Residents across the Denver-Boulder metro areas, especially those living in Longmont, snapped numerous photos of the elegant funnel during its graceful march about the landscape. Hundreds to thousands of images and videos appeared on social media of the unusually tall tornado, which occurred beneath a developing thunderstorm shortly after 5 p.m. local time.
Passengers departing and arriving at Denver International Airport witnessed the tornado from their aircraft, including Adrienne Vonn, who happens to be a meteorologist at Spectrum News in Texas. She tweeted that she received a tornado warning on her phone while still in the air, glanced out the window and saw the funnel dancing beneath a thunderhead in the distance.
“Nearby tornado warnings and weather causing departure and arrival delays,” tweeted Denver International Airport. The tornado was visible from the concourse.
Power lines were downed off Highway 66, which runs east to west to the north of Denver. Damage to crops and outbuildings was reported. Based on the damage seen so far, it seems likely that the tornado had winds between 80 and 100 mph.
Denver-based storm chaser Jim Tang, who drives thousands of miles in pursuit of tornadoes on the Plains every year, was shocked when the atmosphere brought the show close to home on a day when few expected it.
“[I] was wrong to believe chasing was any less than 70 percent luck,” he tweeted.
His photos show concentric rings of dust surrounding the main condensation funnel. Those rings are caused when dirt and dust are centrifuged outward by the spiraling winds to different radii based on their grain sizes.
The vortex was even captured live by television news helicopters, later headlining the 5 p.m. news:
The highly visible tornado was made all the more dramatic by extremely high lifting condensation levels, also known as cloud base heights. That meant there was plenty of space between the cloud and the ground. The tornado also developed before curtains of rain descended, allowing those far and wide to take in the funnel.
This tornado was apparently a landspout, meaning it didn’t form as the result of traditional supercell processes or from the preexisting spin of a rotating thunderstorm.
Supercell thunderstorms exist in environments characterized by strong wind shear, or a change of wind speed and/or direction with height. That makes for spinning updrafts capable of generating tornadoes. Supercells are comparatively easy to forecast.
Instead, Monday’s tornado in Colorado was probably born from other processes.
A preexisting whirl near the ground, perhaps broad and diffuse, became entrained in the updraft of a developing storm. The resulting “vortex stretching” tugged the vortex all the way to the cloud base, causing the vortex to become narrower and, subsequently, more intense. It’s similar to how you will spin faster on a desk chair with a smaller radius, or with your arms, hands and legs pulled inward. (Try it. It’s a great way to pass the time working from home.)
Weld County, Colo., where Monday’s tornado occurred, is technically the most tornado-prone county in the country, largely thanks to these kinds of events, which peak in summer.
While in part a product of its size, the county’s proclivity to hosting twisters is thanks largely to its placement within the Denver Convergence Vorticity Zone. That’s an area in the lee of the Palmer Divide where the terrain enhances surface winds, causing them to come together. That generates storms, which can vertically stretch eddies that form from the colliding breezes.
There is also a chance that the storm may have, in fact, had some of the characteristics of a rotating supercell thunderstorm, making the tornado a “hybrid” mix somewhere between full landspout and true supercellular tornado. Doppler radar did indicate some cloud-based rotation.
The National Weather Service will be out Tuesday surveying the damage. In the meantime, forecasters and storm chasers are eyeing the potential for more severe weather across the High Plains this week.
Check out some more social media pictures and videos of Monday’s tornado: