Was May the warmest month in Northwest history?

In mid-May, as western Oregon and Washington experienced a multi-day heatwave, while smoke from the Alberta fires rose overhead, there was talk of May being the hottest month of the year. history of the area.

Is that a record? Is May warmer in the Pacific Northwest? Does global warming affect May temperatures?

This will be the topic of this blog.

To start, consider the (abnormal) difference from normal over the past 30 days for average maximum temperatures (below). Much warmer than usual for the northern states, including Washington (where it reaches 6-10F), but MUCH colder than usual in California and the Southwest.

People I know in southern California have complained vehemently about unusually cold and wet conditions in May.

May in western Washington

Let me start by showing you the long-term temperature statistics for Olympia, Washington.

Why not SeaTac?

Due to the addition of a third runway and significant urbanization and development around SeaTac, its observations are NOT reliable for climate studies. Much less change and more rural character to the area around the Olympics.

First, consider the May maximum mean value at Olympia, from the early 1940s. May 2023 was the third hottest month on record. But also note that May 2022 is one of the coldest months and there is no long-term trend for high temperatures in Olympia, suggesting climate change is small for that parameter.

What about the highest temperature observed in May in Olympia (see below)? May 2023 is warm but some months are almost warm. Importantly, there is no tendency to extreme heat in Olympia.

But the story is different for the average minimum temperature for May in Olympia, which rose from about 41F to 43F.

It is well known that increasing greenhouse cases have more of an effect on daily low temperatures than high temperatures. Furthermore, local development (and there have been some near Olympia Airport) affects lower temperatures than high temperatures.

May in eastern Washington

Review the temperatures in Kennewick, Washington in the Tri-Cities. This station has a very long history (since the 1890s) and less local development (below).

May 2023 has warm high temperatures, exceeded several times in the years 1920-1950. Note that there is a DOWN trend in the high temperatures at Kennewick. I repeat: go down.

This plot is also a good illustration of why long recordings are important. A record dating back only to the 1970s shows 2023 as the hottest year ever observed, inspiring all sorts of headlines in local newspapers. But a longer record, such as at Kennewick, shows that such temperatures have often been observed in the past and that the long-term trend is downward.

The monthly extreme heat in May for Kinnewick is DOWN and 2023 is nothing special (below).

In contrast, average minimum temperatures have increased by about 2F over the past century in Kennewick, with 2023 being the second hottest year on record.

Key point

Highs in May on both sides of the Cascades were warm, but not record-breaking. Importantly, there is no trend of high temperature increases on either side of the Cascades, suggesting that global warming/climate change has relatively little impact on the region’s high temperatures.

In contrast, low temperatures have warmed modestly (by about 2F) over the past century, and part of that may be due to anthropogenic warming caused by increases in greenhouse gases (most importantly) is CO2 and methane) and increasing urbanization and development in the vicinity of temperature sensors.