Cliff Mass Weather Blog: Spring “Heat Wave” Coming

During most of May in the Northwest, there is warm weather lasting several days, due to intensification of the sun and favorable atmospheric circulation (high pressure and offshore winds).

During such periods, the western lowlands can rise sharply in the 80s, while the Columbia Basin warms in the 90s.

Such times are ahead starting this weekend.

To give you some perspective, here’s a graph of the warmest annual temperatures in Seattle between May 5 and 25. Most but not all years have at least some. a May day above 80F, with some years seeing a temperature rise of around 85F. Interestingly, there was no upward trend in warm days during the record period.

The latest forecast from NOAA/NWS National Blend of Models, which combines the projections of multiple statistical modeling systems, predicts mid-80s temperatures over the weekend in Seattle, rising to 90F on Wednesday. Two.

Tri-Cities will settle into the 90s starting Friday and will be there for days (see predictions for Pasco below).

The source of this heatwave will be the development of a high pressure band along the West Coast, replacing the depression that has existed FOREVER.

Let me illustrate the changes by showing you a series of upper level maps (pressure 500 hPa, about 18,000 ft).

By 5 p.m. Friday, a warmer, warmer (yellow) band had formed north from California to southern BC.

The pattern gets complicated on Sunday at 5pm, with high pressure/altitude over British Columbia and easterly (from east) flow through the NW

This pattern is in no hurry to disappear, with British Columbia’s high pressure still present 24 hours later.

As we get closer, I’ll provide more details on the warmth, which will be a welcome relief from the incredibly cool Spring we’ve had so far this year.