5 Reasons Weather Played a Dominant Role in California's Blue Cut Fire Explosive Growth

Jon Erdman
Published: August 17,2016
California's Blue Cut fire grew rapidly after igniting Tuesday in Cajon Pass, about 45 miles east-northeast of downtown Los Angeles.
(MORE: Latest Blue Cut Fire News) 
This was only the latest in a string of large wildfires to hit the state, including the Clayton fire, which destroyed at least 175 buildings in Lake County.
The weather factors leading to this explosive fire growth spanned the time spectrum from the multi-year drought to conditions Tuesday afternoon.

California's Long-Term Drought

The state's drought has been well publicized, first developing in late winter 2012, then from 2013 into 2014.

Areal coverage of drought in California from Jan. 2000 through August 9, 2016. The current, long-term drought is highlighted. Darker colors correspond to progressively worse drought categories, as defined by the Drought Monitor analysis.

Repeated wet seasons (late fall through early spring) featured a stubborn dome of high pressure aloft in the eastern Pacific Ocean near or over the Golden State, diverting the Pacific storm track north.
Consider the notable statewide records set in California during this drought (data from NOAA's National Centers for Environmental Information)

• 2015: Second warmest year
• 2014: Record warmest year
• 2013: Record driest year
• 2012: Fourth warmest year

Underwhelming Wet Season Precipitation

California's "wet season" typically runs is November-April. It's during this time that the lion's share of the state's precipitation (rain and mountain snow) accumulates.
This past wet season did provide some drought relief in northern California, particularly north of Interstate 80.

Change in drought monitor analysis in a roughly 12-month period ending in early August 2016. Green (yellow/brown) contours show areas of progressively more drought improvement (worsening).

But that was not the case in Southern California.
Despite a record-tying strong El Niño (further weakening the El Niño/wet California myth), many areas in the L.A. Basin piled up sizable precipitation deficits since last fall.

Southern California Rainfall Deficits: Oct. 1, 2015 - August 16, 2016
(Data: NOAA/NWS)

City	Precipitation Departure From Average	Percent of Average
L.A. (Downtown)	-7.79 inches	47 percent
Riverside	-6.70 inches	45 percent

With fewer Pacific storms bringing soaking rain to Southern California, soil and vegetation dried out faster in the spring.
(MAP: Where Large Wildfires Are Most Common in the U.S.)

It's the Heart of the Dry Season Now

August is firmly in the middle of California's dry season, which spans from, roughly, May through October.

Average monthly rainfall in downtown Los Angeles, based on 1981-2010 averages. August, sitting squarely in Southern California's dry season, is highlighted.

Apart from an isolated thundershower during the wet phase of the monsoon, which sometimes produces more lightning and strong, shifting winds than rainfall, there's virtually no chance of moistening up soil and vegetation for a few months.
(MORE: Why California Wildfire Relief Is Virtually Impossible This Time of Year)

Subzero Dew Points

We've pounded the drum on how dry it's been, but it's not every day you see an air mass as dry as what we saw Tuesday in the mountains and high desert of California.

Surface dew point temperatures the afternoon California's Blue Cut Fire started, August 16, 2016.

In fact, dew points in the several locations dropped below zero in several locations near and north of Cajon Pass in the afternoon, according to the National Weather Service in San Diego.
Yes, that included a -16 degree dew point at Edwards Air Force Base. This pushed relative humidities about as low as you can go.

Daily Winds

Perhaps the key ingredient for this wildfire's explosive growth, however, was strong winds.
Typically in the summer, the hot desert and inland valleys of Southern California and the Pacific Ocean-cooled air near the coast drives winds onshore in the afternoon, persisting into the early evening.
These daily, or diurnal, onshore winds funnel through canyons and passes, and Tuesday was no exception in Cajon Pass.
Radar loops illustrated how the strong canyon winds blew the Blue Cut fire's smoke plume to the northeast, eventually into the Las Vegas metro area by early Wednesday.

If that wasn't enough, toward the back end of the dry season, infamous Santa Ana winds howling through locations such as Cajon Pass, become more common.
Jonathan Erdman is a senior meteorologist at weather.com and has been an incurable weather geek since a tornado narrowly missed his childhood home in Wisconsin at age 7.