STORM TRACK: July 31, 1979 (Volume 2 Issue 5)

When I moved to Boulder from Ames, Iowa, about a year ago, I figured that chances of experiencing any severs weather of consequence nearby were minimal. I had heard of Boulder's reputation for strong winds, but an intense Iowa thunderstorm is hard to beat. My opinion on this latter point hasn't changed, but Boulder windstorms have proved to be both enigmatic and fearsome.

The most wind-prone areas average 5-10 days a year during which the peak gusts are at least 75 mph, and 1-2 times a year on which gusts to 100 mph or more occur. The peak gust reported in the Boulder area during the past 10 years or so is 147 mph on top of a building at NCAR (National Center for Atmospheric Research). January is the month with the highest frequency of very strong winds, but high winds can occur any time of year (August 14, 1978 a peak gust to 88 mph was reported and substantial tree damage).

These strong winds always blow from the west and are -thus- downslope. A schematic sketch of the gross aspects of the flow during a typical event, depicted in cross section, is given in the figure below. The dominant feature is the large-amplitude "mountain wave" shown by the streamlines with arrows. This feature has been observed by research aircraft flying traverses across the mountains. The inversion (or stable layer), indicated west of the Divide, is thought necessary to induce the tremendous amplitude of the wave. Forecasting strong winds in Boulder, then, requires an accurate forecast of conditions over and upstream from the Continental Divide. The west winds east of the Divide rarely extend far out into the plains. Boulder, being adjacent to the foothills, receives more strong winds of this type than any other city in Colorado (or the USA). Denver, 30 miles SE, is almost never affected (although suburbs west of Denver are)

Figure showing schematic airflow in a west east cross section across the Boulder area, The curved lines with arrows are streamlines; the spacing between these is approximately inversely proportional to wind speed. Typical locations of clouds are shown. The cloud over the plains just east, of Boulder is lenticular in form. The cap cloud along the Divide usually obscures the mountain peaks.

The strongest winds I have experienced so far came the afternoon of December 4, 1978. At NCAR, which sits atop a 6,000' mesa jutting eastward from the nearby "foothills" of 8,500' and overlooks Boulder 500-800' below, two gusts to ll5 mph were reported. In Boulder itself, rapid melt of a heavy snowfall 3 days earlier led to intermittent blowing of spray and sand (from earlier street sanding), making for difficult driving. I also heard, for the first time in my life, a continuous "freight-train roar" such as is sometimes described by tornado survivors.

In the parking lot, at. NCAR late that afternoon, I spent several minutes observing the following. The wind was less than 20 mph about 60% of the time! However, there was a low background noise of wind roaring through pine trees. Soon, this would become concentrated to the west and grow louder. Suddenly, a dominating screech would emanate from the NCAR buildings (150 yards west), lasting only a few tenths of a second, as the gust hit wires on the roof. Then the roar grew louder as I prepared for the onslaught by turning around to face east, squinting and bracing myself. Thus prepared, I succeeded with difficulty in remaining on my feet. The onslaught lasted only a few seconds and was over, the roar passing to the east,. This pattern repeated 4-5 times in the 5-10 minutes that I stood in the parking lot, observing it.

Surprisingly, only minor damage was reported in Boulder, mostly broken windows. IS such a storm were to strike Ames, particularly in summer, damage would have been much more severs. My hometown, San Diego, would be completely paralyzed by such a storm. In Boulder, many storms of similar or greater intensity have occurred over the past 10 years, leading to strict building codes and widespread public awareness

of wind related hazards.