A Beautiful Start to October

October 3, 2017

6pm Tuesday

It’s only the 3rd of October, but we’ve already seen more sunny days than ALL OF LAST OCTOBER.  Today is considered a “clear” day and last year we didn’t have a single clear day.  Remember October 2016?  The rainy season began a month early last year.  Lots of clouds…

And of course it was a big soaker as the jet stream ramped up early.  Last October was the 2nd wettest on record in Portland

In a typical October here, the first half (or more) of the month is reasonable; occasional weak weather systems mix with sunny/dry periods.  But last year the rainy pattern began in earnest immediately.  My neighbor told me it was the last straw…he sold his house and moved to the Desert Southwest this past summer.  He should have stayed one more year because the first 10 days of this month still look fantastic.  That’s due to high pressure in the upper-atmosphere wanting to linger nearby or just to our west.  You see the current IR satellite image with the stable “high over low” pattern along the West Coast.  That means the jet stream is going well north, then swinging south around the high and then low. This pattern also gives us a nice & warm offshore flow regime at the surface.


That gave us the warm temperatures in the lower 70s this afternoon.  Expect a few degrees warming the next 3 days.  Now Saturday and Sunday (sorry about the weekend timing) the high breaks down with a strong WNW jet stream just to our north.  That pattern will bring in a very thick marine layer plus lifting.  TranslationExpect a very gray and cool day Saturday with a good chance of drizzle or light showers.  That will linger in the mountains through SundayResultThis weekend will NOT be warm & sunny.

But then high pressure develops right over us again for a quick return to offshore flow and sunshine next Monday.  This shows up nicely in the ECMWF relative humidity cross-section.  I’ve circled the weekend.  Thick saturated airmass up to Timberline Lodge or so = gray days.

The 2nd period of warm and mainly sunny weather may last a good chunk of the 2nd week of October.  Yes, it’s possible most of the first half of October will dry and pleasant!

The ECMWF ensemble 24-hr precipitation chart shows that 2nd dry period, then quite a bit of “noise” from the 13th onward.  That’s because the basic pattern appears to change after the first two weeks of the month.  The ECMWF/GEM/GFS models all show upper-level troughing (low pressure) developing in the Eastern Pacific for the 3rd week of October

And here is the corresponding weekly precipitation anomaly (drier than normal is brownish) for NEXT WEEK

Then the 3rd week of the month…looks like wetter westerly flow may come back after this two-week period of mainly dry weather.

By the way, we will be close to the frost point in only the coldest outlying areas west of the Cascades tonight.  I think 99% of us westside will remain frost-free for at least the next week though, and that’s not unusual.  Most areas west of the Cascades don’t get a first frost until the 2nd half of the month or even early November.

Enjoy the sunshine!

Chief Meteorologist Mark Nelsen