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Don Paul: Wildfires get worse when downslope winds go Diablo

Anyone who has followed my work over the decades has probably heard me use the phrase “downslope wind” more than any three other meteorologists combined.

I say that only half-jokingly.

Downslope winds are those which heat up and dry out as they descend from a higher altitude to a lower altitude, and I have spoken of them nearly ad nauseum. For Western New York, our hottest days with rare upper 90s usually occur under the influence of a downslope wind. That’s because such a wind does not cross the relatively cooler water of Lake Erie where it would be modified to gain added humidity and some cooling from the marine influence. The marine layer is the reason we’ve never hit 100 and, amazingly, the reason Miami and Tampa haven’t either—at least not officially. If we ever do hit 100, it will almost surely be accompanied by a downslope wind. The downslope warming we experience here pales beside downslope warming east of the Rockies.

Downslope winds in the western United States can be another matter on impact. The much steeper and larger slopes can allow downslope winds to accelerate to higher velocities, to the point of becoming a threat to property. When a large ridge of high pressure builds in the Great Basin, west of the Rockies and east of the Sierras, winds moving up the west slopes of the Rockies can tumble violently as they move over the ridges and start to race down the eastern slopes, all the while gaining momentum, heating and drying dramatically.

Such winds have also torn roofs off buildings in places like Boulder, Cheyenne, and Fort Collins.

As for heating, Spearfish, South Dakota once experienced its temperature rising from -4 degrees F to +45 degrees F in two minutes as the downslope wind, called the Chinook in that part of the country, arrived on Jan. 23, 1943. That was a world record.

It was a destructive downslope wind which so gravely worsened the wildfire tragedies in California. In part of northern California, the wind is called “Diablo” because prominent Mt. Diablo has such a large slope which lends itself to damaging downsloping when a strong high pressure ridge builds in the Great Basin in just the right position and strength.

After the wettest winter in five years, lush vegetation grew as if in a rampage this past spring across much of what had been a critically dry landscape during a five-year drought preceding the heavy rains and snows. In the spring, the vegetation was rich in moisture. But California summers are virtually always hot and dry inland from the coast. This year was not only no exception, it was extraordinarily hot and dry. During one early September heat wave, downtown San Francisco hit its all-time high of 106 degrees, and Santa Rosa hit 110. The summer turned out to be the hottest in more than 100 years for the region. The formerly moist vegetation turned into a sea of dry kindling in the parched, hot pattern.

When the Great Basin high positioned itself so its winds could begin descending down ridges from the north and east of the Bay Area, such as Mount Diablo, the setup for the spreading and intensification of more “routine” wildfires exploded.

The downslope winds accelerated with hurricane-force gusts, drying and heating the air in addition to the fire’s direct contributions to drying and heating. Humidity levels often dropped to 10 percent or less in the affected regions.

How does the process begin? Prior to the downslope process, air is cooled as it ascends to higher altitudes on the other side of the mountain ridges. Cooled air has greater molecular density, and tends to sink. As it sinks down the other side of the slope, driven by the Great Basin High’s pressure, the air speeds up in its descent with the density and weight of these molecules pressing down and compressing the air, which causes the heating and drying.

Diablo winds are not dissimilar to the better known Santa Ana winds which occur in southern California.

The saving grace this past week in bringing some relief from the cataclysmic conditions has been the return of the marine layer from the Pacific and the relaxation of the barometric pressure gradient which led to such powerful winds.

There will be more episodes like this in the future, and there may be a tie-in to climate change. Most climate models project more frequent severe droughts in the southwest and California. We have just witnessed what happens when such severe droughts finally break in a very wet and snowy winter, supplying immense quantities of what eventually becomes kindling fuel in the wildfire season later in the summer and early fall. The climate change connection is not yet conclusive, but there is a growing body of evidence suggesting as much.

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