Last year had a jump in the number of tornadoes thanks to the strong La Nina of the winter and early spring. La Ninas produce a stronger contrast from north to south feeding spring storms and severe weather. Heavy record snows last year maintained the cold north and strong contrast and led to enhanced activity and flooding. A preliminary count of 1690 storms will be reduced when final analysis in storm reports is completed as some storms are counted multiple times.
In the following NSSL analysis you can see the early season activity last year and again this year characteristic of La Ninas. Las year June was especially active as the storms fed off the contrast and wet grounds from snowmelt and heavy winter and early spring precipitation.
Severe Weather Summaries
2009 Severe Weather Summary
2008 Severe Weather Summary
2007 Severe Weather Summary
2006 Severe Weather Summary
2005 Severe Weather Summary
2004 Severe Weather Summary
2003 Severe Weather Summary
2002 Severe Weather Summary
2001 Severe Weather Summary
2000 Severe Weather Summary
The march of the season – climatology of tornadoes normally follows this depiction (source here)
Some of the monthly data shows this tendency for an early season peak in the south and later in the north as the jet stream lifts northwards.
We don’t yet have a number of strong tornadoes (F3+) for 2008 but there was likely a spike in those much as there was in the strong cold PDO La Ninas of the 1950s to 1970s most notably 1973/74.
Strong tornadoes are mainly found east of the Rockies.
Every state but Alaska has a statistically measurable probability of tornadoes.
More on tornados in weeks ahead.
Storm Prediction Center
The Online Tornado FAQat SPC
SPC Tornado Safety page
Enhanced Fujita Tornado Scale
Fujita Tornado Scalewith photo examples from the Tornado FAQ
SPC lists of deadliest U.S. tornadoes, F5 tornadoes since 1950, tornadoes hitting major downtowns, and websites devoted to historical tornadoes
Tornado Odditiesand Tornado Mythsdiscussed by Tom Grazulis
A page dedicated to the 50th Anniversary of Tornado Forecasting
Questions and Answers about Thunderstormsby NSSL