2010 looks to begin very dry by Bay Area standards with a blocking pattern that may hold up until the middle part of the month.
“So where’s El Nino?” many might ask.
Well, this is very El Nino-esque. As San Francisco Weather Examiner Jan Null (CCM) points out quite eloquently in this article, the Bay Area is prone to dry spells in Winter regardless of a El Nino, La Nina or “average” Winter season.
What has our rain chances blocked up for now is what looks to be a tremendous ridge of high pressure building upward into Western Canada and down most of the West Coast. At the same time a fairly persistent upper level feature sometimes referred to as a “Hudson Bay” low will anchor itself across Eastern Canada. This creates a highly amplified upper level pattern that is slow to break down in most instances.
Now here’s where “El Nino” should start looking more like what most folks see as being an El Nino Winter (heavier rains and milder temps for California). As this large West Coast high builds deeper into Western Canada, the Pacific jet should begin to break through the lower half of the high pressure ridge around mid-month. Given its lower latitude placement, the jet is likely to be infused with subtropical energy that should yield some pretty juicy rainfall potential that appears to be aiming in mainly along the Central Coast southward.
While this pattern may be a good reservoir filler for parts of the state, its warmer trajectory means there could be a potential for snowmelt if strong warm air advection rains boost snow levels to near 6-7,000 feet at times. So this ridge-busting break though back to rain may be a case of one step forward and two steps back for the state’s on-going drought situation.
The Climate Prediction Center’s projections for California and most of the southern tier of the US is holding the El Nino-centric line for rainfall expectations: milder + wetter than average especially for Southern California now through March.
For More Information: Climate Prediction Center