El Nino forecast this year

REDDING, Calif. - Word on the street is an El Nino is on the way.  And while that is true it does raise some questions as to what exactly an El Nino is, and what does it mean for Northern California.

Well, an El Nino is a natural semi-climatic shift caused by oscillations in the Pacific Ocean.  It's caused when warmer water starts to shift eastward and piles up along the western coast of South America.

These temperatures anomalies can be in the area of 5 to 10 degrees above average and have all sorts of different implications on the weather around the globe.  El Ninos bring warmer weather to some places and cooler weather to others along with moving moisture around to places that are normally drier.

For the west coast it really depends on where you are and the exact patterns that set up to see whether areas will get more or less precipitation.

Generally, the Pacific Northwest will see drier weather whereas the Desert Southwest will see wetter conditions during the winter months during an El Nino.

That means that areas like Los Angeles, San Diego and Las Vegas will see more rain than normal whereas Seattle and Portland will get less rain and snow.

Redding, San Francisco and Sacramento are in the awkward transition zone.  They aren't quite far enough north to be consistently drier than normal, but aren't far enough south to get the rainfall benefits the desert states see during El Nino years.

It's all caused by a shift in the jet stream.  In an El Nino year an anomalous area of low pressure forms in the Gulf of Alaska.  That allows the jet stream to push farther south and become more zonal (flowing straight east to west).

The zonal flow allows more moisture to be entrained into the flow and directs the subtropical fire hose directly at California. 

In addition, an upper level trough forms with its axis situated directly over the west coast.  That trough directs the storms southward and pushes them onshore farther south than they normally would.

It all adds up to more frequent storms and more frequent atmospheric rivers flowing off the ocean, bringing more rain and snow to California.

The part that gets complicated to forecast – and climate studies done by the national weather service have shown this – is what exactly will happen to Northern California.

On average, northern California receives 9.6 inches more liquid precipitation than average during a strong El Nino.  But on the same token, two years on record have received 14 fewer inches of precipitation during a strong El Nino.

That story holds true for both moderate and weak El Nino.  A moderate El Nino there is no statistical difference between it and a normal year – with years seeing as much as 35" more precipitation and as little as 21" less.

A weak El Nino averages plus 2" but also has a pretty crazy spread.  1977 was an El Nino year and stands as the driest water year on record for California with Northern California receiving 31 inches less precipitation than average.

In conclusion, El Nino means an extreme of something for the Northstate. The jet stream can certainly result in a storm track that favors an increase in precipitation for Northern California, but it is far from certain.

Currently the equatorial pacific is in a weak El Nino, with sea surface temperatures between 1° and 5° above average.  Models are also forecasting that to increase to between 2° and 8° above average by the middle of the summer with an 80% that an El Nino will form by November.

Let's hope this El Nino is the wet type and pushes California out of the extreme drought most of the state is experiencing.

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