On July 9, the temperature rose to 98F at Seattle-Tacoma Airport, a record for the day and 22F above normal.
But that’s not what surprised me…..SeaTac has been this warm before.
That’s HOW warm the weather gets.
I have done a lot of research on heat waves in the Northwest and have published about ten papers on the subject in the peer-reviewed literature. Traditionally, to get warm temperatures in Puget Sound, you need easterly (easterly) winds blowing over the Cascades, winds blowing down the western slopes of the Cascades. (see diagram).
The easterly winds benefit the warmth of the Puget Sound lowlands in several ways. First, they cancel out the influence of the cool Pacific Ocean, which is only about 50F. Second, air coming from the interior of the continent is generally warmer. And finally, air sinking west of the Cascades warms due to compression.
Strangely enough, the situation on July 9 was not like the usual hot weather situation.
Why?
Because there is little evidence of eastward flow. Seattle Tacoma Airport Gets Very Warm without strong support of atmospheric compression.
I wasn’t shocked at all when I realized this. Can I prove that this wasn’t just a random fluke?
So I decided to do a little research… which I’ll write about later this year. I found all the times when the temperature in SeaTac reached 95-99F. (Remember, the highest temperature recorded there was 108F in 2021.)
I then looked at the regional weather patterns associated with the first five events at these temperatures and the last five events reaching the same temperature (again 95-99). Here are the two day collections.
The first group appeared before global warming became significant.
The second group after human impact was predicted.
For each of these date collections, I found mean sea level pressure models and 5000 ft winds (actually 850 hPa winds).
The results were astonishing.
Let me show you the above levels (again, winds around 5000 ft) before and after global warming for such events. For earlier events (below global warming, GW), large heat waves required easterly winds (look at the little arrows). The shaded part shows the wind speed. After GW, the warming was aided by strong easterly winds unnecessary.
Why not? The atmosphere is getting warmer to begin with, with summer temperatures now 2-3F warmer than they were 50 years ago. And there are other underlying factors associated with global warming, too, such as drier soils and changing weather patterns.
Then I asked another question: What about really extreme heat waves, like the 2021 heat wave in SeaTac that reached 108F or the 2009 heat wave that reached 103F?
Do they need easterly flow?
I checked…..and the answer is yes. To get the extreme temperatures, the support from the compression warming process goes downhill obligatory.
Anyway, this is a fascinating topic: what are the weather patterns that are necessary for such extreme temperature changes? I plan to look into it in detail.
