Did you notice how dry the air was yesterday over much of western Washington?
Oddly enough, you can’t blame it on a major storm over the Pacific Ocean!
To show you what’s going on, below is a graph of the minimum relative humidity west of the Cascades on Tuesday. In some locations, the relative humidity drops below 20%!
The relative humidity forecast for Tuesday 2pm predicts very low values (below 20%, brown) in western Washington and southwestern BC. Low values on east WA as well.
But why?
The satellite image visible yesterday at around 2 p.m. was dramatic, with a deep low-pressure area in the center of the vortex and a broad frontal band of clouds extending around the lowlands to northern Oregon. and then south into central California.
Accompanying this storm was an active low-level wind circulation. Here is a forecast for wind and altitude (like pressure) of about 5000 ft (850 hPa). You can see the extreme low center outside the CA/OR contour. OH.
Now, look at the barb. They point to very strong easterly winds (from the east) blowing over Washington and British Columbia.
Such winds not only carry dry air from within the continent, but as the air travels down the western slopes of the Cascades and the coastal mountains, it is heated by compression. Warm air can hold more moisture than cooler air, and this causes the relative humidity to drop drastically.
Another factor is that the air going down starts higher, where the air usually has less moisture.
In fact, subduction-induced warming was evident at yesterday’s high temperatures, with maximum temperatures rising from the lower to mid-60s in northwest Washington (see below)
The giant depression is currently weakening and slowly moving south (see infrared satellite image below at 8am today) and its eastward (from the east) flow to the north is weakening . We’ll have another warm/dry day north of Tacoma, while southwards bands of cloud and humidity will continue to surround the lows.
One
Needless to say, large amounts of moisture are currently flowing from the Pacific Ocean into California. If only they built more reservoirs…
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