Fall Is Here

September 16, 2013

Between the last 2 days of weather (west of the Cascades) and looking ahead on the maps & models…it’s pretty obvious Autumn is arriving a bit earlier this year in the Pacific Northwest.  Two reasons for that:

  1. I don’t see any weather pattern in the next 7-10 days (just about to the end of the month) that would get temperatures above 80 west of the mountains.
  2. Rain will be an occasional to regular visitor the 2nd half of this month, based on the same information.

We had an early start to the warm season with fantastic warm temps and sunshine the first half of May, so apparently the last half of September will be makeup time…seems reasonable enough.  Plus the first half of September has been unusually warm, running 4 degrees above average at PDX.  So if the 2nd half of the month ends up 4 degrees below…that’s how we’d get TO average.

Lightning yesterday was exciting for some, but disappointing for others.  No one south of Portland or eastside of the metro area saw/heard anything, I wasn’t even aware of it until 1-2pm when I happened to briefly hop online.  But what a show in Puget Sound again and east of the Cascades.  I really like the 99 degree day at Pendleton Sunday followed by a severe thunderstorm with a southwest wind gust to 70 mph!  Crazy…

Looking ahead, we’ve got a round of convective showers tomorrow afternoon.  Could be some real good downpours or hail/thunder possibly.  These showers will be from an upper-level trough passing overhead.  Then not much happening Wednesday as high pressure builds over and north of us at the surface.

Thursday is by far the nicest day of the week with offshore flow (really, it should happen this time!) and sunshine.

What about Friday and beyond?  We get into an increasingly zonal (west to east) flow or “troughiness” over and west of the West Coast.  This is a wet weather pattern in general and it appears it’s going to stick around awhile.  Take a look at Week1 of the ECMWF ensemble maps:

500za_week1_bg_NA

A huge negative anomaly over us and to the west for next week.  The pattern is more like October than September.  Look at the 850mb ensemble chart from the 12z ECMWF and you see the below average temps (except for Thursday and Friday) through the period.

tseries_850t_000-360_Portland

Pretty decent agreement through at least the middle of next week too.  The following 3 weeks show a similar, although less amplified troughing setup in the Eastern Pacific.  I get these maps on Monday and Thursday afternoons from the previous 00z ECMWF run:

500za_week2_bg_NA

500za_week3_bg_NA

500za_week4_bg_NA
At least we’ll be able to shut off the sprinklers and irrigation much earlier this year!

How about snow in the mountains?  At this point there isn’t a cold enough trough forecast in the next week or so to bring it much below about 6,000′.  The earliest measurable snowfall at Government Camp (4,000′) is September 23rd.  That was in 1984.

Not looking forward to the report card tonight.  I totally missed Thursday and Friday.  The only saving grace for Saturday is that I didn’t forecast that day since I had a day off Friday.  Otherwise I would have screwed that one up too!

Chief Meteorologist Mark Nelsen