Cliff Mass’s Weather Blog: Heatwave Ahead!

Almost every May, we enjoy a “heat wave” mid-month, a few days in which temperatures rise into the upper 70s or lower 80s west of the Cascade Crest.

This year is no exception.

For gardeners frustrated by the cool, rainy weather, get your plants and tools ready!

To “start” the conversation, consider the daily temperature forecast at SeaTac Airport from the National Weather Service’s National Modeling Group, a statistical combination of weather forecast models main (see below). Usually highly skilled.

This forecast system predicts temperatures will warm steadily this week, with highs on both Friday and Saturday near 80F, then cooling down to around 70F.

East of the Cascades in the Columbia Basin? Same story, but add about 10°F and a one-day delay (see Tri-Cities temperatures below). Around 90°F on Saturday and Sunday.

The heat wave in the Northwest is associated with the development of a high-level high pressure center (also known as a ridge) in southwestern British Columbia (see map below as of 2 p.m. Thursday in level 500 hPa—about 18,000 ft).

Shading indicates departures from normal in climate, with red indicating altitude/pressure above normal. While significant, this high is not in the same class as those that produced record warmth, such as in June 2021.

A high pressure gradient is associated with sinking air, which heats up due to compression. It is also associated with low-level offshore (eastward) flow, which exerts greater stress as it sinks over the slopes of the local terrain. And the easterly flow blocks the cooling influence of the Pacific Ocean.

The mid-May heatwave is a favorite among locals

Do you remember last May? A heat wave around May 13 (see chart below for Seattle, blue is temperatures from May 2 to 23, red represents record highs).

This year will not be as warm.

Or consider the highest temperature each year between May 7 and May 21 in Seattle since the late 1940s (see below). Two-thirds of the years have at least one day above 75F. There was only a slight upward trend in peak temperatures in mid-May.

Why do we often have warm spells in mid-May, followed by cooler weather?

First, the sun is very strong and the days are quite long at this time of year. Quite similar to the end of July. Therefore, solar heating has strong potential.

Second, weather disturbances such as low and high temperatures are still quite strong (they are strongest in mid-winter) and these disturbances are important to trigger a major heat wave. To have a strong heat wave in the Northwest, one needs a well-defined area of ​​high pressure over coastal British Columbia.

Third, in mid-summer, low-level high pressure builds west of our coast, creating strong, cool onshore currents. Often accompanied by low clouds, such onshore currents cause temperatures to drop from late May to mid-summer.

Either way, I’m looking forward to the warmth. And I know others feel the same way: