Wednesday, January 25, 2012

[californiadisasters] La Niña Signature: Wet Fall, Dry Winter

La Niña Signature: Wet Fall, Dry Winter

By Miguel Miller

Last winter's wet season will be remembered as a wet one, mainly for the torrential rains received during December 2010. But what is easily forgotten is that the rest of the winter, from January through March 2011, was drier than normal. Except for that anomalous week in December 2010, La Niña made her mark.

This season La Niña is back, albeit slightly weaker (see graphic on following page). So far this wet season we have loaded our heaviest rainfall rather early, in November. During that month three whopping storms dropped 309% of normal rain in San Diego, while December ended up at only 56%. Almost nothing has fallen since mid December, climatologically our wettest time of year. The season to date rainfall, which was running much above normal, has trickled down to almost exactly normal by late this month.

So far this winter has been typified by a large ridge of high pressure over the Pacific Ocean off the coast of California. Storms have been forced up and over the ridge, then down from western Canada into the southwestern U.S. These weather systems come from the north from a drier, land-based source region instead of from the wet oceanic source region. The result of this deflection of storms has been below normal precipitation. Many locations in California and Nevada experienced record or near record dry conditions during December 2011. Reno, Nevada, reported its lowest December total (not even a trace) in 129 years (1883 and four prior years also had no precipitation).

Fresno, California, reported no measurable precipitation for the second time since 1878 (tied with 1989). Downtown San Francisco received 0.14 inch, the third driest December since 1849, behind only the 0.00 readings of 1876 and 1989. San Jose (2nd driest since 1874) and Sacramento (6th driest since 1849) also had a notably dry month, and an index of 8 Sierra Nevada stations recorded its second lowest December total since 1920, and 4th lowest July -December.

During the last two wet seasons, La Niña has shown us a wetter first half of the season and a drier second half. But watch out for those wildcards, like atmospheric rivers, that don't need an ENSO connection.

Then with January two thirds over, a noticeable change occurred in the jet stream pattern. The blocking ridge of high pressure broke down and allowed a couple storms to move through Southern California. Then the ridge returned. Our current La Niña is forecast to weaken this spring and become a neutral ENSO phase by summer. If La Niña treats this late winter as it has in the past, including last winter, we will be much drier as the season winds down this summer.


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