The Wettest Month Ever in Portland

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


25 Responses to The Wettest Month Ever in Portland

  1. Tyler Mode in Battle Ground says:

    Beautiful day with all of the fresh snow so low!

  2. JohnD says:

    Pretty thoughtful and righteous NOAA AFD this afternoon. We take what we get–right? It is SPECTACULAR out there for now–and that’s for sure. Who knows, longer term!

  3. Washougal 1100ft says:

    It’s about to get chilly out! I wounder what roll the snow covered ground here and east will do for the upcoming east wind event? 33 out in the sun with a brezzy east wind. No snow melt in shaded areas sstill about 4″ or 5″ on the ground.

  4. Tyler Mode in Battle Ground says:

    Hello offshore flow…Humidity 93% at 1 PM, now 63% at 2 PM

    Temp is 42

  5. boninepaul says:

    If I don’t get any more rain- and I won’t I’ll be at 16.34″ for December in North Portland. Sodden.

  6. W7ENK says:

    17.56″ of rain at my place in Milwaukie for December. With some dense fog overnight and this morning condensing on and dripping down the inside of my rain gauge, does that count as a Trace of precip? If so, tomorrow will be my only dry day for the whole month.

    • Tyler Mode in Battle Ground says:

      Same here Erik. I could see the mist falling this morning out of the fog. 0.01″

    • W7ENK says:

      Yup, I had to use my windshield wipers once the car warmed up enough to keep it from freezing. Areas along the road that hadn’t been run over by cars’ tires had a bit of a sparkly white sheen to them, and any spots that remained wet from yesterday’s rain were very slick. Looked like ODOT had sprayed deicer on McLoughlin not 10 minutes prior to when I drove through.

      Crystal clear, no frost when I got to downtown.

  7. Tyler Mode in Battle Ground says:

    27.5 here in BG with fog and icy roads. Coldest morning of the month so far.

    15.11″ of precipitation for the month. My wettest month ever for my 5 years out here.

  8. Clark County is a mess with icy roads this morning. It doesn’t help that folks are still driving at or above the speed limit despite some roads being glistening white.

    Slow down folks!

    • Unfortunately, many people who own four-wheel drive vehicles operate under the misconception that such vehicles possess special stopping ability in addition to their special getting-go ability.

  9. donaleen says:

    I think this site has a more accurate look at Portland rainfall. The closest gauge to us says 18.03 for December. The average for Portland rain gauges is 17.27 for December. This is a better way to look at the rainfall totals rather than just looking at the airport.

    • Mark Nelsen says:

      That’s an excellent site. I’ve seen those individual HYDRA sites but not the compilation of monthly/yearly totals. Nice average. The Portland Fire site is right downtown and it has around 17″ too.

  10. chiefWright (Marquam) says:

    Mark, how is it that temperature observations are easier to screw up that rainfall? And by that I assume you mean total measurement error (which is the difference between reported value vs true value) as a percentage of measured range? (The technical definition of screw-up 😉 )

    Without getting into all the different measurement technologies that have undoubtedly been used over the years, here’s my take on the two manual methods

    Having done both manually with a min/max thermometer and a rain bucket, I need to be more careful with the bucket than the thermometer, and both can be messed up if I’m not on the spot at 7 AM (depending on whether it’s raining, or the min temp is occuring around a 7AM dawn). So both seem to have their own difficulties with reading error.

    Or is it gauge accuracy? Even if a rainfall has better single-point gauge accuracy, monthly rainfall is a cumulative of 30 individual readings. So gauge accuracy + reading error will also accumulate. And most errors have some offset, so they don’t cancel out as they accumulate.

    Whereas temperature is a single-point and has no cumulative error.

    So do you mean location error (getting a true value, but not of the thing you intended to measure)? Maybe it’s siting, but one look at a CoCoRAHS map will suggest there can be big differences in daily rainfall at several sites within a 1 mile radius. Just as a look at Wundermap will show the same thing for temperature.

    So it’s not obvious to me, anyway, that rainfall is tougher to screw up.

    So why do you say so?

  11. Paul D says:

    Mark: how about wrapping up the year with a blog listing all the records we broke this year. I’ve lost track of all of them!

  12. Tyler Mode in Battle Ground says:

    27.5 here in BG with fog and icy roads!

  13. JohnD says:

    Mark: Haven’t the specific “airport” recording locations changed several times over the years since 1940?! I think they may have.

    • Mark Nelsen says:

      You might be thinking of Swan Island Airport. That was before 1940. If observation location changed at the airport it was by less than I mile I would think

  14. W7ENK says:

    Roads in Milwaukie are icy and slick AF this morning!

  15. That’s quite the rainfall total. I haven’t counted my amount for the month yet, but it should be close to about 11 inches. Meanwhile, I had other things on my mind last night; We felt an earthquake at about 11:40 pm. No damage to be seen yet, but at one point I heard a “crack” in the house.

  16. BlazerFan32 says:

    Hi Mark

    Based on the fact that there temperature readings at least for each day on that chart I am going crawl out on a limb here and say that when the total is put in the next following time that those are multi-day readings. Something that happens quite commonly at CoCoRaHS and also that was done at Mount Rainier National Park too. One other possibility is that the observer may have simply left the column on particular days empty because they saw no rain in the gauge BUT more than likely too there are probably a number of multi-day readings since we basically know there were many days of rain in the Portland Metroplex this month. I wish observations were at least a little more reliable sometimes than they are. As the coordinator for CoCoRaHS Pierce County here in Southeast Tacoma I am always having to check on my observers around here.

    • BlazerFan32 says:

      Looks like the 23rd they forgot totally to observe anything. Maybe they were more concerned about getting home for Christmas or maybe they were already on vacation. Speculation…Speculation… 🙂

  17. jimbo says:

    Looking at some serious black ice in am

  18. Matt (Vancouver 98683 @150ft) says:

    That’s some sweet record keeping and info! Thanks, Mark! =)

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