Rainy April? Doesn’t Feel Like It!

April 29, 2010

Looks like today is day #24 of measurable rain this month in Portland.  While making a graphic for that I thought “it doesn’t seem like it’s rained that many days!”. 

So I did some checking between shows tonight.  Our total rainfall is slightly above average, and will end that way tomorrow.  I see Salem and Seattle are more noticeably above average.  The record wettest Aprils are in the 4-5.25″ range here in Portland though…how did THOSE months not have more days of rainfall?  How is this nearly average rainfall month going to either tie or break a record for days of rain?  Well, we’ve had lots of light rainfall days…no long-lived (3-5 day) ridges of high pressure, and a wet period to start and end the month.  Apparently everything worked out right to get lots of light rainfall days.

Now notice I said “days”…that’s not really correct…and it goes with my thoughts about the month not “seeming” so wet.  Sure enough, we had a mainly dry two week period in the middle of the month where the only significant rainfall fell in the overnight hours twice.  On the 12th and 14th, more than a tenth of an inch fell both times, but during the dark hours of the day.  That means we went 17 days in the middle of the month with no significant daytime rainfall (9th through the 25th)!  That might explain why I’ve heard very few complaints about a “rainy April” and why to this long-time Pacific Northwest resident it seems like a very typical April.

I do notice we’ve only hit 73 so far this year…seems a bit strange that we haven’t hit 77 or 82 or something along those lines at least once.  That would go with the lack of decent upper-level ridging over the region.

The weekend looks slightly wet to start Saturday and then much better later Saturday and Sunday.  The ECMWF is still very cool and showery Monday while the GFS seems to be out in it’s own little world…unsure how strong ridging will be over us the beginning of next week.  We are now within the 84 hour forecast of the NAM (Monday) and it totally agrees with the big cold trough of the ECMWF.  Can’t wait for that GFS upgrade in June!

By the way, I don’t think the GFS is going to be extended in hours.  The T382 to T574 numbers refer to the resolution of the model, not the time period.  Someone correct me if I’m wrong.  I remember when it was exciting and new as the GFS was upgraded to T126!  Getting older…

Chief Meteorologist Mark Nelsen