The Autumn Weather Transition is Occurring

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.   

By September, the situation is very different, with the southwest high pressure weakening and moving inland, and a trough (lower pressure) moving towards the west coast.  The weather implications of these changes are profound.

Consider what is going to happen during the next 15 days using the best forecast available (the European Center AI forecast), looking at the same type of upper level chart.

During the next 5 days, the upper-level ridge over the Southwest is apparent.

During the next 5 days, the SW ridge weakens and starts to shift eastward, and a strong trough moves into our region

And the map for the subsequent five days is downright scary with a very deep trough of low pressure (purple color) offshore:  cool and wet for us!

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.

Enjoy the "fall transition" in August.  One of the treats of living in the Pacific Northwest.

PS: The latest radar shows showers moving in!