October Begins Cool, & Water Year Ends

September 30, 2018

6:30pm Sunday…

It’s been a real quiet weather weekend with just a few sprinkles/showers yesterday and today. Officially we’ve had all of…wait for it…  .01″ in Portland.  That’s weekend total.  I know a few of you had more, but overall boring for weather geeks like me.

Tonight (the end of September) wraps up the “Water Year”.  If you’re new to the Pacific Northwest there’s a very good chance you’ve never heard such a phrase.  Here’s the deal…our rainy season is late October through March; during the cool season.  As you know, we have a very dry warm season here compared to areas east of the Rockies.  That’s May through September.  All that winter rain (in the ground) and snowmelt (coming down rivers) is used to sustain our lives/properties/crops through the dry season.  So it’s important to know how wet our rainy season is each year.  But each rainy season crosses from the end of one calendar year into the first part of the next calendar year.  So for the purposes of ranking our wet seasons, we use a “water year”.  That year begins on October 1st and ends September 30th.  Make sense?

How did we do this year?  We ended up drier than average by 6″or so.  Here’s a nifty little chart from the folks over at Portland NWS.  Winter rainfall was just about normal until we hit February.  But that month and March were drier.  Then a very wet first half of April brought us back to close to average again.  And you know the rest of the story…we’ve pretty much flat-lined since that time.


Comparing this water year to recent years shows we had two very wet “wet seasons” and now we’ve experienced a dry one.  You can see the big year to year variability.

Rain WaterYearPDX

The water year gauge “resets” at midnight and we begin a brand new year.

Here’s what I see for this first week of October:

  1. Next few days feature a splitty flow with a southern system moving into California giving them a much needed rain.  We get a weak system dragging through here tomorrow night and Tuesday morning; sprinkles/showers at most during that time
  2. A large upper-level ridge wants to develop out in the Gulf of Alaska (just to our west) later this week and through early next week.  This means a real lack of storminess in the eastern Pacific but also quite chilly air (for early October) just to our east.  Notice all 3 big models (GEM/ECMWF/GFS) show the same pattern for NEXT Monday, the 8th…one week from tomorrow

This slideshow requires JavaScript.

A slight move westward and we’re into an unusually chilly (early frost?) pattern all across the Pacific Northwest.  A slight move east or a weaker ridge (ECMWF) would mean cloudier, but mild and occasionally wet weather.   We’ll see how it works out and how much rain we get next weekend.  ECMWF was somewhat wet so I went with that model.

3. No sign of a stormy October weather pattern.  With a large ridge out there we’re sure not going to see an extra-early start to the Pacific storm season here on the West Coast.

4.  Cool temps, or at least cooler than average.  We’re done with 80 degree temps and at this rate there’s no reason we couldn’t be done with 75 too.  Check out those ECMWF ensemble forecast highs the next two weeks…pretty cool this weekend and early next week


So the next week will be cooler than average with more showers toward the weekend but not a “start of the wet season”.

Chief Meteorologist Mark Nelsen