La Nina influenced seasons in the past has had resulted in major flooding in CA. So if flooding concerns you, do not let your guard down just because a moderate to strong La Nina event is expect to occur or the long range forecast experts told you that due to La Nina, it should be a below normal season precipitation season. Here are the reasons why.
1) In La Nina seasons, the odds that are based on past La Nina events, sis that CA (especially southern CA will have a better chance of staying dry that we will have a below normal precipitation wet season.
However, that is not always true. The reason why we usually stay below normal is that during a La Nina, the jet streams over the over the Western Pacific will be amplified. If the longwave pattern (the flow of the strong upper level jet pattern) shifts or forms an upper level pressure trough just west to southwest off the west coast, this could briefly put a portion of the west coast in a brief and very wet period. And adding the fact that that highly these highly amplified patterns will usually means the pattern will be slow to change.
For this first situation, if the jet stream takes a deep dive far enough southward and "picks up" warm, moist, and unstable air. This flow will bring this into the west coast. With a cold surface frontal system, once it passes the upper trough axis, the front will slow down. Its orientation of this cold front will usually be from the northeast extending to the southwest. With the surface front parallel to the southwest upper flow, the front will become stationary.
In these types of situation, showers and thunderstorms will form at the southern end of the cod front and move along the frontal boundary. This will continue to happen as long as the pattern does not change. And in a amplified pattern of a La Nina, chances are good that change will be slow. Weather forecaster will call this a "training pattern" because the stationary frontal boundary looks like railroad track, and each strong cells will act like a long train of railroad cars. Ii is because the same area is being hit over and over, flooding is likely, with flash flooding being the biggest concern.
2) Another common pattern we see in the La Nina wet season would be the splitting of the upper level jet stream as it exits
The pattern is better known to meteorologists as a Madden Julian oscillation (MJO) . The public will know its common nickname usually used by the media as the Pineapple Express pattern. With pattern can bring a series of strong winter storm with less than a day apart breaks and can last over a week. The main threats are both local flash flooding and widespread areal flooding. Main stem will run high with these very wet and warn storms. The levees will be especially punished by these windy and wet storms.
3) Both of these types of storm situations can happen in a La Nina year. And the pineapple express type event will have a higher chance of developing during a La Nina wet season over the west coast, including the entire state of CA. These events can last for hours, days, with the pineapple express lasting up about 10 days. If the remainder of the season remains dry, the rainfall season can easily be below normal even though we may have a month where the CA residents will be fighting the flood waters and trying to stay dry.
I just wanted to add my two cents worth on this topic.
NWS Meteorologist (retired)
--- In email@example.com, Kim Noyes <kimnoyes@...> wrote:
> *La Nina effect*
> By Miguel Miller, graphics and research by James Thomas
> We know that La Niña usually causes Southern Cali-fornia to receive
> rainfall below normal during a given winter. But is that always true? James
> Thomas, a meteorologist Intern here at the NWS in San Diego, did some
> research to find out. He used San Diego Lindbergh Field as the point of
> reference, although similar correlations can be inferred for anywhere in
> Southern California.
> In this study, James used the strength of La Niña as defined from the
> Oceanic Niño Index (ONI), which uses a running three month average of sea
> surface temperatures. He broke these seasonal events into Weak, Moderate,
> and Strong events. The rainfall season is defined as July 1 to June 30, and
> the latter part of that season's year is annotated in the above graphic.
> Normal rainfall for San Diego Lindbergh Field is 10.34".
> We found that San Diego has experienced lower than normal precipitation in
> 18 out of the last 20 La Niña events. That's a 90% dry bias!Note how the
> moderate La Niña of 2010-11 sticks out like a sore thumb. In all six of the
> previous moderate La Niñas, San Diego ended the water year with well below
> normal precipitation.
> During the water year of a weak, moderate, and strong La Niñas, San Diego
> averaged 7.37", 8.01", and 6.47" respectively. These averages all fall
> below San Diego's rainfall average of 10.34" (see chart at left).
> We have a clear signature: La Niña produces below normal rainfall in San
> Diego. Recall that in the anomalous La Niña season of 2010-11,
> an"atmospheric river" setup brought copious amounts of rainfall in December
> 2010. It was a very wet fall and early winter in 2010 before a drier
> pattern followed.
> This illustrates another interesting find. La Niñas seem to "kick in" and
> reduce rainfall most during the normally wettest part of the season from
> January to March. This was especially true for the 2010-11 season. And it
> has been true in this 2011-12 season. Conversely, this is something that
> has been noted about El Niño: it seems to enhance rainfall most from
> January to March.
> Source: http://www.wrh.noaa.gov/sgx/newsletter/current-newsletter.pdf
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