Saturday, October 8, 2011

La Nina Has Returned

The latest ENSO discussion issued by the Climate Prediction Center shows that La Nina conditions have strengthened and suggests that we are likely going to see a double-dip La Nina this winter.

Below average sea-surface temperatures now extend across much of the eastern and central tropical Pacific, consistent with La Nina conditions.

Source: CPC
About half of the models used to predict the ENSO cycle call for La Nina to strengthen during the fall and winter.

Source: CPC
One model, the NCEP climate forecast system (NCEP CFSv2) calls for a fairly strong La Nina to develop, although this is an outlier.

Based on the model guidance and an analysis of this and past events, the Climate Prediction Center concludes that "a weak or moderate strength La Nina is most likely during the Northern Hemisphere winter.

It should be noted, however, that this does not necessarily mean a repeat of the 2010/11 winter, for reasons we have discussed in a previous post.  The dice are merely slightly loaded for above average snow in the Pacific Northwest and below average in the far southwest, as indicated in the CPC precipitation outlook for Jan-Mar.

Source: CPC
Wasatch skiers should note that we sit in an area where there is an equal chance of above or below normal precipitation.  In other words, we don't really know how good this ski season will be in Utah relative to average.

4 comments:

  1. And the ski season in Flagstaff, Arizona? Stormy, troubled by steady opposition to Snowbowl's plan to make fake snow with dirty water....

    ReplyDelete
  2. I happened to be doing some comparisons recently between the ONI index (http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/ensostuff/ensoyears.shtml) and SNOTEL winter precipitation data in Utah. I found quite consistent results using ONI averages for the November - April period, showing that the associated precipitation was higher by about 15-30% in the northern/central Wasatch for all seasons with a negative index than those with a positive index number. Snowbird was about 20% higher on average for the years with a negative index for example, so La Nina winters definitely seem to be wetter there. The bias toward higher La Nina winter precipitation seems to extend down to at least the middle of the state, perhaps around Sevier County from what I can tell. South of there (and possibly in much of eastern Utah), it appears that El Nino winters tend to wetter.

    ReplyDelete
  3. Interesting. How far back are you going with the SNOTEL data. Most stations have only been in since 1980, Snowbird since 1990. Thus, statistics of small numbers could be an issue here. On the otherhand, hopefully you are right and we get a big snow year!

    ReplyDelete
  4. I looked at both SNOTEL and pre-SNOTEL storage gage data for 1970-2011, so a total of 42 years. The second portion of the period (1990-2011, the period that Snowbird has SNOTEL data for) had very similar results to the first portion at nearly all the sites, so even though 2011 may be an outlier I think the results are pretty representative.

    ReplyDelete