In March, Colorado got a harsh early reminder from Mother Nature that peak severe weather season is just around the corner.
Two tornadoes – one in Weld County on March 22nd, and another on March 29th near Peyton – served notice that severe weather season, and the accompanying potential for tornadoes, is just a few weeks away.
Colorado averages 47 tornadoes per year, according to the Storm Prediction Center’s (SPC) official 25-year average. More than three-quarters of these come during the three peak months for tornadoes in Colorado: May, June and July. That’s when there’s usually enough warm air in place to support the potential for long-lived supercell thunderstorms, and the jet stream is still roaring strong enough that one of the ripe ingredients for tornadoes, wind shear, is still present as well.
Colorado had 39 tornadoes in 2018, the 12th-highest figure for any state last year. The majority of Colorado’s tornadoes last year, as is often the case here, were the result of so-called landspout tornadoes. These tornadoes form from the ground upwards towards the base of a storm, as opposed to the usual top-down method of a traditional tornado that you might see in Oklahoma or Texas. Landspouts tend to be weaker and shorter-lived than a traditional supercell tornado, but they’re usually more difficult to forecast and, as a result, can have less warning time associated with them.
That said, even a weaker tornado can be deadly, as winds inside a landspout can exceed 100 mph.
Since 1950, tornadoes in Colorado have killed a total of five people, according to archives from the Tornado History Project. The most recent killer tornado was the 2008 Windsor tornado, a mile-wide twister that tore through a swath of Weld County with estimated peak winds of 165 mph. It was rated an EF-3, placing it among Colorado’s strongest tornadoes on record.
Three tornadoes of F-4 (in early 2007, the official tornado measurement scale switched from the regular Fujita to the Enhanced Fujita) have hit Colorado, the strongest tornadoes on record in the state. They all touched down on the same day, May 18th, 1977 in southeast Colorado. Twenty-three EF-4 or F-3 tornadoes have been recorded in Colorado since 1950, meaning we average one roughly once every three years. These tornadoes usually present the greatest threat to life and property. A 2012 study estimated that more than two-thirds of all tornado deaths came from EF-3 or greater tornadoes, even with EF-3 or greater tornadoes only accounting for less than a quarter of all twisters.
In Colorado, the areas of higher risk are along the eastern Plains of the state. It’s clear that the flatter Plains are more susceptible to tornadoes, due to a variety of factors. It’s here that warm, humid air masses originating in the Gulf of Mexico can meet colder air masses to the north, and ample wind shear can create an environment capable of producing a tornado.
The most tornado-prone county in the United States, based on raw numbers from a 2017 study, is Weld County. The third-most tornado-prone county nationwide is Adams (mainly the eastern part away from Aurora). This is largely due to an area of natural spin known as the Denver Convergence Vorticity Zone, formed by our natural topography. That said, many of these tornadoes are of the weaker landspout variety.
In a tornado, the National Weather Service recommends taking cover in the lowest floor or a sturdy shelter, putting as many walls between yourself and the outside as possible.
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