California Wildfire Season Is Stretching Year-Round

By Jon Erdman
Published: May 20, 2014
  


California wildfires. We're used to hearing about them in summer and fall.

Consider, however, some notable wildfires in the Golden State, and nearby locations, since December 2013:

  • Late December: Big Sur fire destroyed over 900 acres and 34 homes destroyed. It was the first December red flag warning ever issued for the Bay Area
     
  • ​Mid-January: Fires erupt in the delta northeast of the Bay Area, including one on Kimball Island, prompting evacuations.
     
  • Late January: Rare January red flag warning in the Sierra; two wildfires in the coastal range of northwest Oregon, one of the most reliably wet locations during that time of year.
     
  • Late April and mid-May: Several fast-spreading wildfires in parts of Southern California.

(MORE: Carlsbad, Camp Pendleton Wildfire News)

San Francisco average monthly rainfall

Average monthly precipitation in San Francisco. Note the dry season running from May through September.

Typical Fire Season Peak

Summer into early fall is typically the peak for California wildfires, particularly for Southern California. The state's 20 largest wildfires on record dating to 1932 have all occurred from June through October.

Vegetation dries out during the summer, the state's dry season. Shifting winds and lightning strikes from occasional afternoon summer thunderstorms can start new fires or spread existing ones. 

By late September or October, hot, dry Santa Ana and Diablo winds return, capable of quickly turning any small brush fire into an inferno. Santa Ana winds are most common from October through March, but can occur anytime from September through June. 

As the wet season sets in later in November and December, the number and frequency of fire events tends to diminish as snow blankets the Sierra and moisture returns to the soil through the winter months.

Again, that's a typical scenario.

California Apr-May 2014 setup

Setup leading to enhanced fire danger in California in from late April through mid-May 2014.

2013-2014: Not Typical At All

What happens when California's wet season essentially doesn't happen?

The year 2013 was California's driest on record, dating to 1895. Persistent high pressure aloft over the eastern Pacific Ocean and West Coast effectively blocked most moisture-laden storms from reaching the Golden State by diverting the jet stream well to the north in the fall, winter and early spring of 2013-2014.

This was also the third dry winter in a row in the Golden State.

Prolonged warmth helped sap any moisture in the ground. May 2013 through April 2014 was the hottest such period on record in California. 

Most anomalous was the prolonged warmth in January 2014. According to Christopher Burt from Weather Underground, this heat wave may have been the most anomalous heat event in the U.S. since the March 2012 exceptional warm spell

By the typical April 1 seasonal peak, snowpack in the Sierra was a paltry 32 percent of average. This not only meant more of the Sierra and foothills were free of snowcover and available for drying earlier than usual, but was also a worrisome sign for those dependent on spring snowmelt for water supply and reservoir replenishment.

As of mid-May, just under 77 percent of California was classified as in extreme drought, the second-worst category in the weekly Drought Monitor analysis.

The result of all this: more fires occurring throughout the calendar.

As of May 10, 1,351 fires had been documented in California so far in 2014, almost double the five-year average for that period (711 fires), though the acreage burned is near average.

Unfortunately, with the wet season now over, no significant drought-relieving rain or mountain snow can be expected until at least October.

MORE: San Diego County Wildfire May 2014

Jeff Brown waters the roof of his home as vegetation smolders during a wildfire, Thursday, May 15, 2014, in Escondido, Calif. (AP Photo/Gregory Bull)


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