There is a lot of attention given to the heavy precipitation that falls on the windward side of Northwest mountains. But perhaps the most remarkable aspects of Northwest meteorology is not where it is raining but where it is not.
Our region is also the land of rain shadows....some of the most profound in the world.
Most are aware of the classic climatological rain shadows of our region: northeast of the Olympics and east of the Cascades (see below). This is why folks retire to Sequim and Port Townsend--Los Angeles rainfall in the Northwest!
But during the past few days, one of the most dramatic rain shadows in memory has occurred in a different location than normal: over central Puget Sound.This blog will describe and explain why.
Consider the accumulated rainfall over the past 48 hours (ending 6 AM this morning) shown below.
Over 5 inches over the windward (western) side of the Olympics, but only 0.01 inches over Puget Sound.
You read that right. Over FIVE HUNDRED TIMES more precipitation on one side versus the other. Mama Mia....that a world-class rain shadow.
The Cascades were almost as impressive. Roughly 5 inches on the western side and 0.01 inch on the other.
The weather radar images have been stunning in showing the profound rain shadow effect (see below for a few samples).
You could even see a rain shadow "cloud hole" in the visible satellite image from yesterday afternoon....some luck folks around the Hood Canal were able to see the sun, while rain was falling around them.
Why did the rain shadow move over central Puget Sound? And why was it so strong?
Rain shadow location and strength are dependent on the wind speed and direction of the air approaching the associated mountain barrier.
As air approaches a barrier it rises, resulting in enhanced precipitation. As it sinks on the downstream (leeward) side it dries out, producing a rain shadow.
For most of the winter, moist air approachies the Olympics is from the southwest, producing a rain shadow to the northeast of the barrier. Thus, the most significant drying is around Sequim and Port Angeles.
But yesterday, the winds approaching the barrier were not only strong, but coming almost directly out of the west, thus producing an intense rain shadow nearly due east of the Olympics.
It illustrate, here are the winds around 5000 ft during mid-day Sunday (the barbs show wind direction and speed...almost 50 kt at the level). Strong westerly winds hitting the Olympic crests and then descending over Puget Sound.
Enjoy the weather contrasts....it is why meteorologists love this region.
What's also very interesting is that the convergence zone often forms in the same place.. directly downwind of the Olympic mountains. So the same area either sees a lot more, or a lot less, precipitation than nearby areas.
ReplyDeleteCliff, can you do a blog on what factors have to be in place to create "convergence" rain (more rain) for some systems and a rain shadow (no rain) for others, both in the same place?
I'd agree with JHK. It is not always obvious why the convergence forms where it does. If the mountain barriers where symmetrical, like a round rock in a stream, I would expect the rain shadow to form immediately behind the mountains, and the convergence to form further downwind, in line with the rain shadow. That does not always seem to be the case. Recently, northern Bothell, where I live, has been rain shadowed from the last two atmospheric rivers.
Delete1.78 inches in West Gig Harbor, Washington in 36 hours since Sunday at 12:01am. One event for the records books. Just to the north in Silverdale and Poulsbo, nearly nada! Thanks for the excellent blog, Cliff.
ReplyDeleteHey Cliff, wondering if you have any thoughts on the Wednesday weather this week other people are predicting tornadoes?
ReplyDeleteBob in Sequim
ReplyDeleteGood post. It’s hard being a weather nut in Sequim, where forecasts are often rubbish. From this last ‘atmospheric rive’ we’ve had about 0.1”/day. The forecasts are for several times that. It’s very common. One would think that given the weather direction, rainshadow country could have better forecasts.
ReplyDeleteI live in Port Townsend and use three different weather apps. At 100% chance of rain, it will be raining. Anything less than 100% means a few sprinkles at best. Cliff, what's the challenge with getting a good rain forecast in the shadow?
DeleteWonder if you have any insight about the potential for severe thunderstorms/rain/hail that’s headed our way tomorrow?
ReplyDeleteYour illustration triggered a memory from my Jr. High days in Seattle, around 1961 or so. A TV set was brought into class, and set to Channel 9-the local "Public"/information station. Some geeky-looking man proceeded to explain to us kids, about the "rain shadow effect"--the first time I had ever thought about it! (and growing up in Seattle, that made sense)...He had a model of our region in front of him, and took a wet sponge, jammed it against the "Olympics", squeezing it too hard, spraying water over himself!...But his demonstration did stay with me, and I did learn a thing or two about our "different" local weather conditions. Personally, now living in Everett, I resent the Rain Shadow, as I actually enjoy a rainy day. It does turn out, that we in Everett get about 6-8" less rain than Seattle does. I am reminded of that Cat Stevens tune, "I'm being followed by a Moon Shadow"...in my case, a Rain Shadow!
ReplyDelete