Coastal areas of California had a cool gray May while statewide rainfall for the month ranged from to zero to over 12 times normal.
A saggy trough of low pressure aloft made the marine layer a nearly permanent fixture for the month. Consequently cities near the coast were mostly 2 to nearly 6 degrees below normal, though San Diego managed to be just 0.6 below normal. Inland it was also cooler than normal in most areas, though Redding in the northern Sacramento Valley was three degrees above normal while Sacramento, only 150 miles to the south, felt the persistent marine layer and was 3.2 degrees below normal.
In San Jose, after an 84 degree max on May 1st not a single day exceeded normal. For the month San Jose's average May 2015 maxima of 68.6 degrees was 5.7 cooler than its normal of 74.3 degrees, the coolest May since 67.97 degrees in 1977. (Their record coolest May is 66.81 inches in 1915.) It was also the first month below normal in San Jose since last September.
San Francisco was similarly cool, with only May 1st topping 64 degrees all month. And it was the 1st time since September 2013 that the average monthly maxima was below normal!
The persistent trough also produced several significant rain events on the South Coast, with San Diego's 2.39" for the month was their wettest May since 2.54 in 1921, and 1292% of normal. Elsewhere rainfall was hit or miss and highly variable with generally drier amounts north.
Jan Null, CCM
Golden Gate Weather Services