Diablo Winds are warm dry winds originating when strong high surface pressure builds over the northern Great Basin. The resulting flow from high pressure inland to lower pressure off the California coast is warmed and dried by compressional heating as the air sinks from the Great Basin, which is nominally between 4000 and 5000 feet, down to sea level. The primary impact area is over and downwind of the Coast Range and Diablo Range, from about Lake County in the north to San Benito County in the south. [This pattern also produces strong north winds down the Sacramento Valley and downslope northeast wind in the northern Sierra Nevada.]
These areas of high pressure in a favorable position in the Great Basin are most common in the fall and winter months as the polar jet stream makes its seasonal progression southward. Troughs and ridges moving along the jet stream begin affecting the northern portions of the Great Basin. In certain configurations, the jet stream pattern encourages strong sinking motions and the development of surface high pressure areas in the northern Great Basin. In that position, these highs can generate Diablo Winds and when they move or extend farther south Santa Ana winds can be the result.
The seasonal timing of Diablo, and Santa Ana, Winds is extremely important in regard to wildfire danger. One of the chief characteristics of California’s predominantly Mediterranean climate is its protracted dry period from about May through October. Consequently, these strong, warm and dry winds occur after months of very little, if any, precipitation and when fuels (i.e., grasses, shrubs and forests) are at their driest.
The exact trajectory and strength of Diablo Winds is due to the strength and location of the high in the Great Basin and likewise the strength and location of lower pressure to its southwest. During a Diablo Wind event this trajectory will often begin as a more northerly wind and shift to one from the northeast. This flow is significantly altered as it flows over and down first the Sierra Nevada and then the Coast Range, with the channeling of the wind over ridges and down canyons sometimes drastically increasing its velocity. The vertical temperature patterns associated with the Diablo Wind weather type often have the result of constraining the wind flow against the mountains, almost as if there were a “lid” just over the top of the coastal mountains. Winds pushing from northeast to southwest are then squeezed through and augmented at the ridge line and on the lee, southwest, slopes in what is known to meteorologists as the “hydraulic jump” phenomenon.
One common “rule of thumb” diagnostic for Diablo Winds in general is to look at the pressure gradient (i.e., difference in pressure) between Winnemucca in northern Nevada and San Francisco; with the higher the difference meaning stronger wind speeds. This comes with the large caveat that the exact location of a Great Basin High and offshore low can dramatically alter the resultant wind speed and direction.
The current and previous 24 hours of Winnemucca (WMC) to San Francisco (SFO) pressure gradients can be found in the lower half of https://www.wrh.noaa.gov/mtr/versprod.php?pil=OSO&sid=001, under SFO-WMC. The more negative the number, generally the stronger the offshore winds.
Historically, here are several significant SFO-WMC surface pressure gradients days:
Oct. 20, 1991: -13.1 mb Tunnel Fire
Oct. 8, 2017: -17.8 mb North Bay Fires
Oct. 24, 2019: -16.3 mb Kincade Fire
The etymology of the term “Diablo Winds” dates to shortly after the 1991 Tunnel Fire which devastated a large area of the Oakland and Berkeley hills. Myself and another forecaster (John Quadros), working at the National Weather Service office in Redwood City at the time, found that after the 1991 fire, calling offshore wind events in the Bay Area as “the northern California version of Santa Ana winds” was awkward at best, and meteorologically fuzzy at worst. We somehow fell upon the name Diablo Winds as a double entendre because they generally blow from the direction of Mt. Diablo in the far East Bay, and "diablo" translates from Spanish as “devil”; thus, devil winds.
Special thanks to Dr. John Monteverdi for his suggestions and editing.
Jan Null, CCM
Certified Consulting Meteorologist
Golden Gate Weather Services