As upper-level high pressure strengthens into Tuesday, our regional air conditioner is turning off. The cool onshore winds that keep us comfortable will shut down, shifting to a weak offshore flow (moving from land to sea).
To understand why this causes our temperatures to skyrocket, it helps to look at the mechanics of our local terrain. Onshore flow brings cool, dense air straight off the Pacific Ocean and the Strait of Juan de Fuca, keeping us insulated from high heat.
But when winds turn offshore, that marine air gets pushed away. Instead, air moves from east to west, sinking down the slopes of the Cascades. As air sinks, it compresses and warms up rapidly.
That thermodynamic “hairdryer effect” will allow temperatures to soar Tuesday afternoon. While the coast, north sound, and the islands will stay in the 70s, areas around the Puget Sound basin will easily climb well into the 80s.
The hottest spots tomorrow will be located nearest the Cascades and down into Lewis County, where that compressing air hits first and hardest. Expect highs there to top out close to 90 degrees Tuesday afternoon!
My forecast high for Seattle is 84 degrees. For context, that is exactly ten degrees shy of the daily record high of 94 degrees set back in 1970—so that particular record is perfectly safe.
Enjoy the heat while it lasts, because the marine push returns almost immediately. Onshore flow kicks back in on Wednesday, bringing more clouds, a slight chance of rain showers, and much cooler highs dropping back into the 60s to low 70s.
We also have a slight chance for a mountain thunderstorm, mainly during the afternoon hours as the shifting air mass creates some instability.
We will dry out briefly on Thursday with highs remaining in the mid-60s, before another rain chance enters the picture by Friday afternoon and evening.
Looking further ahead, an early outlook for Saturday indicates a possible upper-level low-pressure system moving directly across the state.
If this timing holds, we can expect widespread rain showers and isolated thunderstorms on Saturday, before we finally see a drying trend and slight warming early next week.