Weathernotes Archive October 2007

Upper Level Low Wetter Than Expected
Weathernotes for Saturday, October 13, 2007.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

An upper level low that was expected to produce only a chance of scattered showers south of Pt. Conception has resulted in more rain at Downtown Los Angeles (USC) in an 8 hour period than was recorded in any month of last year's rain season!

As of 10 a.m. Downtown Los Angeles has recorded 0.95 inch of rain, and precipitation amounts in Los Angeles County have generally ranged from about 0.20 inch to about 1.00 inch. Here's an archived copy of a NWS Public Information Statement with some preliminary rainfall amounts for Los Angeles, Ventura, and Santa Barbara counties.

This morning's 12z GFS shows a series of storm systems moving into the west coast over the next two weeks, but at the moment the precipitation from these systems is forecast to remain north of Pt. Conception. We'll see!

More information concerning Southern California weather can be found using our WEATHER LINKS page.

Southern California 2007-2008 Winter Precipitation Outlook
Weathernotes for Tuesday, October 9, 2007.
(Any updates will be found at the end of this discussion. Last month's notes are archived here.)

According to the Climate Prediction Center's Weekly ENSO Update, issued October 9, La Niña conditions are present across the tropical Pacific and are likely to persist through early 2008. As a result, several longer range U.S. precipitation outlooks have been similar to this Nov-Dec-Jan Precipitation Outlook, issued September 20 by the CPC.

NOAA released its initial Winter Outlook for the U.S. today, including this Dec-Jan-Feb precipitation outlook. The precipitation pattern projected for the western U.S. is typical of a La Niña, with an increased chance of higher than normal precipitation in the Pacific northwest and an increased chance of lower than normal precipitation in the southwest.

But will the impacts from this La Niña be typical? In his blog Atmospheric Insights, Ed Berry suggests the possibility that a shift in Eastern Hemisphere tropical forcing from the Indian Ocean to the west central Pacific is imminent and if this shift occurs, the western U.S. could see more strong full latitude troughs starting sometime week after next. If the west Pacific becomes the dominate area of forcing this Winter, then a somewhat atypical La Niña precipitation pattern may be the result.

On the other hand, the La Niña has has become significantly stronger in recent weeks. The most recent value of the Multivariate ENSO Index (MEI), plunged from -0.43 for July-August to -1.11 for August-September. This was the second largest one month drop for the season on record and moved the MEI below the moderate La Niña threshold. Virtually all international dynamic coupled climate models are forecasting cooler equatorial Pacific SSTs will persist through this Fall, trending towards neutral equatorial SSTs this Spring and Summer.

The two driest water years recorded in Los Angeles since 1877 have occurred in the last seven years. During the most recent, from July 2006 to June 2007, downtown Los Angeles (USC) measured only 3.21 inches of rain.

Updated January 10, 2008. The ESRL-PSD Composite ENSO plots page was updated today to correct an issue that resulted in the wrong set of years being used for its Winter La Nina composites. This updated precipitation map shows the mean Nov-Mar precipitation for the U.S. during 9 La Niña events from 1948 to the present. Note that the average La Niña rainfall indicated for coastal Southern California is in the 7.0-10.5 inch range, rather than the 10-15 inch range previously indicated in the ESRL-PSD graphic. Even so, in comparison to last rain season's paltry rainfall, this would seem quite wet! We'll see...

More information concerning Southern California weather can be found using our WEATHER LINKS page.