New tool for predicting Santa Ana winds in California

Offshore flow severity index
Offshore flow severity index issued October 7, 2010. Click to enlarge.

A new tool is available that predicts the severity of Santa Ana winds in southern California six days in advance. Called the Offshore Flow Severity Index, it combines the relative humidity with the 1:00 p.m. wind speed predicted at the Saugus weather station to rate the events each day on a scale from one to four, with four being the highest. A category four event requires winds in excess of 30 mph with a relative humidity of less than 9%.

The term “offshore” is used because Santa Ana winds usually blow from a high pressure area over the Great Basin to a low pressure area off the southern California coast. These north, northeast, or east winds blow from the land to the ocean, or “offshore”, warming and drying as they move from high elevations to coastal elevations. Many of the larger and most devastating wildfires in southern California have occurred during offshore or Santa Ana wind events.

This forecasting tool will enable wildfire managers to modify their staffing or move fire resources into areas as needed to deal with the potential for large fires.

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UPDATE: October 26, 2011. Access to the site now requires a password, which seems very odd.

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Author: Bill Gabbert

After working full time in wildland fire for 33 years, he continues to learn, and strives to be a Student of Fire.

One thought on “New tool for predicting Santa Ana winds in California”

  1. Another “model” from the GACC that hasn’t been field truthed by the fire managers in the field, nor had collaboration from experts in fire behavior and fuels. I’m very concerned that we keep using models to make decisions that fire managers USED TO MAKE… such as proper staffing and the movement of resources.

    Currently, our live fuel moisture in chamise are running 52% (old) and 61% (new). Chamise will stay dormant through the winter until its next “green up” period which typically begins in March/April. During dormancy, chamise will keep its low end fuel moisture +/- 10%.

    Some of the largest historic fires in SoCal have come from “cold Santa Anas” following frontal passage and previous precipitation.

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