LFM Forecast Variables Related to Santa Ana Wind Occurrences

Local and synoptic mechanisms causing Southern California’s Santa Ana winds

“…Two recent studies have also focused on flow acceleration in idealized gaps, and found that strong gap flow developed primarily because of downward mass and momentum fluxes, with strongest flows when geostrophic winds were parallel to the gap Durran 2004, 2006). These results provide a mechanism relating the synoptic scale pressure gradient to the strength of Santa Anas, and are somewhat consistent with the picture painted by previous research: Case studies have suggested that Santa Anas form because of an upper level northwest to southeast pressure gradient (Sommers 1978;Schroeder et al 1964), with a high sea-level pressure anomaly in the Great Basin (Conil and Hall 2006;Raphael 2003).…”

“…Two recent studies have used large scale surface pressure conditions, a high over the Great Basin and a low southwest of Los Angeles, to identify Santa Ana events (Miller and Schlegel 2006;Raphael 2003). Operational definitions combine this pressure gradient with two other parameters, the relative humidity in coastal Southern California, and wind direction in major mountain passes (Sommers 1978;Danielson 2007, personal communication). Conil and Hall (2006) used a mixture model cluster analysis of the daily mean wind anomalies to classify the winter circulation into three regimes, one of which was Santa Ana events.…”

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

“…Certain synoptic-scale conditions interacting with local topography contribute to Santa Ana occurrence (Yoshino 1975;Sommers 1978;Hughes and Hall 2010). This mid-February Santa Ana wind event commenced around 0000 UTC 15 February 2013, as maximum sea level pressures exceeded 1028 hPa in the Great Basin (Fig.…”

“…The Santa Anas are very dry, sometimes hot, offshore winds directed from the Great Basin and Mojave Desert over the mountains and through the passes of Southern California (cf. Sommers 1978;Small 1995) that can produce gusts exceeding 45 m s 21 (100 mph) in favored areas. 1 The winds evince terrain-associated amplification of the mountain gap and downslope varieties (Huang et al 2009;Hughes and Hall 2010).…”

“…Although the Santa Anas tend to form most frequently in midwinter, the most dangerous events often occur in autumn, before the winter rains have begun (Sommers 1978;Westerling et al 2004). At that time, the vegetation tends to be extremely dry, and fire danger is elevated owing to the combination of low to very low humidity and strong winds that can spark and spread flames.…”