The Meteorology Behind the Marshall Fire: Explaining the Mountain Wave

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The most destructive fire in Colorado’s history occurred on December 30, 2021, destroying more than 1,000 homes. The Marshall Fire tore through the towns of Superior and Louisville with intense speed driven by extreme winds created by a mountain wave.

The Boulder National Weather Service created this page, with detailed descriptions of weather conditions that led to gusts reaching over 100 mph.

Much of what follows below has been copied from this page, with some minor altercations.

“Strong winds developed mid-morning on Thursday, December 30, 2021, following a mountain wave that developed as very strong westerly winds passed through the mountains and foothills of the Front Range and crashed on the plains. The mountain surge remained virtually unchanged for the remainder of the day, resulting in very persistent and extremely high winds. Mountain waves are generally concentrated very close to the base of foothills and adjacent plains. Sustained winds of 50 to 60 mph gusting to 80 to 100 mph were felt along Highway 93 and pointing east around Superior and at times, Louisville that day. It just takes the right combination of weather parameters, including stability, wind shear and wind magnitude to create a powerful and damaging windstorm like this. To better visualize what a mountain wave would look like, see the image below.

The diagram below depicts strong westerly winds flowing down the mountainside (in this case, the mountains and the foothills of the Front Range) and accelerating to the base of the foothills. From there they spread east into the Superior and Louisville area, before suddenly weakening east (call it the jump region). It was interesting to note in this case that easterly winds were observed at times immediately east of the “jump” area around Broomfield and Lafayette. It can also be called a rotor.

The video below from Chris Holder, shows the jump region discussed above, perfectly illustrated by the rising smoke.

“The fire started shortly after 11 a.m. and quickly spread east, fueled by gusty winds induced by 80-100 mph mountain waves and extremely dry grass. The descending portion of the windstorm held the strongest winds and smoke very close to the surface. The smoke then rose vertically into the hydraulic jump region of the mountain wave, indicated by a linear, nearly stationary cloud cap. This region of rising smoke also roughly indicates the eastward extent of very strong westerly winds.

“Time-lapse video in Figure 8 (replace with images above) visualizes the flow of the mountain wave, with strong winds crashing to the ground from Highway 93 east toward Davidson Mesa and Superior, before flowing vertically upwards in the “jump” seen in the diagram of the previous section. This view gives us a mirror-like image of the mountain wave diagram, since we are now looking south towards the wave rather than north as in the concept diagram.

“In the early afternoon, the mountain surge was fully established and clearly visible in the smoke and cloud pattern. The time-lapse images in the video above, taken from the southwest side of the metro area from Denver, were shared by Mike Nelson and Mario Gori, showing the smoothness of the mountain wave. Note the reflection shown in the first strongest wave, then the second and even the third wave following on the far right of the l Smoke is clearly visible in grey, while clouds are also shown on the crest of the wave where vertical upward motion has been maximized above the “jump” area.

The plume of smoke from the fire is clear in scanned radar images from Denver International Airport. While the fire was most intense, reflectivity values ​​in the yellow hue are apparent, indicating the highest cloud tops created by the smoke plume and mountain surge.

For more information, please visit the original page with this detailed breakdown of the dynamics of this terrible fire.

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