Tuesday, January 31, 2012

Wayward La Nina

This has been a La Nina winter, but you might not know it.  In fact, the La Nina has been intensifying as indicated by the cooling waters of the tropical Pacific, as shown by the plot of the sea surface temperatures Nina 3.4 area of the tropical Pacific seen below.  The figure actually snows SST anomalies...differences from normal...which are now more than 1C cooler than normal.

La Ninas are usually associated with wetter than normal fall and winter over the Northwest, but so far we have been a bit drier than normal (see plot for Seattle):
And snowfall in the mountains, which should be above normal, particularly after January 1, is near normal over Washington and about 60-70% of normal over Oregon, even after all the snow of late January.   The latest forecasts for the next week suggests a VERY dry situation after we get past the rain tonight and tomorrow AM.  The National Weather Service long range forecasts for the entire winter have been for typical La Nina conditions:  wetter and colder than normal, but except for two weeks in late January, this has not materialized.

So what is going on?  The answer is that the typical La Nina circulation has not set up consistently so far this winter.   Why is that?  I can't tell you and I am not sure anyone can.
Typically, La Ninas are associated with major ridging (high pressure) over the Aleutians and troughing (low pressure) over the Northwest.  This brings cool northwesterly flow towards our area and troughing is associated with precipitation:  thus,cold and wet.  Here is a figure showing the typical configuration aloft for La Nina (500 hPa, around 18,000 ft).  This figure actually gives you the difference from climatology.  The classic La Nina pattern is evident.

But this year the pattern has been different.  Yes, the ridge is there, but it has moved much farther east, leaving us in a much drier position.  Only for a few weeks in January has a La Nina pattern been observed.   This figure illustrates the evolution of the upper level pattern for the last two months to show you what I mean (colors give the anomalies from climatology).

Now here is the interesting part.  The next week should be dominated by a huge ridge over the West coast and we have some confidence in this because nearly all the forecasts, ensembles and others, and showing it.  Just to give you a taste from the UW modeling system from Thursday to Sunday.   And the ridging continues in the following week (although I don't know if I can believe it that far out).

Seattle winter really ends in the beginning of March...greatly reduced chances of major windstorms, floods, snowstorms.  So we are running out of time for major lowland action.

And did I say what the models are suggesting for next winter?  No more La Nina and a move to neutral conditions.  Although infrequent, the biggest and baddest storms occur in neutral years.  Something to look forward to.