Mainly Or All Dry Next 7 Days

September 24, 2017

6:45pm Sunday

It has been a beautiful fall Sunday with temperatures reaching the low-mid 70s across the metro area.  Quite a change from exactly one week ago!  Remember that’s when the first rain was arriving.  You may also remember models were forecasting up to 2″ or so rain in the valleys and 3-8″ in the Cascades, followed by sunshine.  That’s exactly what happened:

Portland has now seen more than its typical allotment of September rain.  Typically September is our 3rd driest month (after July & August).  But this year we had a very dry start to summer with just over an inch of rain both in May and June.  So this was a much-needed soaking.  Notice the final rain totals in the mountains show a “season-ending” event for fire season in most of the Cascades…well before the typical time we’d see that happen.  I just checked the morning fire briefing and every single fire is showing minimal fire behavior.

Of course with a dry week ahead I’m sure some fires will produce some smoke, but the woods don’t dry out easily the last week of September so it appears unlikely any fire will suddenly start charging the lines again.  Good news!

The extra September rain has cemented this “water year” as the 3rd wettest in Portland’s history.  This follows those crazy wet back to back years of 1995-1997.  Water year is how we measure rainy seasons in the West.  On this side of the Rockies most precipitation falls during the cool season, which means it is dropped into two different “yearly buckets”.  So by starting each “year” on October 1st, all the rain/snow from one season gets put into “one bucket”.  Then looking back we can easily compare rainy seasons.  In this case we are wrapping up our 3rd wettest rainy season on record!


Looking ahead, the general plan for this next week is a building upper-level ridge over us, then it gets flattened next weekend.  We get a somewhat strong “thermal trough” west of the Cascades late Tuesday through early Thursday.  That means gusty east wind through the Gorge and over the Cascades, maybe out over the Coast Range too.  This will be the first time since early in the summer that easterly wind will bring blue (instead of smoky) skies.  Enjoy the warmup…a +17 degree (C) temperature at 5,000′ combined with offshore flow should push us well into the mid 80s for the last time this season.  Then onshore flow and much cooler temps return Friday and into next weekend.

As the ridge builds tonight and tomorrow, a weak system dies as it squeaks by the ridge; Monday will see lots of clouds and maybe even measurable rain in spots north of Portland.  So tomorrow may be a relatively gloomy fall day before the bright sun returns Tuesday.

For you gardeners, of course you don’t need to water right now (soil is wet) and you can probably get by all this week without watering.  That’s because soil is slow to dry out in late September with the weak sun angle.  But pots/containers will definitely need a soaking by midweek.

The next real soaking, or wet fall storm, will likely show up at some point early/mid next week.  Using the ECMWF ensemble forecast system, you see the ridging still overhead next weekend:

But by the following Wednesday (10 days out) troughing with the associated showers & cool weather is back

Then at the 2 week point, ALL 3 MODELS (GFS, ECMWF, GEM) show a wet pattern with upper-level troughing filling the far eastern Pacific and Pacific Northwest.  This is a wet mid-fall pattern

The message here is: USE THIS WEEK TO WRAP UP OUTDOOR PROJECTS, NEXT WEEK MAY TURN QUITE WET ONCE AGAIN.  Now a bit more rain is fine, but let’s hope we don’t have a ridiculous October like last year.  The entire month was a washout.  28 out of 31 days saw measurable rainfall.  It was so bad that my (ex) neighbor told me that was the final straw to get him to move.  Now he’s in Las Vegas…that’s quite a change!

Chief Meteorologist Mark Nelsen