A Late Summer This Year

August 29, 2011

We’ve only got 2 more days to go this month and it looks like August will end up right around average.  That’s due to cooler than average temps the first part of the month, then a very warm 2nd half of the month.  Now this workweek appears to be quite chilly with marine air dominating west of the Cascades.

That cooling is just in time since all the lightning late last week started dozens (maybe hundreds) of fires across central and north central Oregon.  The latest (the Dollar Lake fire) flared up the last couple of days and is now burning southwest of Laurance Lake and a few miles north of the summit of Mt. Hood.  A huge plume of smoke was towering over the Upper Hood River Valley all day today.  I see temperatures at the 5,000′ elevation have plummeted into the 40’s this evening up there as a cooler airmass moves in.  It’ll be here through Thursday, and maybe Friday, so firefighters have plenty of time to get caught up.  And I don’t see any lightning pattern either.

Starting this weekend, we appear to be headed into a very warm early Fall pattern with easterly surface flow at times and a dry airmass.  This is one of my favorite weather patterns of the year.  It’s not uncommon to have a crisp low around 45-50 and a high of 90; a huge daily range due to the dry air and long nights with a warm airmass overhead 24 hours a day.  We went somewhat conservative with our 7 Day forecast due to models uncertainty about placement of a thermal trough.  For example one run of the GFS showed a strong thermal trough west of the Cascades for quite a few days beginning Saturday.  But the ECMWF showed weak onshore flow, or at least a loss of offshore flow, Sunday-Monday.  That can make a huge difference in high temperatures.  You need easterly flow at the surface to get hot after the 1st of September.

Chief Meteorologist Mark Nelsen