La Nina & Spring

March 6, 2008

SnapshotI had some free time tonight between shows (remember we do a 4-5pm show now), so I looked up some La Nina stats.  I heard somewhere, either on TV or in print, that we tend to have cool & wet springs in La Nina years.
The data doesn’t support that for the Portland area for moderate to strong La Nina events.  There are 13 of these events since 1950.  6 of those springs had normal rainfall, 4 were wetter than average, 6 below average.  I don’t see any significant trend there.  A closer inspection shows only 2 really wet springs out of the 13.  What I really notice is that in the wetter than average springs, most of the time the wet conditions were in March or April, mainly March.  Maybe it’s the "left-over" of the active winter storm track that lingers into the beginning of spring? 

Something else I noticed…a wet May seems to be quite rare.  Only 2 of the 13 years produced above average rainfall in May….NONE more than 3"!  That’s a bit strange…because every few years we have a very wet May and you would think one of those would have occurred during a La Nina year. 

Well, I really messed up the forecast today…too little (or no) offshore flow, and a thick shield of high clouds until 3pm or so.  Ah, at least in meteorology you get another "stab at the weather pin~ata" every day!  Mark Nelsen