UC Irvine

The consecutive occurrence of three large wildfires in 1 year is rarely seen in northern California including the deadliest case during 8–25 November 2018 in Butte County. They happened under favorable fuel and meteorological conditions including long-term warming trends, normal summer-autumn warm-dry climate, and extreme heated air masses with strong offshore wind. The former two meteorological conditions contribute a background for large wildfires but extreme heated air masses and strong offshore winds are critical potentials for a fire spread. The relationship between extreme heated/cooled air masses and the three large wildfires was carefully examined from their starts to extinguishing. An anomaly-based synoptic analysis method is used by separating atmospheric variables into climatology and anomalies. A 3-dimensional anomalous synoptic pattern of atmospheric variables is established for the three wildfire cases in 2018. The analysis showed that one anomalous warm-air mass in the mid-to-low troposphere dynamically associated with a positive center of geopotential height (GPH) anomalies at the upper troposphere (150–300 hPa) is an anomalous synoptic pattern indicating a potential large wildfire spread, and conversely by negative anomalies when the fires were extinguished. The ECMWF model seems to be capable of predicting such anomalous temperature-pressure patterns to indicate possible fire spread and extinguishing.