James Waller, PhD, Research Meteorologist
As a commentary on the 2014 severe convective season and resulting “noise” in the media, severe weather reports remain below the average. This is true for each of the sub-perils of tornadoes, hail and straight-line (nontornadic) wind.
As of May 1, 2014, U.S. Storm Prediction Center (SPC) preliminary reports for the 2014 year stood at only:
- 276 tornadoes compared to the 2005-2013 average of 533
- 1007 versus 1729 for hail
- 1688 versus 2239 for nontornadic wind
This follows the recent and deadly outbreak from April 27-30. The question on severe weather activity still bears some scrutiny.
Severe convective events such as tornadoes, hail and wind require enabling conditions of:
- Moisture – Air from the Gulf of Mexico is ideal.
- Instability – Warm at the surface and cold at flight-level. The sharper the contrast the more unstable it becomes. When you think of a hot air balloon, it rises because the surrounding air is less dense or “lighter” than the warmer air in the balloon. On a hot day rising air currents can act in much the same way. Spring frontal systems can bring very cold air left over from the poles over hot muggy air, and ideal conditions for instability.
- Wind shear – changing winds with height. Changing wind speeds with height bring a rolling motion to the atmosphere, and rising air currents in a thunderstorm tilt this rolling motion into the vertical (with circulation looking like a corkscrew). Changing wind direction together with speed is ideal, prompting even better conditions for severe weather.
Once enabling conditions are in place, a trigger is needed such as a cold front or a sea-breeze to force air to rise.
The missing ingredients for the 2014 season prior to April 26 were warm, moist air. Between the calendar beginning of spring and April 26 we saw the ongoing grip of winter weather lasting longer than usual (much longer for the Midwest and Great Lakes).
In other words it was too cold.
As spring progresses in earnest, warm moist air has pressed further north, and enabling conditions for severe weather have and will continue to combine more frequently. The quiet spell we have seen in severe weather for the 2014 season has ended
The quiet 2014 season is a seasonal anomaly and is not evidence of any long-term trend in tornado frequency.
Thinking about tornadoes over the longer term, Simmons et. al. (2012) (1) normalized tornado losses against economic factors over time. The article suggests a decline in normalized tornado losses. The paper also suggests significant normalized losses in notable outbreak years of 1965 and 1953, and to a lesser degree 1974. The article attributes this to increasing wealth, property value and net exposure (and possibly improved building stock since the 1970’s). Recent media articles speculate that this may be due in part to a declining trend in tornado frequency (although this is not supported by the data).
Chuck Doswell (2012) (2) (a pioneer of severe convective research), stressed that there is no evidence to suggest a long term trend in tornado activity. The events of 2011 are not unprecedented. Outbreaks of this nature have happened before, and will happen again. The statements are supported by significant outbreak years of 1965, 1973 and 1974, amidst great volatility from year to year.
The threat of strong tornadoes is an unfortunate fact of life for those living in North America (anywhere south of the permafrost line or away from the mountains). Tornado preparedness, including attention to our building stock and construction codes, should be an ongoing priority in light of this threat.
1. Simmons, Sutter and Pielke (2012) “Normalized tornado damage in the United States: 1950 – 2011”, Environmental Hazards -Vol12, Issue 2.
2. Doswell, Carbin and Brooks (2012) “The tornadoes of spring 2011 in the USA: an historical perspective”, Weather – Vol 67 No 4
Local NWS watches and warnings, and statements from local emergency management agencies supersede this update, and should be closely heeded concerning matters of personal safety.
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