A Warm September, But Not a Record

September 27, 2011

I did some good old style number crunching this evening:  The average temperature at PDX through today is 67.98 degrees.  That is warmer than the warmest September on record (67.6 in 1994).

So then I took our forecast for the next 3 days, out of the 7 Day forecast, and recalculated the average temperature for the month.  If our forecast is correct, the average temperature should end up at 67.5 degrees.  That would just barely edge out 1991 for the 2nd warmest September here in Portland.  We’ll see how that works out, but it’ll be close.

As for weather forecasting the next few days; two items of interest.

One is the gusty east wind coming in tomorrow.  High pressure develops east of the Cascades and the pressure gradient through the Gorge ramps up from about zero at daybreak to 6 millibars by 8pm according to the WRF-GFS.  It shows 45 kt (50+ mph) wind up around 2,000′ exiting the Gorge.  That should easily give gusts 40-50 mph at the west end of the Gorge and 25-30 mph across a good chunk of the metro area after sunset.  I bet we get our first 60-70 mph gust at Vista House this season between 7-10pm tomorrow.

The wind dies off quickly Thursday afternoon, then it’s dullsville weatherwise through Sunday.

Models then show item of interest #2 for early and middle next week.  A strong westerly jet punches right into the West Coast for the first time this season.  Both the GFS and ECMWF show widespread significant rainfall from SW British Columbia all the way down to about San Francisco.  This will likely be the “season-ending” rainfall that extinguishes the fire season here in the Pacific Northwest.   Confidence is high in the big change next week, for example check out the 12z ECMWF Ensemble plot: 

The green line is the average 850mb temp the next 16 days.  The thick blue line is the actual operational ECMWF run the next 10 days.  The ensemble average is red.  You can see that beginning Sunday the 2nd 850mb temps go below average not only on the operational run but almost all the ensemble members too.  That continues through at least the 12 day mark.  If you’re looking for warm and sunny early October weather I sure don’t see it here.

The brand new 00z GFS version is similar:

Below average upper-level temps from later this weekend through at least the following 8 days or so.

Chief Meteorologist Mark Nelsen