Updated Trends and Synoptic Patterns for Pacific Northwest Heat Waves

There have been several extreme heat events in the Pacific Northwest over the last decade, including the major heat wave that occurred in June 2021. The record-breaking June 2021 heat wave set a new all-time maximum temperature record in Washington (120°F) and tied the record in Oregon (119°F), set or tied 268 all-time maximum temperature records at select individual weather stations across Washington and Oregon with long and relatively complete data records, and had significant impacts on human health, infrastructure, forestry, and coastal ecosystems. Both Washington and Oregon recorded their warmest year on record in 2015, which also featured a warmer than usual summer. A previous study by Bumbaco et al. (2013) assessed long-term trends in historical heat waves in western Washington and western Oregon, but given the recent extreme heat in the region, have significant trends emerged in heat magnitude, frequency, or duration in the last decade?

This study assesses trends in heat waves throughout Washington and Oregon by a few different measures. The first methodology takes a regional view by examining hot days and warm nights separately for the western and eastern portions of the states using aggregated station observations. The second methodology examines a subset of individual weather station datasets in which hourly temperature and humidity data are available so that trends in high heat index days can be calculated. There are statistically significant increasing trends in most aspects (i.e., frequency and duration) of the warm night heat waves but the results are mixed for the measures including daytime heat.

We also assess similarities and differences in synoptic patterns (e.g., 500 hPa geopotential height, sea level pressure, vertical motion) of the heat waves that occur only west of the Cascade Mountains and those that occur only east of the Cascade Mountains. Similarly, while many of the daytime and nighttime heatwaves occur simultaneously, we examine the synoptic characteristics of those that are solely daytime heat waves and those that are solely nighttime heat waves.