Thursday, April 28, 2011

Record Heat Fuels Historic Tornado Outbreak

Video (click to play): Tuscaloosa, Alabama tornado by Chris England from University of Alabama, via YouTube

For related record temperature posts, see:
6 PM Update: NCAR Currents has just published an analysis of the factors leading to such a high death toll in yesterday's tornadoes:
Recipe for calamity: The ingredients of a horrific tornado outbreak

The overall death toll has now been reported as 272 across 6 states.

Original post:
OK, let's get the caveat out of the way right up front: No individual temperature ever caused a specific storm.

When a pattern persists over a large area for an extended period of time, however, it's worthy of notice. The latest reports indicate hundreds of people killed, 162 in the state of Alabama alone, from the tornado outbreak which pounded the Southeast yesterday (April 27). The National Weather Service's Storm Prediction Center has a preliminary count of 164 tornado reports for the day and 3 additional reports so far today. This is the culmination of a month which has undoubtedly broken the April tornado record with 453 separate storms tallied by The Weather Channel. This total, which apparently does not yet include the widespread outbreak of April 27, smashes the previous record of 267 in 1974 and approaches the all-time monthly record of 543 set in May, 2003.

Temperature is only one factor in the development of severe storms, but heat provides the energy which makes thunderstorms and their associated tornadoes possible. So far this month, NOAA's National Climatic Data Center reports 1759 record high temperatures in the U.S. vs. 310 record lows, a ratio of nearly 5.7 to 1, exceeding even March's 5.3 to 1. This is the highest since the ratio of 6.1 last April. There have now been 13 out of the last 14 months with more heat records than cold records. The cumulative ratio since January 2010 is 2.2 to 1.

As shown on the map to the right, April temperatures to date have been above the long-term average across much of the U.S., particularly over the southern portion of the country.

Images (click to enlarge):
  • Radar image at 10:25 am, April 28, shows showers and thunderstorms stretching almost continuously from the Canadian border southward to the Gulf of Mexico, from Intellicast
  • Tornado and severe storm reports for April 27, from Storm Prediction Center
  • Monthly total number of daily high temperature, low temperature, and high minimum temperature records set in the U.S. for November 2010 through April 2011, data from NOAA National Climatic Data Center, background image © Kevin Ambrose ( Includes historical daily observations archived in NCDC's Cooperative Summary of the Day data set and preliminary reports from Cooperative Observers and First Order National Weather Service stations. All stations have a Period of Record of at least 30 years.
  • Cumulative excess of heat records vs. cold records by month for 2010 and 2011 to date, data source as above
  • Preliminary April 2011 monthly temperature departure from climatological average from NOAA/NCEP via ESRL

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