many as 800 died from tornadoes in a single year.
Expect as the La Nina slowly fades, severe weather to shift north to the Northern Plains and Upper Midwest in June (the Dakotas, Minnesota, Iowa, Wisconsin and Michigan).