Long-term Rainfall Deficits Accelerate West U.S. Fire Risk

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Highlight: California (and much of the West) fire season 2021 concern.

Discussion: Comments made by officials regarding a brush/forest fire near Los Angeles the past couple days indicated the fire took place while relative humidity was near or above normal and wind was light. These conditions are usually not related to fire risk. Cited was the enhancing effect of long-term drought that propelled this particular fire.

Long-term drought is the issue! The recent updated 30-year climatology by NOAA cites steep decadal precipitation deficits concentrated on California (Fig. 1). The longer-term rainfall deficits are reflected in deep layer soil moisture loss which propels enhanced fire risk. Most of the 30-year precipitation deficit is recent and possibly related to the development of the “warm blob” of SSTA in the northeast Pacific in 2013.

During the past two decades the North Atlantic SSTA has turned warmer than normal and the increased low atmospheric moisture related to that warming leads to a wetter East U.S. climate.

There is a specific characteristic of “climate change” in this analysis. The warmer SSTA in the Northeast Pacific and the North Atlantic are associated with a warmer than normal middle atmosphere which means increased high-pressure ridging. Increased high-pressure ridging leads to higher risk of anomalous heat and dryness during the warm season and California certainly is targeted. However, warm SSTA also increases low-atmosphere moisture so when it rains it pours. Tropical cyclones moving into the U.S. over warmer waters of the past two decades also have a tendency to over-achieve with rainfall (Harvey and Florence are recent examples).

The incidence of dry extremes in the 1910-2020 West U.S. climatology has increased sharply during the past 10-15 years (Fig. 2). Dryness in California and the Southwest U.S. is frequently caused by La Nina/cool Pacific decadal oscillation (-PDO). However, more recent years has shown the “warm blob” as the primary catalyst to the drier climate.

The 12-month rainfall deficit in the western states identifies the recent problem…historical rankings of record to near record rainfall shortages (Fig. 3). Areas within or near the record dry zones are in danger of historic fire danger for the 2021 season.

Fig. 1: Updated precipitation trends on a decadal basis for 1991-2020.

Fig. 2: Western U.S. Palmer Drought Severity Index (PDSI) extremes 1910-2020.

Fig. 3: The 12-month divisional precipitation ranks for the U.S.