An hour ago, I felt it.
The winds picked up and the accompanying air was cooler and moister. My wind chimes started to sound. After a second day with highs around 80F, it was clear that tomorrow would be cooler.
An offshore push or marine push had begun.
Not a particularly strong one, but enough to bring down the high temperatures tomorrow by 5-10F within the interior of western Washington and northwest Oregon.
The visible satellite image late this afternoon showed a tongue of low clouds moving up the Oregon and SW Washington coasts, a sign of the changes to come.
Whether we have warm, dry offshore-directed flow or cooler onshore flow is all about pressure.
The low-level winds tend to flow from high to low pressure.
Friday afternoon at 5 PM the pressure pattern was favorable for warmth, with an area of low pressure, the thermal trough, along the coastal zones of Oregon and Washington (see below, lines of constant pressure…isobars…are shown by the solid lines, color shading shows temperature).
The pressure was higher to the east of the Cascades resulting in easterly (from the east) wind across the Cascades.
But check out the situation today (Sunday) at 5 PM–quite a difference. The lowest pressure has jumped to the east of the Cascades and higher pressure had built up along the coast. The result is an onshore-directed pressure difference (or gradient) that produces westerly winds from off the Pacific.
A plot of the winds (red numbers are gusts) and air temperature at 8 PM clearly show the onshore winds, with southwesterly winds gusting to 24 mph at Shelton and 22 mph gusts from the northwest at Port Townsend. Cooling westerly winds are also starting to push over the Cascades and show rev up the winds on the eastern slopes of the Cascades.
Admittedly, this is not an extreme or threatening weather event, but it is one of the subtleties of local weather that is enjoyable to experience and understand.
The Northwest Weather Workshop, the annual gathering to talk about Northwest weather, climate, and major meteorological events, will take place on May 1, 2021. This year we will have a special session on the meteorology of the September 2020 regional wildfires. The meeting will be online. More information, the agenda, and registration information is found here: https://atmos.uw.edu/pnww/
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