Downslope Windstorms of San Diego County. Part I: A Case Study

Abstract

The “Santa Ana” wind is an offshore flow that affects Southern California periodically during the winter half of the year, typically between September and May. The winds can be locally gusty, particularly in the complex terrain of San Diego County, where the winds have characteristics of downslope windstorms. These winds can cause and/or rapidly spread wildfires, the threat of which is particularly acute during the autumn season before the onset of winter rains. San Diego’s largest fires, including the Cedar fire of 2003 and Witch Creek fire of 2007, occurred during Santa Ana wind events. A case study of downslope flow during a moderately intense Santa Ana event during mid-February 2013 is presented. Motivated by the need to forecast winds impinging on electrical lines, the authors make use of an exceptionally dense network of near-surface observations in San Diego County to calibrate and verify simulations made utilizing the Advanced Research version of the Weather Research and Forecasting (WRF) Model, which in turn is employed to augment the observations. Results demonstrate that this particular Santa Ana episode consists of two pulses separated by a protracted lull. During the first pulse, the downslope flow is characterized by a prominent hydraulic jumplike feature, while during the second one the flow possesses a clear temporal progression of winds downslope. WRF has skill in capturing the evolution and magnitude of the event at most locations, although most model configurations overpredict the observed sustained wind and the forecast bias is itself biased.

A Surface Observation Based Climatology of Diablo-Like Winds in California’s Wine Country and Western Sierra Nevada

“…Despite the massive destruction wrought by the Diablo winds, associated with the Northern California firestorm of October 2017 and the Oakland Hills firestorm of 1991, very little is known quantitatively about them, especially relative to the Southern California downslope winds that favor wildfires, such as Santa Ana [1][2][3][4][5][6][7][8][9][10] and Sundowner [11][12][13][14][15] winds. A prescient preliminary analysis of 'Santa Ana-like' winds in the Oakland Hills was performed in 1973 [16], some 18 years prior to the Oakland Hills fire of 1991, and analyses [17][18][19] subsequent to that fire confirmed their localization at the eastern San Francisco Bay Area.…”

Some Requirements for Simulating Wildland Fire Behavior Using Insight from Coupled Weather—Wildland Fire Models

“…Southern California windstorm-driven fires are associated with Santa Anas-seasonal pressure-driven offshore flows-and sundowners. Curiously, although the phenomenon has long been recognized (i.e., [27]) simulations of the structure of Santa Anas (e.g., [73]) and Santa Ana-driven fire events (e.g., [17,59]) have been overlooked until recently. Similarly, the Diablo winds of northern California have historically driven some of the most destructive events (e.g., the 1991 Oakland Hills fire and the 2017 Napa and Sonoma County fires) but other than general descriptions of cause and appearance, have not been modeled in fine detail.…”

The Santa Ana Wildfire Threat Index: Methodology and Operational Implementation

“…Most notable was the Witch Creek fire in San Diego County, where wind gusts of 26 m s 21 were observed at the Julian weather station along with relative humidity values of '5%. However, high-resolution model simulations at 667 m showed that wind velocities were much higher in unsampled areas (Cao and Fovell 2016). This event became the catalyst for the development of a comprehensive Santa Ana wildfire potential index to better inform fire agencies, first responders, private industry, and the general public about the severity of an approaching event.…”

“…Although not frequent, significant fire activity associated with these winds in this zone has occurred in the past, which is why this geographic area is now represented in the index. Santa Ana winds across zones 1 and 2 are primarily a result of offshore surface pressure gradients (locally and/or synoptically) interacting with the local terrain to produce gap winds through the Soledad Canyon, the Cajon Pass, and the Banning Pass Cao and Fovell 2016). These winds also tend to precede the Santa Ana winds that occur across San Diego County by 12-24 h. Across zone 3, offshore winds take on a more ''downslope windstorm'' characteristic driven largely by the tropospheric stability (Cao and Fovell 2016).…”

“…Santa Ana winds across zones 1 and 2 are primarily a result of offshore surface pressure gradients (locally and/or synoptically) interacting with the local terrain to produce gap winds through the Soledad Canyon, the Cajon Pass, and the Banning Pass Cao and Fovell 2016). These winds also tend to precede the Santa Ana winds that occur across San Diego County by 12-24 h. Across zone 3, offshore winds take on a more ''downslope windstorm'' characteristic driven largely by the tropospheric stability (Cao and Fovell 2016). Other factors that led to the division of the zones were changes in terrain, National Weather Service Forecast Office boundaries, and local news media market areas.…”