Archive for the ‘La Nina’ Category

Rainy Day… Again?!?

Wednesday, June 1st, 2011

Mary arrives at ExploratoriumIt’s June 1 and I got thoroughly soaked on my bike ride into work today. By now in the San Francisco bay area, we’re usually into a spring pattern of mild, sunny days that have school kids and working adults thinking about playing hooky and heading for the beach. But for the last three months we’ve had what seems like relentless cold, rainy weather–more dead winter than a mere 20 days till official summer.

I gave our local National Weather Service meteorologist, Tom Evans, a call to ask him what’s up with the weather (all the while controlling the irrational desire to blame him for my miserable bike ride this morning). He confirmed that our weather, indeed, has been unusual with higher than normal rainfall, especially for a La Nina year. “We’ve been getting a lot of weather systems from the Northwest, picking up moisture from the tropics that’s giving us heavier rain periods than we usually see.”

Let’s back up a second and  talk about La Nina which I have some familiarity with from a webcast project I did years ago. Perhaps less well-know than it’s opposite twin El Nino, La Nina refers to cooler than normal water temperatures in the equatorial Pacific Ocean off the coast of South America. This cooler water disrupts normal climate patterns, with warmer, drier weather than normal in the Southwest and cooler, wetter weather in the Northwest. In the bay area,  climate patterns could go either way, but usually tends to the warmer, drier side. But not this year. Tom did a little study and found only three La Nina winters in the last 50 that  have been significantly wetter than normal in the bay area: 1955-56, 1973-74 and 2010-11. This year has been a real doozy. Our rainfall has continued into May (and now June) with an accumulated total in San Francisco  of over 30 inches or 175% of normal. We’ve had mountain snowfall at nearly twice the normal accumulation and it kept snowing in the Sierras, even into May when the annual Amgen Tour of California bike race had to cancel its first stage in Lake Tahoe because it *snowed* nearly a foot that day.

According to Tom and NOAA’s National Centers for Environmental Prediction (NCEP)  office, we are transitioning away from La Nina to a neutral ocean condition so maybe there’s a glimmer of hope that we’ll have some kind of spring… but not right away. On the NCEP website comes this ominous statement: “Atmospheric circulation anomalies associated with La Nina remain significant.” In plain English, Tom Evans says there’s nothing in the current condition of the atmosphere that will force a change in the jet stream. That means additional cool, rainy weather will be moving in as if on a conveyer belt over the next several days with another strong storm predicted for Friday. “The good news is that the Climate Prediction Center is telling us we should have a normal summer this year, but we have to get into a summer pattern  first… it might be awhile yet.”