Saturday, September 17, 2011

First "major" storm of the year

Saturday, September 17, 2011
10:56 A.M.

9/17/11 10:45 A.M. PDT infrared satellite (taken from UW atmos website)

Ladies and gentlemen, it appears that our first "major" storm of the year is upon us. I say "major" because it will likely be the biggest storm we have seen in months. However, I put quotes around it because in all honesty, it is a pretty weak storm. However, remember how I was talking about that pattern change and that strong jet stream? Although most of the storm is heading up north, the jet stream has helped the cyclone deepen, and we will see some fringe effects of it.

One thing that stands out is that you can see three clear fronts. The cold front is in the far left, and it is trailing the storm. The warm front is to the far right, and it is already spreading some clouds over the area. The occluded front is to the north, and it is bent back in a counter-clockwise direction to the center of the low. For more information on the different types of fronts, refer to my previous blog post.

 Valid 11:00 am PDT Sat, 17 Sep 2011 - UW 12z  36km WRF-GFS 3 hour precipitation

The above model frame, taken from the WRF-GFS model in 36km resolution, shows the current precipitation associated with this storm, as well as the isobars. Isobars show lines of constant pressure, and can be thought of as a "topographical map" of pressure at the surface.

Valid 05:00 am PDT Sun, 18 Sep 2011 - UW 36km 12z WRF-GFS 3 hour precipitation

We are getting drizzle right now, but the storm will really start to affect the interior early Sunday morning. The Olympics will get the heaviest rain, but the lowlands will likely also get some significant rain for this time of year. Nothing too drastic, but more than we have seen in a while. I'm forecasting amounts up to 1.5 inches on the Olympics with most places getting less, and rainfall amounts around a quarter of an inch or less in the lowlands, with more precipitation the further north you go.

I was looking at the extended run of the WRF-GFS, and I saw something that could be a little troublesome.

Valid 11:00 am PDT Wed, 21 Sep 2011 - 102hr Fcst - UW 36km 12z WRF-GFS 3 hour precipitation

 My eye immediately jumped not to the storm affecting British Columbia and Alaska (which is significantly stronger than tomorrow's storm but will not impact us) but to the wave by 40 degrees north and 160 degrees west. After watching these models for years, I've become able to recognize waves that look like they might have potential to develop into strong storms. 

Below is the same wave 15 hours later.

Valid 02:00 am PDT Thu, 22 Sep 2011 - 117hr Fcst - UW 36km 12z WRF-GFS 3 hour precipitation

But look at the storm it develops into 18 hours later!

Valid 08:00 pm PDT Thu, 22 Sep 2011 - 135hr Fcst - UW 36km 12z WRF-GFS 3 hour precipitation

That is a very powerful storm for September! A little further south and we would be talking about a major windstorm. And when I say major this time, I really mean it.
But I started to get concerned after the following frames. Here is the storm 12 hours later.

Valid 08:00 am PDT Fri, 23 Sep 2011 - 147hr Fcst - UW 36km 12z WRF-GFS 3 hour precipitation

You can the cold front "training" into the Olympic Mountains. This setup is called a "Pineapple Express," because it often stretches back toward Hawaii and brings gobs of saturated, subtropical air into our area. To make matters worse, these types of storms often stall over the area and don't move from side to side much. This is a classic setup for major flooding in the Pacific Northwest. Right now, the front is mainly to our north and only dips down for a small period of time, which would give the mountains heavy rain but would confine Vancouver Island to flooding. Also, this forecast is far in advance.

Bottom line: this scenario is too far out to make any conclusions, and honestly, it probably will not develop. But it is something to keep an eye out for. I have already warned my cousins who live on the Snoqualmie River, because if this scenario plays out and the front stalls over central Washington, some of our rivers could flood. 

Regardless, it is interesting to see a storm this powerful this early in the season. Then again, the most powerful windstorm on record, the Columbus Day Storm, was also the earliest major windstorm, striking the Pacific Northwest on October 12, 1962. 

The storm season is upon us!

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