Autumn doesn't wait until September 22 in the Pacific Northwest.
During most years, there is a transition to cooler temperatures and the return of rain during late August, and this year will not be different.
A plot of average temperatures at SeaTac Airport (below) illustrates my point. Today (indicated by the blue line) is near the transition point.
This change is not only forced by changes in solar radiation intensity, but also by changes in the large-scale pressure systems that control our weather.
Let me illustrate these large-scale changes by showing you a series of average upper-air (500 hPa pressure, about 18,000 ft) monthly weather charts.
During mid-summer (July) there is a big ridge (dark red) over the Southwest and a weak trough centered to the north of us. The jet stream (located where the colors change rapidly) is north of Washington State.
During the next 5 days, the upper-level ridge over the Southwest is apparent.
The implications for our weather are profound. The European Center temperature forecast for Seattle shows highs falling into the lower 70s and 60s:
Rain will return as shown by the forecast below for two 7-day blocks of time into the future.
No major heatwaves are predicted in these model runs, and it is rapidly getting too late in the season to get one.
Thanks for the heads up, prof! We still have a few last-minute chores to finish before monsoon season, including clearing the rain gutters and some last-minute painting.
ReplyDelete"Monsoon season" is ending. Check out Cliff's past blogs explaining the monsoon.
DeleteYes- our fall/winter rainy season does not qualify as a monsoon, which has its roots in the tropics or subtropics. The Southwest monsoon brings rain to the SW and southern Rockies, and sometime to the northern Rockies and eastern Washington, mostly via thunderstorms, but (unfortunately) rarely brings much to Western Washington.
DeleteAt long last
ReplyDeleteCertainly feels like it today. Last weekend, my dad and I encountered a couple of fellow trail riders, and I happened to spot one sporting a kit from a Seattle to Vancouver ride. I asked him if it was from 2024, but he said he thought it was from before then. The 2024 STV happened to be very, very wet on both days.
ReplyDeleteThe 2026 Seattle to Vancouver kicks off today, and at least it looks like the afternoon will be dry. I'm not doing the ride, but I will be riding on the same trail (after the commotion passes through Machias, thankfully).
I'm not ready for fall. It feels like summer is just getting underway. There's no need for relief from Seattle area summers, the summers ARE the relief, and they're frustratingly short.
Cliff, is there a corresponding transition point/date from spring to summer in the Pacific Northwest?
ReplyDeleteTraditionally July 5th, although Cliff used to say July 12. But I think it has gotten earlier in recent years.
DeleteI know this was written on Friday and it's Sunday now but I think we got a touch warmer than predicted yesterday (temps I saw forecast were 73, but appeared to be more like 75F) but I got the front yard mowed (mostly the now beheaded dandelions).
ReplyDeleteI still have the backyard to do this morning.
Am noticing it's cooling off, but still have a burst of low to mid 80's, Mon and Tuesday before returning to the mid 70's, at best for the most part all the way into Sept.
Can't wait for fall to arrive.