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Fire Data: Dry Winters Mean More Charred Acres

Tim Walton

Photo: Tim Walton

Last year around this time, I asked some state fire officials what to expect in terms of the fire season and got a definite “It depends.” On the one hand, scant precipitation over the winter had left behind a dry landscape. On the other hand, spring rains had given a boost to rebounding vegetation, providing more fuel for later in the season. I was reminded of the old joke about politicians searching for a “one-handed economist” and found myself wishing for a one-handed forester.

Whether a good spring dousing is more likely to inhibit wildfires or feed them is a common source of confusion, which San Jose Mercury News writer Paul Rogers has sought to extinguish. By dipping into four decades of fire data, Rogers and his researchers at the Merc conclude that dry winters generally make for more intense fire seasons in California.

Rogers writes that “the worst fire seasons come after dry winters, not wetter ones like the one we’ve just had.” That would seem to bear out a conversation I had in 2007 with Crawford Tuttle, Chief Deputy Director at CalFire, fire protection arm of the state’s department of forestry. Walking through the burn zone of a Sierra wildfire that broke out in May of that year, Tuttle said that early-season fire was “a great demonstration of how (the) fire regime–fire severity is expanding in California.” Tuttle told me that when moisture levels in the air, vegetation and soil are lower, earlier in the season, it’s likely that the fire season will be intense.

The preceding winter had been dry by historical standards, with just over half the normal amount of precipitation and indeed, wildfires went on to char almost a million acres in the state that year.

The Rain in May Falls Mainly Near the Mean

Rainbow following late spring rains in Vallejo. Photo: Craig Miller

Rainbow following late spring rains in Vallejo. Photo: Craig Miller

Just a passing admonishment from meteorologist Jan Null, who keeps meticulous, often eye-opening records of weather patterns in northern California: We can stop talking about the “unusual weather” we’ve been having.

In California’s Mediterranean climate, precipitation tapers off to virtually nothing between June and October. So any rain this close to the end of “the rainy season” tends to create some buzz.

But Null, a former forecaster with the National Weather Service and founder of his own weather consulting firm, pointed out in an email this week that “the amount and number of days (with rain) so far in May are right near the 30-year normals for San Francisco and San Jose.” Null confirmed for me this morning that:

“So far in May, San Francisco has had three days of rain for a total of 0.44 inches.  The May normal is 3.3 days of rain for a total of 0.54 inches.  Last year, there were 5 days of rain for a total of 0.80 inches.  Even if there is a little more rain (this week), it will be pretty close to a normal May.

Similarly in San Jose the normal is 0.44 inches in 3.0 days. So far in May 2010 there have been three days of rain totaling 0.19 inches.  Last year San Jose had 0.09 inches over three days.”

By George, I think we’ve got it.

The Bay Area’s weather has been unusually cool, however. Null says the April-May period could end with “dramatic cool averages.” He says average daily highs for the two months could be “on the order of three to five degrees below normal.”

Null regularly updates local weather statistics on his website.