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Early Winter Outlook/La Nina Growing Stronger

August 02, 2010; 12:35 PM

We are barely into August and it's hard to think about the coming winter when there is still a lot of summer (and then fall) ahead. I usually hold off my initial forecast of the coming winter in the West until early September then finalize it in early October. However, this year the signs are so strong and so consistent across the board that I feel comfortable to start talking about my forecast for the winter now. This discussion will still be general in nature with a more specific forecast likely later in September or early October.

Anybody who has been routinely reading my postings know that eastern Pacific sea surface temperatures have been running cooler than normal since June. This has helped to cause the cooler-than-normal weather along coastal California and into the coastal Northwest. Temperatures have been as much as 3 to 4 degrees below normal in portions of Southern California. Instead of the large ridge that usually holds court around the Four Corners, that ridge has been displaced much farther east by a mean trough along the West Coast. While coastal temperatures have been below normal, it has been a hotter-than-normal summer in the deserts areas of the Southwest up into the central Great Basin.

The cooler-than-normal water temperatures in the eastern Pacific west of the U.S. also holds true near the equator from off the coast of South America on west to the Dateline. Temperatures are at least 0.5C below normal with areas of 1.5C below normal in this area.

A look at the entire Pacific sea surface temperature anomalies shows the classic La Nina look. Cool water temperatures in the eastern Pacific and warm in the west.

Remembering back to last winter, we had a moderately strong El Nino in place that produced much needed extra water for a large part of the Southwest. During last winter, eastern Pacific anomalies were in the 1 to 1.5C above normal range. However, this quickly changed between April and May, and now the numbers are completely on the other side of the ledger. This chart shows the rapid change in the upper oceanic heat anomalies since last winter.

The La Nina signature turned so strong, so quick, that it has had a dramatic effect on the summer weather in the West. The trends of the summer to date will likely continue the rest of the summer as well. All the ENSO models are also quite consistent across the board that the remainder of the summer into the fall that the La Nina will strengthen even more then hold all winter long. Some models are showing a strong La Nina signature.

Because of this, the West is probably looking at a moderately strong or stronger La Nina-type winter. This means warmer- and much drier-than-normal conditions in the Southwest and a stormy, wet weather pattern in the Northwest.

This has been the fear all along by water management people in the Southwest, California in particular. The excess water we got this last winter was something very temporary and would only mean a short reprieve in the overall dry weather patterns of the last 4 to 5 years.

Again, this is a preliminary look at the coming winter, but it probably is a reasonably accurate one. I will be making more specific forecasts as we get closer, so stay tuned for that.

As a side note. My forecast was done independent of AccuWeather.coms forecast but both are predicting the identically same pattern for the West.

The views expressed are those of the author and not necessarily those of AccuWeather, Inc. or


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Ken Clark
Ken Clark's Western U.S. weather blog tackles daily weather events with commentary from one of the most experienced and trusted Western U.S. weather experts.