When the mountain wave sets off a storm, pilots learn from it


Pilots based in the Front Range of the Rockies in Colorado know that winter brings the wave to the mountain.

What they not wait in winter? For this wave to fuel a storm of flames on the ground.

On Thursday afternoon, a grass fire likely started by downed power lines around Marshall was fanned by a massive conflagration by these winds. The fuel of a wet spring and extreme drought that lasted all fall – and the fact that there was no snow cover around the Denver metro area in late December – made which appeared to be an abnormal event.

Unfortunately, adjacent suburban communities quickly lit up in the rapidly advancing firing lines. In the neighboring towns of Superior and Louisville, more than 570 homes had been destroyed by Friday afternoon.

Rocky Mountain metro evacuates

The general aviation relief airport on the north side of Denver, Rocky Mountain Metro Airport (KBJC) is located just south of the point where the fire started, on a wide mesa.

Metro is home to several flight schools, Signature and Sheltair FBOs, the Broomfield campus of Spartan College of Aeronautics, and a host of other GA companies.

It is also the site of Pilatus Business Aircraft, the American headquarters and the finishing center for the Swiss manufacturer of the PC-12 NGX turboprop and the PC-24 light jet. OEM’s new 188,000 square foot facility opened in 2018.

As of Thursday afternoon, much of the airport was inside the compulsory evacuation zone and was besieged by the plume of smoke, prompting its closure in the late afternoon. The last METAR was released around 3 p.m. local time, and regular reporting resumed Friday morning.

Early reports from the area indicate that the airport has been largely spared, including Pilatus, as it is about 2 miles from the southern edge of the known line of fire.

How a mountain wave works

The furious airflow that passes over the Continental Divide and descends through the foothills below is generated when the air is cold and stable, forming layers between which the prevailing winds blow halfway up (normally between 12 000 feet and 24,000 feet msl) can accelerate and create a waterfall in what appears to be a sine wave, over the rocks below.

Downwind of the mountain range, this wind drops and creates the powerful gusts that periodically plague the Colorado plains from Pueblo to Fort Collins.

While the mountain wave often generates telltale cloud formations both at altitude (standing lenticular altocumulus or ACSL) and at the height of the pattern (rotor clouds), it is mainly an invisible and difficult phenomenon. to view.

However, with the introduction of massive amounts of smoke and particles in wave or föhn winds, the outlines of their power have caused pilots, meteorologists and weather observers in the metropolitan area to take photos and videos. .

The pronounced leeward wave – so called for its position downwind of the mountain range – has been reported by several weather observers, including distant observers such as atmospheric science professor Neil Lareau.

The plume itself was visible for nearly 100 miles on the otherwise clear winter afternoon, testifying to the strength of the downdrafts and how they might affect airplanes in flight.

Other associated phenomena were also observed, including the formation of breaking Kelvin-Helmholtz waves downwind of the fire.

The recovery begins

Aviation will play its part in firefighting and recovery efforts to the extent possible. Last night, from a base at Denver Centennial Airport (KAPA), a Colorado state-operated multi-mission Pilatus PC-12 scouted over the fire area as local winds have calmed down enough to allow flight operations to resume.


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