Clear skies and dry weather over the past few months have rainfall totals well below average in the Bay Area, and the trend looks likely to hold through the holiday weekend.
National Weather Service meteorologist Tom Evans said today that most cities in the Bay Area have only had between 36 and 48 percent of their average rainfall totals so far this year.
Downtown San Francisco, which usually has 7.8 inches by Christmas, has had a little more than 3 inches, Evans said.
San Jose has only had an inch and a half of rain so far this year, or about 36 percent of normal.
Rainfall in the North Bay is also well below average, with just over 4 of inches falling in Santa Rosa since July. Usually by this time, that total is about 11 inches.
The reason for the dry weather is a mild La Nina pattern in the Pacific Ocean, Evans said.
Cooler ocean temperatures near the equator are creating a ridge of high pressure in the northeastern Pacific, which causes storm systems to pass north of California before dipping into the central U.S.
“We have to wait for that ridge to break down before any change can happen,” Evans said.
As of today, the forecast models show no sign of change before next Wednesday, meaning cool and clear weather is likely to last through the holiday weekend.
There is still time for this season’s rainfall totals to make up for a dry start, Evans said.
Most of the above-average rainfall in 2010-11 came from heavy storms that hit the region in February and March, Evans said.
Meteorologist Mark Strobin said that so far, this is the 27th-driest year in downtown San Francisco since rainfall records started being kept in 1849.
Chris Cooney, Bay City News