Tornado season currently below average in central U.S.

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May is the month when we often see the most tornadoes across the U.S. As of late last week, there have been about 33 confirmed sightings around the country. The average for this month is 276 with the majority occurring in Texas (43), Oklahoma (28) and Kansas (38). Both Idaho and Washington average 1 tornado during the month of May.

Based on the numbers, May is currently reporting a “below average” month for tornado activity in the U.S. However, there have been a number of outbreaks earlier this year including one on Jan. 21 and 22 in Mississippi and Georgia. Feb. 28 had deadly outbreaks in Illinois and Missouri. April was another active month with tornadoes in Louisiana, South Carolina, Texas and Mississippi. There were rare tornadoes spotted in Arizona and New Mexico last week.

For 2017, the U.S. has seen approximately 600 twisters. In an average year, there are about 1,200 tornadoes sighted in the U.S., more than any other place in the world. More than 60 percent of all U.S. tornadoes each year occur in what is called, “Tornado Alley,” which stretches from Texas and Oklahoma northward through Kansas and eastern Colorado into Nebraska and Iowa.

Wind-measuring instruments like anemometers are often destroyed by tornadic blasts exceeding 250 miles per hour as tornadoes are nature’s most destructive storms. Pieces of straw, for example, can penetrate wood at wind speeds exceeding 230 miles per hour. The strongest wind speed ever measured in a tornado was 280 miles per hour in Kansas in 1997.

Usually beginning in early spring, low-level winds from the south/southeast bring copious amounts of moisture and sub-tropical air up from the Gulf of Mexico into the central and southern Great Plains and the Lower Midwest. At the same time, much cooler and drier air spills down the eastern slopes of the Rockies crashing headlong into the warm, moist flow from the Gulf of Mexico. This is a recipe for meteorological disaster as the colder air tries to push toward the ground, it’s heavier, while the warm air attempts to rise through the sinking currents. These huge differences in both temperature and moisture at various levels often triggers strong winds, torrential rains, crop-damaging large-sized hail and, occasionally, deadly tornadoes.

Fortunately, in the calmer Inland Empire, the Cascade Mountains to the west and the Rockies to the east usually protect us from the extremely powerful thunderstorm activity. But, every spring season, and sometimes during the hot summer months, we do see an occasional period of extreme weather conditions.

Tornado intensities are measured using the Enhanced Fujita Scale, with an EF5 being the most destructive. This scale was implemented in early 2007 and has the same design as the original Fujita scale, which included ranges from an F0 to an F5.

Idaho averages 3 tornadoes per year. One of the worst severe weather and tornado outbreaks ever seen in our region occurred on May 31, 1997, across eastern Washington and northern Idaho. On that day, four F1 twisters hit Stevens and Spokane counties with one F1 tornado striking Athol in North Idaho and an F0 spotted near Lewiston. Severe thunderstorms also produced hail up to 2-3 inches in diameter, very heavy rainfall and wind gusts of over 80 mph. Fortunately, there were no deaths or injuries.

An estimated record 10 tornadoes touched down in Washington and Idaho on May 31, 1997. In Kootenai County, an F2 was reported, one of the largest ever seen in Idaho. An F1 was reported in Jefferson County.

IN TERMS of our local weather, we’ll continue to see periods of showers for much of the week. I do expect to see some days with sunshine and very warm temperatures as highs will challenge the 80-degree mark sometime early next week. We’ve already had our first 80-degree day on May 4 with a high of 83 degrees.

As we get close to the “new moon” cycle of May 25 through 31, our region should once again see an increase of showers and thunderstorms. Our normal May precipitation total is 2.37 inches and we should be close to that amount rather than seeing record or near-record amounts of moisture like we had in February, March and April.

More showers and thunderstorms are expected into early June before conditions should start turning drier and warmer than normal. The summer of 2017 still looks very warm with only occasional afternoon and evening showers and thunderstorms, but mainly over the mountains.

Contact Randy Mann at randy@longrangeweather.com

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