What's Up With This Weather?

Unusual cold pattern El Nino's fault? Hail, no!

By Rob Mayeda
|  Tuesday, Mar 9, 2010  |  Updated 11:45 AM PDT
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January 2010 Bay Area Storm Photos

Zelman Brounoff

This week's wild weather is pretty typical of a spring-driven system.

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We have some of the best weather here in the Bay Area. But this week, as hail came down in between sunny breaks with gusty wind pushing the clouds out, many may be wondering, "what's up with this weather?"

But don't blame this month's odd behavior on El Nino. Cold weather, even hail, are not unusual this close to spring as cold air moving into the area mixes with warmer air settling in for the upcoming season. Stronger El Nino patterns of the past have brought in much higher snow levels that resulted in extreme flooding events and washed away our precious snowpack and summer water supply.

In a typical year, you would expect to see some pretty wild swings in the Bay Area 7-day forecast during March as our winter pattern -- typically dominated by the polar jet aiming in on Northern California -- begins lifting northward into the Pacific Northwest and British Columbia.
 
Case in point: Tuesday morning's record low temperatures, which ranged from 30 degrees in Napa to 40 degrees in San Jose, will be followed by what could be 60s and 70s early next week.

But this year has been different, obviously, with a moderately strong El Nino. We've benefitted from above average rainfall which has really helped fill reservoirs around the state, Yet, the subtropical jetstream that powers El Nino has stayed far enough to the south to keep snow levels near 5,000-6,000 feet for most of the winter. This is a huge plus because it has allowed us to build a snowpack with 28" of water equivalent or 107 percent of average statewide as of March 8.

This has been the perfect storm, so to speak, of fairly steady rain events, fairly evenly spaced throughout the winter. Now heading into mid-March, our 'fourth quarter' of the typical rainfall season, we're still seeing some opportunities for rain (a decent rain event for Friday and a couple more storms for the middle of the month in the long-range models) that would seem to indicate as we exit this El Nino winter and start into spring, we'll continue seeing rain off/on at times even as spring begins.
 
March is usually wetter than average during a moderate El Nino, so you'll probably want to keep umbrellas ready through the month (more than average) while temperatures finally start to climb higher through the month.

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