It’s no wonder that the Marshall Fire spread as quickly as it did since winds were gusting 60-100+ mph at times on December 30. High wind events in the Foothills and along the adjacent Plains are not uncommon for Colorado. In fact, just a few weeks ago we had a similar wind event unfold bringing 100+ mph winds to areas near Colorado Springs.

A big question being asked right now is why were the winds that strong? The answer is because there was an extremely amplified mountain wave over Colorado.

The National Weather Service (NWS) in Boulder started alerting the possibility of strong winds in and near the Foothills on Wednesday afternoon due to a mountain wave setting up. The setup showed a classic wind event possible as strong winds flowed down from the mountains of the Continental Divide (commonly known as chinook or Foehn winds). Wind gusts at that time were forecast to reach 75 mph by Thursday afternoon near Boulder and Golden and Estes Park. By Thursday morning, forecasters at the Boulder office were growing concerned about the possibility of even higher winds. The combination of several weather factors were showing that a mountain wave pattern could be amplified – which essentially means that the expected strong winds could be even stronger than initially thought. That’s when the NWS issued a high wind warning for the Foothills and adjacent Plains to the west of Denver. By the mid-morning update on Thursday, 100 mph winds were starting to be reported across Jefferson and Boulder counties.

But how does this happen? Why was this wind event so strong? How did we end up with a truly historic wind event so quickly?

When asked about winds, David Barjenbruch, the Lead Forecaster at the NWS in Boulder said that “forecasting winds and their exact speeds is probably one of the most difficult tasks for a meteorologist.” When you add in terrain like the Rocky mountains and certain atmospheric conditions, you can get the perfect recipe for extreme and damaging winds.

A mountain wave is a weather term that can be compared to water flowing over a boulder. When water rises up and flows over a boulder it speeds up as it descends downstream and creates ripples. When air flows up and over the continental divide in Colorado, it speeds up as it descends towards the Foothills and adjacent Plains.

Although forecasting winds is extremely difficult, there are 6 ingredients to look for when forecasting them. As Barjenbruch explained, you need some but not all of these ingredients to get winds over 100 mph in the Foothills and adjacent Plains. December 30 provided some but not all of the ingredients necessary.

  1. Cross Barrier Flow – These are winds of a certain direction above the surface. On December 30, the winds were almost due perpendicular to the continental divide setting the stage for a perfect mountain wave set up. On top of this, the wind flow above the surface were very strong pushing 50-60 knots (55-70mph) further amplifying the wave.
  2. Mountaintop Stable Layer – This is essentially a point in the atmosphere that acts as a blocking mechanism for air that is being forced up from the surface. When wind hits the Rocky mountains, air is forced up but because of the weak stable layer that was near the mountaintops on December 30, the air was blocked and forced back down creating an up and down motion – or a wave.
  3. Downward vertical motion – this essential relates to whether we’re under a high pressure or low pressure. Under high pressure, air sinks. The opposite is true for low pressure. For December 30, there was not much support showing that there was downward vertical motion. Relating this to a mountain wave, down vertical motion would aid in transporting strong winds aloft down to the surface.
  4. Jet Streak Position and Intensity – We all know the jet stream is that fast-flowing “river” of air above us. Embedded within the Jet Stream are areas of exceptionally fast winds known as Jet Streaks. If a city is located under a jet streak, there are assumptions that can be made in regards to what kind of weather is expected based on which side of the streak they are on. Barjenburch explained that winds are stronger “usually in the subsident right exit region of the upper jet, but instead, it looks like we (Superior and Louisville) were located under/near the subsident left entrance region.”
  5. Strong Pressure Gradients – Winds are created by pressure gradients. The tighter the gradient, the faster the winds. In Colorado, when forecasting wind events, we look at the pressure difference across the mountains say from Denver to Grand Junction. On December 30, the pressure gradient was relatively weak, about half of what we typically look for when forecasting high wind events.
  6. Little Vertical Wind Shear – We usually hear about wind shear when there is a severe weather threat but wind shear, or a lack of it, can be crucial when forecasting mountain waves. The less wind shear there is, the better chance that a mountain wave can “break.” Think about a wave in the ocean, when it breaks, it crashes down with a vengeance. Similarly, when a mountain wave “breaks” it causes winds to crash down to the surface. On December 30, the wind shear was very light making for ripe conditions for a breaking mountain wave.

As we saw on December 30, some but not all of these features need to be in place for a mountain wave to be amplified. “The elevated, or near mountain top stable layers and shear profiles are key to the wave “breaking” and mountain wave amplification. The realized wind at the surface is extremely sensitive to the vertical distribution of these flow features, thus making downslope windstorms difficult to forecast,” Barjenburch explains.

While comparing the winds in the atmosphere to water in a river is great imagery, it’s not an exact comparison. The slope of local mountains and the amount of energy converted due to that slope (known as a hydraulic jump by researchers) can have large impacts on winds and could be a main driver of the wind storm we saw unfold.

In video that was sourced online, you can notice several features happening that were referred to above.

In the timelapse above, winds are flowing down off the mountains. This is the “breaking” mountain wave. The wind hits the surface and bounces back up. You can notice the smoke immediately rising downwind and then descending again.

Another view shows winds blowing from west to east. Once the fire starts, you can see the ripples that the mountain wave is creating downwind.

Overall, December 30 was an extreme wind event in Colorado caused by a multitude of factors and atmospheric conditions. Another factor of this disaster is the fact the area has been under rapidly developing drought and has not received beneficial moisture since June. Looking even further back, the extremely wet Spring that we had led to grasses and shrubs growing tall and healthy creating more “fuel” for fires once the drought set in.

Forecasting is hard work. It’s ever-changing and evolving. Here in Colorado, the joke is that if you don’t like the weather, wait five minutes. If you believe that’s true then you know that forecasting here must be extremely difficult. Staying updated with the weather and having a respected source to get current and up-to-date weather information is key to preparing for what Colorado can deliver.

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