Done With April; On to May

April 30, 2012

By one measure this Spring is doing far better than last year.  Check out the number of 60 degree days so far this year:

Compared to last year, and the chilly spring of 2008, it’s a decent improvement.  Looks like April temperatures ran quite close to average across the Pacific Northwest, here’s a graph showing temperature anomaly over the past 4 weeks across the west from Western Region Climate Center…the rest of the country has been warmer than average too, although not as extreme as March:

Well, the first week of May, or at least the first 6 days, will be quite cool.  This has been a well forecast cool and wet spell (see previous blog postings).  Cool westerly flow in the upper atmosphere turns more into a cool upper level trough as we get to the end of the week.  Then that trough “closes off” into a chilly upper level low and takes it’s time getting out of here this weekend.  As a result, this upcoming weekend will probably be the coolest we’ve seen in a month.  850mb temps Saturday morning bottom out around -4, impressive for May. 

The surface low and associated moisture may give us quite a soaking Thursday; that appears to be the wettest day of the week.

Models are in pretty good agreement on upper level heights rising Sunday and into next week for a return to normal or above average temps.  Here is the 00z GFS ensemble chart:

Interesting to note the last few weeks that as we move into the warm season, even these 5,000′ temps are affected by the strong diurnal heating.  You don’t see that October-March. 

And here’s the 12z ECMWF, showing above average 850mb temps beginning next Monday, at least in the mean:

Chief Meteorologist Mark Nelsen