As a meteorologist and climatologist, I have visited Boulder, Colorado on several occasions, as several NOAA facilities and the National Center for Atmospheric Research (NCAR) are located there. My heart sank when I heard the news of the fast moving wildfires destroying over 500 structures or homes (at the time of writing) in the surrounding area. Chances are I know someone who has it. A colleague asked how this could happen so quickly. In this case, the initial weather pattern indicates strong winds in complex mountainous terrain. Here’s a quick “101” on what happened.
The morning before the fires, the National Weather Service (NWS) – Boulder announced very strong winds. They tweeted, “Confidence is building in very strong winds across much of Larimer and Boulder counties from now until this PM. Gusts can be sporadic and localized, but the potential is there for gusts of 75-90 mph for wind-prone areas (especially the foothills). The NWS Thursday morning discussion noted that “intense flow across the barrier and weak shear aloft” could amplify mountain waves.
Let’s quickly decipher the meteorological jargon of this statement. The flow through the barrier indicates that the wind was blowing essentially perpendicular to the mountainous terrain of the region. Low wind shear means there has not been much change in wind speed and/or direction with altitude. The radar image (above) of the smoke ash plume associated with the Marshall Fire (as seen on the satellite below) is a strong indicator of westerly flow.
A deeper dive into the NWS forecast discussion on Thursday morning hints at the emerging concern. They wrote: “The flow through the barrier is in the range of 55-70 knots so if it mixes I certainly can’t rule out strong winds at lower elevations near the base of the foothills for a few time. The service forecaster pointed out that one of the high-resolution weather models showed gusts of up to 90 mph that could spread eastward. The forecaster wrote: ‘I can’t ignore what I’m seeing so I’m going to issue a high wind warning for areas 36, 38 and 39. The graph below gives an indication of how mountain waves and the mixing of stronger surface winds can occur over complex terrain in these situations.
Media reports that downed power lines (due to high winds) sparked the fires in the already drought-stricken region. The double blow of the winds coupled with the human footprint (downed power lines and structures in the path of fast-spreading fires) led to this tragic set of circumstances. As I write this on New Year’s Eve morning, the winds have died down, but now the area is bracing for a winter storm. Ironically, this will also be an impact event, but it can also help fight persistent fires.