If there is a tornado “season” in Boulder, it would center on the month of June. Since 1975, eight of 14 tornadoes reported within Boulder County occurred during this month, when relatively moist air on the plains can boil up into violent afternoon thunderstorms.
Tornadoes are rated in intensity from F1 (maximum winds of 86-110 miles per hour) to F5 (winds greater than 200 mph), and just two of the Boulder County tornadoes observed since 1975 achieved even F3 (136-165 mph) status. On the evening of May 22, 2008, a severe thunderstorm tracking north from Longmont toward Greeley spawned the infamous Windsor tornado, which destroyed 78 homes within a three-quarter-mile-wide swath during its 40 minute rampage.
A twister with estimated winds of more than 140 miles per hour clipped northeastern Boulder County on June 4, 2015, churning across the prairie for 6 miles while damaging dozens of homes and farm buildings.
Just an hour’s drive east of Boulder County, tornadoes occur more frequently and cause much more damage. A total of 13 twisters have touched down within 10 miles of Limon since 1982, including an F3 that destroyed much of the downtown area on June 6, 1990.
It turns out that the same mountains that spawn Boulder’s legendary winter winds protect us from tornadoes in summer. Cold, relatively stable air over our mountains impedes daytime heating and development of the most severe thunderstorms.
But mountain tornadoes do occur. On June 16, 2015, a twister touched down southeast of Nederland and stayed on the ground for several minutes. A 1987 tornado in the Grand Teton-Yellowstone region achieved F4 strength while churning across 10,000 foot-high ridges.
Tornadoes form when warm, humid air boils up into cumulonimbus clouds and encounters colder air, creating wind shear and rotational energy. The Great Plains region of North America, where cold air flowing off the mountains and southward from Canada meets warm, humid air surging up from the Gulf of Mexico, may experience more thunderstorms and tornadoes than any other region on Earth.
The city of Boulder receives more than 60 thunderstorms per year, but few are severe. The farther east you go toward the Missouri and Mississippi valleys, the more dangerous the storms typically become.
This year’s late-spring tornado season is off to a disturbing start, with more than 200 storms touching down in the Midwest and High Plains during the last week of May. Colorado experienced at least nine tornadoes on May 26 alone. Climate scientists warn that these outbreaks will become progressively more severe as the Earth warms, increasing the amount of water vapor in the atmosphere.
In addition, urban areas throughout the Western Plains are expanding into some of the most active tornado pathways. As areas along the Northern Front Range corridor east of I-25 fill up with subdivisions, more and more people risk being exposed to severe weather.
After summer solstice, as solar energy diminishes and Canadian cold fronts retreat northward, tornadoes become less frequent in Colorado. But severe thunderstorms may occur throughout the summer months. Last July, golf ball-sized hailstones produced by thunderstorms killed nesting birds as large as American kestrels and caused millions of dollars in damage to cars and roofs in eastern Boulder County.
Most Boulder County communities have outdoor warning sirens to alert residents of impending tornadoes, wildfires or floods. If the sirens go off during a thunderstorm, take shelter in an enclosed, interior space.
Other June events
• Bighorn lambing peaks in the high mountains, while mule deer fawns appear in urban gardens. Elk and white-tailed deer also give birth.
• Common nighthawks perform aerial courtship displays, making a “booming” sound as air passes through their wings.
• The first Colorado columbines unfurl in the foothills and mountains.
Stephen Jones and Ruth Carol Cushman are authors of “Wild Boulder County” and “The North American Prairie.”