Tornado Season Update–July 2012

By Paul Homewood

 

 

NOAA have now released preliminary numbers for US tornadoes in July, showing that the remarkably quiet season seen up to June has continued. As the graph above indicates, April through July are by far the busiest months, so it is unlikely that the rest of the year will make any significant difference to the overall trend.

As at 6th August, preliminary tornado reports total 905, lower than any year since 2005. ( NOAA’s Storm Prediction Centre take about three months to finally confirm actual tornado numbers and classification ). Figure 2 attempts to compare trends since 1954, with “inflation adjusting” to compensate for changes in methods of detection over the years. Again, the number of tornadoes is very low by historical standards. (For more discussion on these topics, see here.)

 

 

Figure 1

Figure 2

 

July Report

Figure 3

According to data from the SPC, during July, there were only 24 preliminary tornado reports. This is the least number of tornadoes reported during the month since 23 tornadoes occurred in July 1950 and July 1951. It is possible the number will be revised lower once storm surveys are completed. On average, the U.S. experiences 134 tornadoes during July. This also marks the least active tornado month since January 2011, when 16 tornadoes were confirmed. 

On July 28^th, a tornado touched down along the northeastern slope of Mount Evans in Colorado at an elevation of approximately 11,900 feet above sea level. The location was remote and no structural damage or injuries were reported. The tornado was uncommon due to the high elevation of its occurrence, marking the second highest elevation at which a tornado has been observed in the nation. Mountainous terrain and high elevations typically create unfavorable conditions for tornadoes, making these events rare, but not impossible. The highest elevation of an observed tornado in the country occurred in July 2004 at 12,000 feet above sea level in California’s Sequoia National Park.

Confirmed numbers for March.

The SPC have confirmed the actual numbers for March, with a total of 82 tornadoes of EF-1 classification or greater. As in February, this was unusually high, comparing with the 1970-2011 March average of 42. Since 1950, the March total ranks 4th highest, behind 1976, 1961 and 1991. The total for 1976 of 147 was a good deal higher though.

 

Figure 4

Confirmed numbers for April

There were 81 confirmed tornadoes in April, close to the 1970-2011 average of 85.

 

Figure 5

Last year, of course, April was the worst  on record, so what has changed. NOAA themselves admit that nobody really understands how tornadoes are formed, but do give some clues.

How do tornadoes form? The classic answer–"warm moist Gulf air meets cold Canadian air and dry air from the Rockies"–is a gross oversimplification. Many thunderstorms form under those conditions (near warm fronts, cold fronts and drylines respectively), which never even come close to producing tornadoes. Even when the large-scale environment is extremely favourable for tornadic thunderstorms, as in an SPC "High Risk" outlook, not every thunderstorm spawns a tornado. The truth is that we don’t fully understand.

 

Now take a look at the next two temperature maps from GISS for April, first last year and then this year.

 

 

There has been little change in the Gulf, where it has remained warm, but the cold air towards the North West that was so prevalent last year has simply disappeared this year. It seems that a warm Spring can have its advantages.