Over the past several days we've shown you incredible video of the wildfire on the western boundary of Yosemite National Park. It's a fire that continues to spread and has put the city of San Francisco under a state of emergency. Why? It could threaten the city's water and power supply. The fire continues to spread toward a reservoir that the city gets most of its water from.
I looked more closely into the weather pattern that lead to the blaze and found a situation typical of most large fires.
Problem 1: Lack of rainfall. It's been an extremely quiet fire season in the Carolinas due to excessive rainfall. The opposite is true in California, where much of the state is in a severe drought. This includes the area near Yosemite National Park. In much of California, summer months with next to no rainfall are quite common. It's the winter months that are usually very wet. This year that wasn't so. 30 year averages for Yosemite National Park Headquarters for January and February are about 6.5 inches. Both months this year saw less than an inch.
Other nearby climatologic stations report yearly rainfall deficits of over 4 inches.
Problem 2: Very low humidity. Dewpoints have remained in the 30s, meaning that afternoon relative humidity values are as low as 30%. This very dry air extends throughout the lower (and upper) levels of the atmosphere.
Problem 3: The wind. Wind speeds have come down overnight, but Southwest winds are expected to gust to 30-40 mph again Sunday afternoon as an area of low pressure strengthens off the California coast. That would push the fire further North, closer to more structures.
Problem 4: The terrain. It's not strictly a weather problem, but the rough terrain of the canyons in the area are making it more difficult for firefighters and equipment to get in and out.
As of Sunday morning the fire was only 7% contained.
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