Sunday, November 20, 2011
Carol Snider – Urban Flooding in Seattle – December 3, 2007
Approximately four years ago, one of the strongest storms to ever hit our area, the “Great Coastal Gale,” brought hurricane-force winds to the coast and major flooding throughout the area. While the inland areas didn’t get hit quite as hard, the storm was still one to remember. For old time’s sake, I’ll show you the satellite picture of the storm approaching the area.
07:00 pm PST Sun 02 Dec 2007
While we probably won’t see a storm of this magnitude for many more years, we will see a major storm bringing high winds and flooding rain approach us. A fast-moving front will come through the area Monday morning, bringing a brief dose of heavy precipitation and brisk winds to the coast, but this will not be the main event. On Monday night, the biggest storm we have seen for all winter will slam into the Pacific Northwest. This storm will generate very strong winds, particularly for the coast, and will dump heavy rain over much of the area. To make matters worse, this storm is expected to be a slow-mover, and it is possible that it might not clear the area until Wednesday morning.
For most of us, heavy rain will be the primary concern with this storm.
Valid 04:00 am PST Wed, 23 Nov 2011 – 60hr Fcst – UW 00z 12km WRF-GFS 24-hour precip
Look at all of that rain! The models indicate that much of Western Washington could get inundated with 2-5 inches of rain from Tuesday morning to Wednesday morning.
With all this rain comes flooding concerns, and it will be a close call for many of the area rivers. Although the mountains will get even more rain than the lowlands, the amounts in the models are actually relatively low for an event like this. This seems to be due to the lack of orographic enhancement over the mountains, particularly the central Cascades, which generally get more rain from a westerly flow rather than a southwesterly one. The Skokomish is likely to see major flooding, as it is the most flood-prone river in the state. Many other rivers may see minor to moderate flooding. Thankfully, at this point, it doesn’t look like a historic flood event.
But it will be very, very wet.
Now, let’s look at the winds.
Valid 01:00 am PST Tue, 22 Nov 2011 – 33hr Fcst – UW 00z 12km WRF-GFS 10m wind, SLP
The model above shows the winds expected over our area at 1 A.M. Tuesday morning. You can see two general areas receiving high winds… the area immediately south of the low, and the larger area in ahead of the stationary front off our coast. Since this front will not move much, the area of high winds will not move much either. The result will be a prolonged high-wind event for the coastal strip of the Pacific Northwest. Inland areas will also get windy, particularly the north interior, but the coast will receive the brunt of this storm with respect to wind.
The rest of the week looks rather exciting as well. On Thursday, the model shows a small but intense area of low pressure making landfall at the mouth of the Columbia. This could spell high wind for the Portland area, and will need to be watched carefully.
Valid 10:00 pm PST Fri, 25 Nov 2011 – 126hr Fcst – UW 00z 12km WRF-GFS 10m wind, SLP
The bigger story, however, is that the model I have been using for this post indicates that another major storm similar to the one we will see Tuesday will impact our area over the weekend, giving high winds to the coast and heavy rain throughout Western Washington
04:00 am PST Sun, 27 Nov 2011 – 156hr Fcst – UW 00z 12km WRF-GFS 24-hour precip
We’ll have to keep an eye out for this one as well, but not all of the models show it at this point. In the meantime, button down the hatches and get ready for Tuesday. It looks as though the storm season has finally arrived.