“Normal” September Ahead

September 20, 2010

What a soaker this weekend…record rain Saturday, then a few thunderstorms scattered around Sunday (although drier than I expected in general), just a bit gloomy today.  Things have returned to normal now with the moist subtropical airmass no longer in the Pacific Northwest.  As a result we’ve got cooler nights ahead and some sunshine the next couple of days.

The big picture over the next 7+ days is definitely not set in stone, but it appears that we PROBABLY have much more pleasant September weather in store for the last week of the month.  The main problem is: where will a warm ridge of high pressure in the upper atmosphere set up shop?  Will it be along the West Coast, or farther east, in the Rockies, next weekend and beyond?  I always like to watch for trends in the models;  the trend today and this evening is for the ridging to hang on at least through the first part of the weekend.  The 12z ECMWF and 00z GFS have very little rainfall Sunday, with ridging kicking in again Monday.  I don’t trust that for now, so we’ve left the rain in the 7 Day forecast Sunday and Monday.  It COULD be a very warm weekend (best in weeks!) if the cold front and rain holds off.

Late last week and today I pulled out the La Nina graphics, cleaned them up a bit from 2007.  and put them in the 10pm show.  Tonight I talked about the Fall/Winter rain possibilities.  Interesting to see how the winter of 2007-2008 lined up with all the graphics I made the Fall before.  Things turned out about as expected except no big windstorm in the Valley and no big arctic freeze.  Plenty of mountain snow, valley rain, and coastal windstorm otherwise that year.  Plus the hills around town really got pounded.  I had 39″ of snow at 1,000′, yet PDX only had a trace.

So what about rain the rest of the Fall?  Most likely above average; same for winter.  Very few La Nina Fall/Winters end up below average.   Temperature is a tossup.  As we just saw, lots of cloudy/rainy weather can be mild.

More La Nina tomorrow…

Chief Meteorologist Mark Nelsen