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.

The Rain in May Falls Mainly Near the Mean 20 May,2010Craig Miller

3 thoughts on “The Rain in May Falls Mainly Near the Mean”

  1. San Franicsco picked up another 0.20″ today (5/25), pushing the monthly total to 0.64″, a tenth of an inch above normal. This makes it only the 55th wettest may since SF rainfall record began in 1849. Even with maybe another .10″ on Thursday it won’t be all that memorable for the amount of rain.

    The wettest was 4.02″ in 1925. See

  2. Ted – not sure where you were going with that?

    sounded pretty mean to me, except the temps, they may be soon filming the Deadliest Catch near the Oregon border soon.

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Craig Miller

Craig is a former KQED Science editor, specializing in weather, climate, water & energy issues, with a little seismology thrown in just to shake things up. Prior to that, he launched and led the station's award-winning multimedia project, Climate Watch. Craig is also an accomplished writer/producer of television documentaries, with a focus on natural resource issues.

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