What About La Niña?
So what good is the Climate Prediction Center's Winter Outlook that called for below average precipitation in Southern California? The November through Janu-ary outlook gave us equal chances of above, near, or below normal rainfall. For this we would probably have to give the CPC a grade of B. Not bad, not good, since they were non-committal about which way to lean through this early winter period. But the outlook we all know and remember was for the dominating La Niña to produce a drier than normal winter overall. To be fair, we'll have to withhold our judgment until winter is over: grade I for incomplete.
It is important to understand the difference between deterministic forecasts, which we make out to one week here at the NWS, and probabilistic outlooks, made by climatologists at the CPC. In the above graphic, we can see that probabilities are broken up into three categories: above nor-mal, near normal, and below normal. This graphic is the "local outlook" calculated for one point, such as San Diego Lindbergh Field (currently, this is only done for temperature, not precipitation, but the principle is the same). So if La Niña makes the blue pie slice larger, the other two slices would shrink accordingly. They never go to zero, so there is still a chance. This is how the CPC can indicate "probably" a below normal rainfall season, but still keep in a chance for above normal.
In their defense, they never said, "It will be drier than normal in Southern California this winter." But they did in-dicate that had the highest probability of occurring. Still, they have some ex-plaining to do. Even if not another drop of rain falls through June, a few sta-tions have already exceeded their entire seasonal rainfall! Recently we had a chance to ask the CPC: where was La Niña in December (and this wet season so far overall)? The answer: the typical La Niña long wave pattern across the Pa-cific did in fact set up (as shown in the graphic above), but it was shifted westward, allowing the jet stream this Fall and Winter to direct storms into Southern California. So then what caused the westward shift? They don't know. We don't know.
La Niña has been on our weather stage for months now, she just hasn't performed...yet. Has she started to dance this month? In early January we had one medium rainfall event and nothing since. And nothing looks too promising through most of January. During our normally wettest month a drying trend has begun, but will it continue into February and March?
The La Niña outlook continues strong through the Spring, and CPC sees no reason to deviate from the higher probability of below normal precipitation through the rest of our wet season.
Source: http://www.wrh.noaa.gov/sgx/newsletter/current-newsletter.pdf
--
Check out http://groups.yahoo.com/group/californiadisasters/
Read our blog at http://eclecticarcania.blogspot.com/
Visit me on Facebook at http://www.facebook.com/derkimster
__._,_.___
No comments:
Post a Comment