Are we pumping in cooler air way too early this fall?

Eye on Climate

PORTLAND, Ore. (KOIN) – You may have noticed that the start of October has been quite cool this year.

We’ve only had 11 days in the 70s from Sept. 1 to Oct. 13. It has been nearly below average for the last two weeks in Portland. Talk about a harsh jump from summer to fall for the Willamette Valley. Some of those 70-degree days came with rain too.

Most of September should be spent in the 70s. From the start of September to the start of October, there should be somewhere around 28 days in the 70s. Early October does have an average high of around 80 degrees. By the time we start the month of December, the average high will be in the upper 40s.

The graphic below displays the climatic high transition from September to December. The larger numbers are the temperatures we begin the month with (Sept. = 80, Oct. = 71, Nov. = 58, Dec. = 49). That’s a 30-degree drop from the start of the fall months to the start of winter.

The sunrise and sunset from that point in time goes from 6:32 a.m. / 7:47 p.m. to 7:30 a.m. / 4:28 p.m. It’s that seasonal change that is reflected by the temperatures for us in the Pacific Northwest (PNW).

What happened to the fall transition? How can our 30-year average high be off this year? Of course, those average high temperatures aren’t going to play out every year. A climate is over a span of time and this is a 30-year average. That means we may have a year that is colder or warmer than the previous. This year, after a summer that finished well above average, we are watching the pendulum swing in the other direction.

We went back and checked out the coldest October high over the previous 30 years, and many years we find temperatures in the lower to mid-50s. There are a handful of years when the month of October may offer a day in the 40s. Back in 1991, there was a day when the daily high temperature was just 43 degrees. Talk about a winter day tucked into October. Last year, we had a day in the 40s too, which is around 15-20 degrees below average. There is a 20% chance that the daytime high is in the 40s in October. That hasn’t happened this October, just yet.

There is a 20% chance that the month of October also doesn’t have a morning low temperature that dips below 40 degrees. That happened six times over the last 30 years in Portland. Although our average morning temperature doesn’t fall below 40 in October, most years, we have at least a day or two that hits the 30s. That is exactly what has happened here the last few mornings in Portland. If 80% of the time we do drop into the 30s, what about freezing?

From 1991-2020, we have hit 32 degrees or below 26.6% in the month of October. We have dropped below 30 degrees only 13.3% in the month of October in that 30 year timeframe. It doesn’t happen often, but mother nature can bring in that freezing air from time to time. The coldest temperature in October over the last 30 years was 27 degrees in 1991.

The graphic below is a representation of the coldest October lows from 1991 to 2020. I do want to point out that in 2014 the coldest low temperature was 47 degrees (mild). It is noticeable that in the last decade, the coldest October morning low has been 40 or above more than the previous 20 years. The last two years did bring in back-to-back years of 29 degree lows for the month of October.

We’ve been close this year!

Frost 10.12.2021 (On Disney Frozen Kids Toy)

Multiple morning records were broken on October 12, 2021. It was a morning that brought frost to the car windshield and the grass in the yard. If you happened to leave the kids vehicle out, it likely had some frost too. You can see how the frost built up on Natasha’s daughter’s car in the photo to the left. Yes, we’re aware of the irony.

The earliest 32 degrees or below temperature in Portland is October 8, 1985. Now that we know frozen temps happen in October, we can also stress that on average, it usually occurs in November. From 1981 to 2010 the average first 32 degree day is November 15. From 1991-2020, that has shifted one day later to November 16. No big deal, but if you cycle through the slideshow below, you can see the big differences between our current 30-year normals (1991-2020) and the previous 30-year norms (1981-2010).

You’ll notice the trend since 1981 of our coldest low temperatures, frost (36°) to below freezing (24°), is happening later in the fall season on average. Our coldest temperatures are delayed by one to two weeks on average.

  • 11-day difference for <=36°
  • 1-day difference for <=32°
  • 6-day difference for <=28°
  • 16-day difference for <=24°

I mentioned that we recently broke & tied records in the morning. You can swipe through the slideshow of record morning low temperatures from around the state. Portland and McMinnville tied previous records, but out to Astoria and central Oregon, many records were lowered. Astoria hit 33 degrees, meanwhile, Sunriver had a record low of 10 degrees! For temperatures to drop that low in the month of October, you usually need a clear night. We don’t usually have snow in October because the temperatures do not drop low enough when the moisture comes in during the Fall. As I mentioned, you need the sky to clear out after an already cool afternoon.

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