Sunnier, But Cool This Weekend

September 22, 2012

A nice surprise on visible satellite imagery this morning.  Even though we still have a very thick marine layer, the upper level low passing overhead must have “stirred things up” a bit; you can see plenty of clearing west of the Cascades.    This is a sign of a much brighter day ahead.  And it will still be a cool day so everyone should be happy.

Astronomical Fall started this morning (about an hour or so ago).  Meteorologically we consider Fall to be September-November.  That’s because across most of the northern hemisphere the warmest three months are June-August and the coldest are December-February.  That leaves the 6 months in the middle as Fall and Spring.  It’s all very arbitrary and there are only 3 weeks difference between the two so it sure isn’t worth an argument.  But here’s a curious fact that adds to the debate:  West of the Cascades in Oregon and Washington (and parts of California too), September is slightly warmer than June.  A quick check across the rest of the USA (including eastern Oregon and Washington) shows that we are alone along the west coast with this phenomenon.  June, July, and August are the warmest just about anywhere else.  This likely contributes to the general feeling that summer is “delayed” here.  The past two years sure have helped that along too!  Part of the reason is the slow to warm and slow to cool Pacific Ocean, and the general weather pattern often features more upper-level (cool) disturbances passing overhead. 

But think about the main difference between the two months (June and September).  We get far more cool onshore flow in June.  In a normal September we get more offshore (easterly) flow events in September than June.  That would mean that if all else is equal temperature-wise in the atmosphere between the two months, then September should average warmer west of the mountains.  We’ve seen what a difference the wind direction can make this week.  The 5,000′ temperature was about the same Monday-Thursday, yet we had a high of 90 one day and 63 another just due to a low-level wind switch.  Occasional episodes like this are enough to make September slightly warmer.  And why do we get more offshore flow in September?  Because the continent starts to cool with the longer nights.  That equals more episodes of higher pressure to our east.  So I suppose one could argue that because the interior USA is cooler in September, we end up slightly warmer (on average).

Chief Meteorologist Mark Nelsen