During the past week it has been completely dry over the western WA lowlands, but not over the crest and eastern slopes of the Cascades, where convection (cumulus, cumulus congestus, and cumulonimbus--thunderstorms) have developed each afternoon. A satellite picture from Friday afternoon shows an example (see image). I have also include radar images from the NWS radar at Camano Island and Spokane. You can see some convective cells over the Cascades and immediately to its east.
So what is going on? During the past several weeks we have had a persistent flow pattern with low pressure to our south and easterly flow (from the east) approaching the Cascades. This air has been only marginally stable, which means it has the potential for convection with sufficient surface heating or mountain lift. Anyway, as this air has started to rise on the eastern Cascade slopes cumulus convection has developed, increasing in strength over the crest. Then as the cells descent the western slopes they tend to weaken...since sinking causes warming and drying that works against convection (see my book for more on this).
There is another way that mountains encourage convection. When slopes are heated, upslope flow often occurs and for a mountain crest upslope flow occurs on both sides. The upward currents join together at the crest with strong upward motion resulting...such upward motion can initiate convection.
Want to see a nice time lapse of convection forming over the Olympics by this mechanism?..check out http://www.drdale.com/lapse/lapse090610.mov
..from a HD cam near Silverdale (Dale Ireland)
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