A Cool Weekend, Then Warmer Weather Ahead

September 20, 2018

7:00pm Thursday…

Today has been another very nice early fall day with temperatures reaching the lower 70s.  Here’s the late afternoon view from our camera looking over the Tilicum & Ross Island bridges

Capture

Weather the past 11 days has been amazingly consistent; cooler than normal.  Every day has been below average; topping out between 68 & 72 degrees.  Typically in September we see more up & down movement.  For example very warm days in the upper 70s and 80s and then quick drops into the 60s.

High Temp Last 13 Days

Tomorrow should be even a bit warmer, likely reaching the mid-upper 70s. That’s due to increasing southerly wind just ahead of an approaching cold front.  So enjoy a warm and mainly sunny Friday.  A few light showers arrive around sunset in the western valleys of Oregon and SW Washington; but it’s a weak front so don’t expect a soaking.  A few light showers may pop up Saturday, but Sunday looks dry.  Overall kind of a slow weather weekend; not much happening and no one should get soaked either.

Now about that warmer weather ahead; remember 5 days ago I posted that we were in a cool pattern for the next 10 days.  Apparently it will stop at around 8 days.  It’s pretty obvious that we’re going back into a warmer pattern starting Monday.  You can see the cool upper-level trough over us this weekend

Jet Stream Forecast 2017

then the big ridge coming back next week

Jet Stream Forecast 2017_2

This is the classic early fall warm weather pattern that many of us like.  Lots of sunny days, along with warm temps.  Bright blue skies since the fire smoke is long gone too.  This time of year nights are so long that we drop well down into the 50s and 40s at night so there’s no need for air conditioning.

Here’s a glance at the 24 hour rainfall forecast from the ECMWF ensemble.  Each horizontal line on the upper chart is one ensemble member.  The bottom chart is the average of all those members.  There appears to be almost no chance for showers from the 24th-28th (next week)

KPDX_2018092012_eps24_precip_360

How warm will we get next week?  We know 850mb temps will reach into the +15 or higher range.  That can put us into the lower 80s in late September.  But to get warmer than that (closer to 90) we would need a bit warmer temps overhead plus easterly offshore flow with a thermal trough.  The GFS indicates that will happen at some point next week but ECMWF is keeping the ridging a little to our west.  That doesn’t give us a significant offshore flow; you can see the effect on the model surface temp forecast:

KPDX_2018092012_eps_min_max_15

In a nod to the EURO I didn’t go wild on high temps, keeping them around 80 or so most of next week.

By the way, mark your calendar for SATURDAY, OCTOBER 27TH.  It’s the 26th annual WINTER WEATHER FORECAST CONFERENCE at OMSI.  It’ll be starting at 10am.  You can see last year’s conference presentations here:  https://oregonams.wordpress.com/2017/10/28/presentations-25th-annual-winter-weather-conference/ 

 

Chief Meteorologist Mark Nelsen