The Last 80 Degree Day?

September 30, 2010

What a day!  But was this it?  The last taste of Summer?  There’s a good chance it was if you look back at history.  The average last 80 degree day of the season here in Portland is September 27th.  We’ve gone through 5 Octobers (since 2004) without seeing that 80 degree mark.  In fact last year we only had two days above 70, the year before only 3!  71 degrees was the best we could do in October 2009.

This points out how quickly our weather cools (and turns wet) during the month of October.  It’s the fastest changing month in our area.  Average high temps cool from around 70 on the 1st to upper 50s by Halloween.  No other month sees the temperature change so quickly.   The quickly decreasing sun angle, long nights, and cooling continent finally catch up and overcome the warm late summer atmosphere.

I dropped a bunch of the 7 Day forecast numbers because I felt they were just too warm for this time of year and the weather pattern.  For example, it’s obvious that we’ll be socked in a westerly flow and saturated atmosphere below 5000′ on Sunday.  On October 3rd that’s well down into the 60s…maybe ONLY 60.  Tomorrow too, I think 79 is way too warm.  It took great effort (offshore flow) to get us into the mid 80s today.  Flow goes onshore tomorrow, so a 10 degree drop seems reasonable.

Chief Meteorologist Mark Nelsen