The key element of other weather the past few days has been strong easterly flow aloft. Such strong offshore flow typically produces fine visibility (as long as you are not embedded in low-level fog) along the western Cascade slopes and this morning is no exception (see below at 7:30 AM from the Space Needle cam). Below that is the view at noon Friday.
Why are the mountains so crystal clear under easterly flow? Because the air sinking down the western slopes warms and dries as is descends, so no clouds and low relative humidity. Low relative humidity prevents the growth of water-loving (hygroscopic) particles.
But why is there easterly flow?
We start with a large-scale upper level atmospheric configuration with high pressure to the north and and an unusually persistent low to our south. This figure shows 500 hPa (roughly 18,000 ft )heights, winds, and temps forecast for 4 AM Saturday morning. The wind barbs show strong easterly flow (from the east) over our region.
Closer to the surface, the easterly flow is revved up by the large difference in pressure across the Cascades, with higher pressure to the east. Here is the seal level pressure and low-level temperatures at 4 AM this (Saturday) morning. The black lines are isobars (lines of constant pressure). A very large pressure difference across the Cascades (about 10 hPa), which drives easterly flow down gaps, such as the Columbia River Gorge. Why the big difference in pressure? The big driver is temperature, with cold dense air (blue colors) trapped east of the Cascades.
To show this temperature contrast, here are the minimum temperatures the last 24 hours across our region. Lots of single digits in eastern WA, and even some below zero, while twenties were dominant over western WA.
What about wind? Here are the maximum gusts for the last 24 h (ending 7 AM). Very strong winds over the western side of the Columbia Gorge (51 mph) and the western side of the Strait of Juan de Fuca. Air is accelerating down the pressure gradient in these gaps. And you will also notice some strong winds along the eastern slopes of the Cascades.
But the sunny weather has a subtle down side: our air quality is starting to decline. Clear skies allow the earth to radiate infrared energy to space, thus cooling the surface and low-level air. The easterly flow, and associated warming, is stronger aloft. The result is the development of inversions, where temperature increases with height. And inversions lessen vertical mixing and result in declining air quality. You can see the changes by looking at the vertical temperature soundings at 8 AM, once a day, for the last several days at Seattle (see below). Today (black line) there is a decided inversion. Not so a few days ago.
But changes are in the air. A system will approach on Sunday, bringing clouds and rain to Oregon (but will hold off until Sunday night in most of Washington). But nothing heavy. Splitting systems. Classic post New Year's situation under a strong El Nino. But California is going to get hit, very hard. Here is the GFS model total precipitation forecast for the next 10 days. Biblical levels of rain.
The Northwest Weather Workshop, the big regional meeting on Northwest weather will take place in Seattle on March 4-5, 2016. At this meeting we talk about the latest advances in understanding our local weather. There will be a special session on the OLYMPEX project. It is open to all. For more information go here.
If you want to give a presentation, abstracts are due on February 1.
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