Monday, November 21, 2011

Thin Band of Heavy Rain

Monday, November 21, 2011
12:03 P.M.

Valid 01:00 pm PST Tue, 22 Nov 2011 - 33hr Fcst - UW 12z 4km 1-hour precip, 10m winds

I just got out of my atmospheric sciences class, and I want to talk about a specific feature associated with this front that will make a huge difference in determining which areas get the greatest rainfall. If you look very closely at the model above, you can see a very thin line of extremely heavy rain. This image shows the rain pointed right at us midday Tuesday. This band is expected to move around a bit, but not a whole lot.

The bottom line is that the heaviest rain totals will be found where this specific band hits. If you look closely, you can even see some greens, which denotes rainfall amounts from .64 to 1.28 inches in ONE hour. That's incredible. Much of the band is blue, which means around a half inch an hour. The rainfall rates weaken over the Puget Sound area, but they are still very high. The Seattle area will probably experience some urban flooding from this storm, but the results could be much worse if this band stalls over our area.

At this point, it looks like the rain will start to impact our area in force by 3 A.M. Tuesday morning. You can see that heavy band of precipitation over the Olympic Peninsula.

Valid 03:00 am PST Tue, 22 Nov 2011 - 23hr Fcst - UW 12z 4km 1-hour precip, 10m winds

That band will sweep over us a couple hours later.
Valid 06:00 am PST Tue, 22 Nov 2011 - 26hr Fcst - UW 12z 4km 1-hour precip, 10m winds

After this, the heaviest precipitation will dip south for a bit (although we will still be getting rain), but that band looks to head back up into the general Puget Sound vicinity by noon and hang around until Wednesday morning.

Last, let's look at the 48-hour precipitation totals expected over the area.

Valid 04:00 pm PST Wed, 23 Nov 2011 - 60hr Fcst - UW 12z 4km 48-hour precip, 10m winds 

Right now, it looks like the North Cascades and Olympics will be the hardest hit. Mt. Rainier looks pretty rainy as well. In this model run, the Central Cascades look ok as far as flooding goes. This is not the "classic" pattern for flooding in the Central Cascades, which get more precipitation from a more westerly flow. Nevertheless, it will be close for some spots, and I encourage people who live there to prepare for the possibility of minor flooding.
I'm particularly worried about the Chehalis River. The models point towards some especially heavy rain in the Chehalis River drainage. In the December 2007 flood event, a 20-mile stretch of I-5 from exit 68 to exit 88 was closed because floodwaters from the Chehalis inundated it with up to 10 feet of water in spots. While we won't see 10 feet of water on the road this time around, we could easily get a couple feet, causing I-5 to be closed for Thanksgiving travel. This  would surely put a wrench in many people's holiday plans.

I've done all my homework for the next several days, so I'll be updating this blog as more information about this coming storm surfaces.

Thanks for reading!

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