The Pacific Northwest experienced an extraordinary heat wave last summer. The heart of the heat wave lasted three days; however the days leading up to and trailing those three core days were also very warm. The average daily temperatures at Seattle-Tacoma International Airport (KSEA) for June 26-28, 2021 exceeded normal daily temperatures by 22˚F, 25˚F and 23˚F respectively for those three hottest days. The actual daily maximum temperatures for this period were 102˚F, 104˚F and 108˚F. This event was covered prominently in local, national and international news and continued to be for months afterwards.

However, in the Seattle area (just a relatively small area impacted by the heat wave), aside from another relatively hot period towards the end of July, the rest of summer was very pleasant. In fact, by mid-August, mean daily temperatures began to fall short of normal August temperatures. September and October, often pleasant and warm months in the Seattle area, were both cloudier and cooler than normal. And aside from a warmish period in late November 2021 when atmospheric rivers from the tropics swept in warm winds and huge rainfalls causing massive flooding in parts of the Pacific Northwest and British Columbia, the past winter and spring has continued to be rather cool and showery.

It has been 317 days since the heat wave broke. I plotted the average daily temperature departures from 30Y-normal mean daily temperatures at SeaTac (KSEA) for these 317 days. I also included the 317 days prior to the heat wave as well in the plot. Including the 3-day heatwave, a total of 637 days are plotted in Figure 1.

Figure 1. Daily departure from normal temperature trends, August 13, 2020 through May 11, 2022.

The temperature trends in Figure 1 appear to show a certain seesaw symmetry around the brief heat wave. The linear fit curve (for just the shown data range) crosses the X-axis about a month after the heat wave broke. The pre-heatwave period had 183 warmer-than-normal days at an average of about 4.8˚F warmer per day. The post-heatwave period had 183 cooler-than-normal days. at an average of roughly 4.3˚F cooler per day. One key difference is that the pre-heatwave period had almost twice the number of neutral days than the post-heatwave period.

I suspect this symmetry is likely more coincidental than anything else and likely had little to do with the large heat dome that set in place last summer and caused temperatures to peak. It likely has much more to do with the strong La Nina pattern we’ve been in this winter and spring. Atmospheric systems are huge and are likely slow to change. Though 637 days sounds like a lot of data points, it’s less than two years and it’s perfectly normal for warm and cool trends to settle in for long periods. But just looking at the chart and thinking in medical terms, it appears the “fever broke” after the extraordinary heat wave.

The truth is, if you look at the longer record and extend this chart backwards for many years as shown in Figure 2 the cooling trend appears much more moderate. This chart goes back to 2000. And a 365-day smoothing curve of average daily temperature departures shows a much more gradual cooling trend (after an extended warming trend reaching back to at least early 2017).

Figure 2. Daily temperature trends, Seattle-Tacoma International Airport, 2000-2022.