An Unusual September Rain Event

Saturday, September 28, 2013
5:50 a.m.

There’s no sound quite like moderate rain.

Light rain doesn’t make much of a sound at all, and heavy rain can be overpowering at times, especially if it is related with a thunderstorm. But moderate rain… that hits the spot. Moderate, consistent, soaking rain is great for just sitting by a fire with a warm bowl of Top Ramen. Hot chocolate is so early 90s.

Let’s take a look at our current situation. It’s been a while since we’ve seen such a colorful map. We’ve got flood watches, flood warnings, wind advisories, high wind warnings, gales, and even storm warnings well off the coast and from Willapa Bay down south to Cape Blanco, Oregon.

Watches, Advisories, and Warnings over Western Washington. Retrieved from Seattle NWS. Site URL: http://www.wrh.noaa.gov/sew/

So far, the Cascades in Southern Washington and Northern Oregon have seen the heaviest rain. Timberline Ski area has seen 5.17 inches of rain in the last 24 hours. Much of the mountains northward have not seen quite as much rain, but amounts have still been quite significant. As of 3:50 p.m., Sea-Tac has accumulated 1.15 inches of rain in the gauge since midnight. I cross-checked this statistic with some on-site observations while my mom was driving me down Alder to our house, and the 1.15 inches seems to be correct… perhaps even a bit low. Take a look at the UW WRF-GFS below. It shows over 10 inches of precipitation in many locations over a 72 hour period. That’s pretty incredible. Thankfully, while some rivers will flood, the rivers started out pretty low, so any flooding will be minor.

Valid 05:00 am PDT, Tue 01 Oct 2013 – 72hr Fcst: 72-hour precipitation (inches), 10-meter wind (knots). UW WRF-GFS 4km Resolution: Initialized 12z Sat 28 Sep 2013. Retrieved from the (University of Washington) Pacific Northwest Environmental Forecasts and Observations website. Chart URL: http://www.atmos.washington.edu/~ovens/wxloop.cgi?mm5d3_wa_pcp72+///3

One of my favorite aspects of the UW atmospheric sciences website is their “Northwest Radar Loop,” which is a composite loop of images from a variety of standard NWS WSR-88D radars around the region. I wish they kept a continuous record (or at least a longer one) of past radar images, but the loop extends back eight hours, and that’s not too shabby at all. The image below shows that most of the recent rain has been over Washington, and it will continue well into the evening hours over Seattle before coming to halt.

Andddd… the power just went out. 4:38 p.m. But that’s ok.

I was reading some Seattle NWS discussions and looking at some satellite imagery over the past couple days, and it became clear that there was some tropical moisture entrained within this storm. While it is uncommon for strong storms to roar ashore this early in autumn, they are definitely not unheard of. I remember a strong, compact, poorly forecast storm that came through on October 18, 2007, and I remember a much stronger storm that curled off our coast on the 5th of that year. And of course, the earliest windstorm in the Pacific Northwest – the Columbus Day Storm – was easily the strongest. Downtown Portland had a gust to 116 mph. ‘Nuff said.

We’ve got an even stronger storm on tap for Sunday, and although this one will be quite the soaker as well, the main threat will be the wind. A HIGH WIND WATCH is now in effect from from Sunday afternoon to Sunday night over much of Western Washington, including areas from Seattle northward in the Puget Sound region. As is tradition, the coast and north interior will see the highest winds, but even the Seattle area could see a few gusts to freeway speeds. This will no Columbus Day storm… but it’s forecast to be stronger than anything I’ve ever seen in September.

Last year’s storm season was apocalyptically boring. Will this year help to nudge the short-term average back to the long-term average? It’s too early to know. But from all the thunderstorms earlier this summer to a possibly record-setting soaking of a September, I can rest easy, knowing that I’ve already been blessed to see some unusually interesting weather in our region. šŸ™‚

And at 5:48 p.m., the power is back on. šŸ™‚

I will post an update tonight with the latest forecast for the Sunday storm. If you take a model average, we’ll probably see a 970-980 mb low coming in somewhere between Forks and Winter Harbour, BC. This is quite a spread for an event expected to occur in less than 24 hours.

Windstorm-ily yours,
Chuck

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