La Nina? - conditions only, Western US snowpack dry, short-term forecast, water supply
By: DWR meteorologist Elissa Lynn

The last newsletter appropriately explained the cool equatorial ocean conditions now in place, and gave links for graphs of falling ocean temperatures. However, these are merely cool conditions that have arisen in recent weeks. Like a recession, it takes some time to call an official La Nina. Professional apologies, as I jumped the gun on saying it’s ‘back’ last week during snow surveys. Better to say, it’s cooling down again, as it was cool last winter. At present. But it may be temporary.

The Climate Prediction Center has a weekly update on Jan. 5, 2009 saying:

“Atmospheric and oceanic conditions reflect La Nina. Recently, negative equatorial SST anomalies have strengthened across portions of the central and eastern Pacific Ocean. Based on recent trends in the observations and model forecasts, La Nina conditions are likely through early 2009.”

http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/lanina/enso_evolution-status-fcsts-web.pdf

That’s not the same as calling an official La Nina, in spite of their text sounding so. I, myself, was caught by the technical definition versus the sentences issued above.

From Golden Gate Weather, Jan Null:

“Despite reports to the contrary we are NOT yet in a La Nina. The waters have continued to cool in the equatorial eastern Pacific but the lower threshold for a La Nina is that the 3-month ONI (Oceanic Nino Index, see http://ggweather.com/enso/oni.htm) is -0.5 deg C or colder. The most recent ONI for Sept-October-November was only -0.1, though indications are that it will fall below -0.5 degrees when the Oct-Nov-Dec figure comes in.

HOWEVER, to be classified a historic La Nina there needs to be 5 consecutive 3-month means that are below the threshold. The bottom line is that while we may reach the threshold, we are several months from this being a “La Nina”.”
The possibilities of what La Nina can lead to in California (although several scenarios are possible) are covered in recent Weather and Climate Newsletters. Southern California is often hit with a dry winter during La Nina, but was not last year.

Their fall has been wet so far this season. For more on Southern California perspective, see the San Diego Union Tribune:
http://www3.signonsandiego.com/stories/2009/jan/06/1n6weather0030-la-ni241-blamed-more-drought/?zIndex=32485

IF La Nina stays in place, and IF it does what it did last winter, we could have a 3rd dry year (NorCal and Sierra impacts); that’s the concern. But other’s quotes and the headline of the article do not provide the 5-month stretch caveat. Change “La Nina is back” to “A La Nina-like cool down is in place.” Too technical for some, not technical enough for others.

Onto current forecast: A system cuts across Northern California Thursday. Not a big storm, just a couple tenths here and there.

The 10day forecast over the Feather River Basin is unimpressive, especially for a January stretch:
http://www.water.ca.gov/floodmgmt/hafoo/hb/csm/docs/QPF.pdf

A ridge is in place over the Eastern Pacific. Still hoping for that normal to above normal year that seemed to be indicated as the season got underway by the Sac NWS Office, and Winter Outlook Workshop presenters in December.

January has been fairly dry. We’ve now fallen below last year for Northern Sierra precip; now at 80% of average, while last early January we were at 85% of average. Other than October, which is not a big producer, each of the months this water year has been below average for the Northern Sierra. The Southern Sierra is at 90% of average for precipitation.

Current Snowpack (snow water content) measured by automated snow
pillows:

Northern Sierra 50%
Central Sierra 75%
Southern Sierra 85%

None of these numbers are drought busters. But there’s still about 60% of the season to go. With the next 10 days looking dry, and a pretty good ridge in place, we’ll have to hope for a change in the pattern. A Miracle March would be good. Or a Fabulous February. Or a Generous January. Anyone for all 3? Missing out on a mid-winter month is hard to make up for.

Western U.S. Snowpack:

The National Water and Climate Center creates and transmits the weekly Western US Basin snowpack and cumulative precipitation report every Monday. This report is based on an analysis of data collected at over 680 NRCS SNOTEL sites in the Western United States.

ftp://ftp.wcc.nrcs.usda.gov/data/snow/update/ws.txt

Arizona seems to be doing well, as does New Mexico, in terms of percent of average to date.

Water Equivalent Maps (Western Regional Climate Center):

http://www.wrcc.dri.edu/snotelanom/basinswe.html (color)
http://www.wrcc.dri.edu/snotelanom/basinswen.html (numeric)

The next Water Supply Index (seasonal water supply forecast) will be out within a week. Look for another newsletter shortly thereafter, with reservoir status, as well.