The Wettest Month Ever in Portland

December 29, 2015

It has finally stopped raining!  Here are (what should be) the final numbers for December 2015.  Wow…

BMAC Rain Two Day Total

The only 3 days without measurable rain will be the 26th, 30th, and 31st.

This blows away the previous wettest month of all-time at PDX, which was 13.35″ in December 1996.  The records go back to around 1940 at PDX.  That’s a 75 year period.

Now over the past few weeks I’ve had several people email (one called…yep, old style communication!) that I was missing the older records and it sure isn’t the wettest of “all-time”.

  1. It is at the airport.
  2. It’s quite possible it’s the wettest month downtown since the 1882.  Most likely it’s the 2nd wettest of all time downtown!

Here’s the problem:

  • Records HAVE been taken at various downtown locations since the early 1870s
  • Downtown over time has shown to be about 15-20% wetter than the PDX location.
  • As a result you can’t compare both locations directly.
  • Rainfall observations are tougher to screw up than temperature, so I do feel pretty good about using those numbers, even though I think downtown temperature records are a bit more suspect in the past.

What does Downtown show?  Here are the 3 wettest months:


So how much this month?  Short answer is…we don’t know.  The official observations are sent in once per month and this month hasn’t been sent in yet of course.  One more reason I rarely mention downtown weather stats…quite a time lag.

Preliminary data IS available, but in the case of this month there are lots of days missing:


There are 10 days missing in this data for downtown.

So let’s do some guessing at what the total might be.  The total WITH the 10 days missing is 14.42″.  If you take just PDX numbers for those missing days and plug them in, the total would rise to 17.86!  Remember that’s using the “drier” PDX numbers.  If you increase those missing days by 15% (because downtown is typically 15-20% wetter), then the monthly total would end up at 18.38″.

As a result, I think most likely this month was the 2nd wettest on record in downtown Portland and the wettest since 1882.  

That’s the wettest in 133 years!

Chief Meteorologist Mark Nelsen


More Precipitation = Lower Snow Level

December 29, 2015

10:15am Tuesday…

It’s rare, but I was actually surprised by a dumping of snow out the window when I woke up this morning.  About 1.5″ new so far.  Now I live at 1,000′ just east of Corbett, but I see snow is falling (not sticking) even into the east metro area near the Gorge.  That’s due to slightly drier/cooler air out there.

Models didn’t show this much moisture making it north, and as of now I don’t see the precip suddenly changing over to snow for most of us.  BUT, the snow level near the Gorge at least is right at 1,000′ at this hour.  Notice it’s been dropping from 1800′ to 1100′ the past few hours on the Troutdale profiler:


I don’t think the snow level will get much lower since we don’t have a cold/dry source of air flowing through the Gorge.  That said, offshore flow increases through the afternoon/evening as the low pressure offshore slides to the south.  I’ll be watching closely!

Showers will taper off this afternoon/evening and then we go dry/cold the rest of the week as mentioned in the previous post.

Chief Meteorologist Mark Nelsen