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.
A major warm-up around May 9, followed by cooler temperatures the rest of the month. Classic.
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.
The flowers and the newly planted veggies really perk up with the sun and warm temps. Then everything stalls out for a month during June Gloom. Sounds about right!
ReplyDeleteThat "June Gloom" situation is one that I remember so well, as a young teen in the very early 1960s...School in Seattle would finish for the summer during the first week of June...which meant that me, and a few friends, would hit West Green Lake bathing beach, around 11 am, most every day. The sun was fickle, and the temps were usually in the low-mid 60s...but we simply shivered thru it all, enjoying our dominating presence at that beach...because most people were not so brash as to deal with those cooling clouds!...by later June, scores of people would show up, as the temps were hitting into the 70s!!! Transistor radios were blaring, by then!
ReplyDeleteI am concerned about the dryness. BC coast and interior, it's been over 3 weeks since we've had a drop of rain and the two week forecast is bone dry. Warm, sunny, pleasant days. Cold and crisp nights, which at least is holding the mountain snow pack.
ReplyDeleteIt seems to me that June Gloom has declined in recent years, am I right? I hope so.
ReplyDeleteBut that does not mean I am rooting for drier summers which up the fire risk. In fact, a good summer rain once in a while- not a light drizzle but a real thunderstorm- is something I miss about living in the East and, while in school, in the Rockies.
Some years, yes. We definitely had June "gloom" in 2020, 2022 and 2024. However last year, 2023 and especially 2015 did not see any June "gloom".
DeleteThink about what it means to long for the climate in one place to be like it is somewhere else.
DeleteThat is very interesting. I remember the same thing happened in 2023 as well. Warm to hot first part of May. Then second half cooled down significantly.
ReplyDeleteCliff - how unusual is it to have so much dry weather this time of year? It hasn't rained in over 8 days in the Seattle area, and there is no rain in the 10-day forecast either. I'm getting a sense of déjà vu from spring 2025, when it also seemed to stop raining in late April.
ReplyDelete