Can you imagine a region in which rainfall can vary by a factor of 100, with 50 miles or less?
No need to imagine if you are living in Western Washington!
The map below shows the 24-h precipitation totals ending at noon today (Wednesday) over western Washington.
Over two inches over the southern and southwest sides of the Olympic Mountains, but only 0.01 inches around Sequim and Camano Island. A factor of 200 between the rainfall at the two locations!
High-resolution computer model forecasts of rain (the total precipitation for the 36h ending 5 AM this morning) accurately predicted the huge variation (see below), Two inches or more on one side, virtually nothing to the NE of the barrier.
There is a reason that the zone from Port Angeles to Sequim to Port Townsend to Whidbey Island is popular among retirees.
So what happened? Moist southwesterly (from the southwest) air flow was forced upwards by the southern Olympics, producing lots of rain, but north of the mountains, the air descends, causing drying and little rain. (See graphic below).
Rising air cools by expansion as air goes from high pressure at low levels to lower pressure aloft. That causes cooling--like you note when air escapes from a car or bicycle tire. Cool air can't hold as much moisture as warm air, so water vapor condenses out as rain.
Sinking air warms by compression, causing clouds to evaporate and precipitation to cease.
You could see the drying yesterday on the northwest side of the Olympics in high-resolution visible satellite imagery, with a big hole in the clouds to the NE of the barrier (see visible picture).
With all the precipitation, all of the Olympic Peninsula rivers are now running at normal levels (green dots) or above normal levels (light blue and blue).
Good that the Bear Gulch fire is knocked out, Cliff. But precipitation totals in the central Sound so far are disappointing. Less that an inch at most locations. We're still way behind. The forecast you gave us a few days ago was more optimistic. But currently, I see no forecast for heavy rain in the near future.
ReplyDeleteHeavy rain is not a good thing - it causes flooding and other unwanted problems. The nature of precipitation is that it's normal for it not to track along the "normal" line. It's the multi-year average that's important.
DeleteJHK- You're entitled to your opinion! But being from the East Coast, I have never gotten over not being able to watch a torrential rainstorm with thunder and lightning but once in several years! Fifty years and I still miss them. Its good entertainment. Residents there expect it. Happens in the Rockies sometimes too.
DeleteI did some logging out of Forks in the 80’s and we always laughed about the rainfall totals. It’s an inch a mile of yearly precipitation difference going from PA to Forks.
ReplyDeleteYesterday evening, here in lovely S. Everett, my neighborhood got blitzed with high wind, and a heavy dose of rain...but only for maybe a half-hour. It was a welcome sight, the resulting scoured air smelled really fresh, and my cars got a free power washing! Hope we get more rain mini-squalls!!!
ReplyDeleteDr. Mass, I know the United States is in the Dark Ages when it comes to measurements, and I comfess that I'm a US citizen and have lived in this country my whole life, but I really wish we could start measuring using the metric system, especially with rainfall. For example, I can easily visualize 2.5 mm versus 0.1 inches.
ReplyDelete