Warm But Not Hot

 During the first two weeks of May, there is often a short warm period that surges to 80°F or more, followed by a serious cool down.

This year will be no different.

Below are the forecast temperatures for Seattle.  Warming to 80F on Monday, followed by cooling into the upper 60s, with no major heat through May 11.

To illustrate this typical temperature pattern, below is a plot of temperatures in Seattle for April 1- May 31, 2024.  Observed temperatures are in blue and record highs in red.  Record lows in blue.

A major warm-up around May 9, followed by cooler temperatures the rest of the month.  Classic.  

You will notice that May brings the end of any frost threat.....good for gardeners to keep in mind.

So why do we often see a spike in temperatures in early May followed by cooling?

First, the sun has become strong in May, as strong as it will be in August.  Solar radiation at noon is summer-like, and days have become longer.  

Below is the solar radiation reaching Seattle since January 2025.  By May 1, we are really cooking.

To get warm temperatures, even in summer, we need a period of offshore flow, since onshore flow off the chilly Pacific will not allow us to get out of the 60s.

To produce the upcoming warming, this weekend will bring offshore-directed (easterly) winds, as illustrated by the winds, temperatures, and heights (pressures) around 5000 ft (850 hPa pressure) on Saturday morning (below).

Why do we typically have fewer really warm days later in the month?  

Because later in May brings extensive low-cloud areas over the eastern Pacific as high-pressure builds offshore.  This high pressure pushes the cloudy/cool air into western Washington. 

We call this "June Gloom," but it typically starts to move in during mid-May.

Here is an example of June Gloom cloudiness from last May.  Enough to make me want to grab a sweater.