Wednesday, October 19, 2011

Change is in the Air

Wednesday, October 19, 2011
5:41 P.M.

Yesterday, I wrote about the variability of autumn weather in the Pacific Northwest and how we can see big changes in short periods of time. Today, we didn't see a monumental change in the weather, but it definitely wasn't like yesterday. As I was walking back from the IMA at the University of Washington, there was a nice, refreshing breeze and plenty of leaves flying around. It wasn't super windy, but it was nice to feel some gusts ruffle my hair and send leaves in those small little whirlpools we often see in blustery conditions.

This diagram above shows the change in winds over the past 12 hours for different heights in the atmosphere as recorded by the Sandy Point profiler. The change is not quite as dramatic as yesterday, but you can still see a shift from northerly winds to southerly winds in the lowest levels of the atmosphere. You can also see a change in the winds in the upper atmosphere from southeasterly to south-southwesterly, which generally means that a low pressure system is progressing westward. We had a weak low pressure system give us some clouds today, but we will see stronger systems Friday through the weekend.

These systems will be directed to us via a zonal jet stream pointed right at us. Check out the graphic below... it shows the winds at the 300mb level forecast for 8 P.M. PDT Friday.

Valid 08:00 pm PDT Fri, 21 Oct 2011 - 63hr Fcst - UW 36km 12z WRF-GFS 300mb Isotachs

This jet stream is not going to move much, and it is going to direct a series of weak systems into our area. The mountains will get lots of rain because the winds aloft will be favorable for orographic enhancement. However, what goes up must come down, so while the mountains are getting enhanced rain due to upslope winds, northern Puget Sound will get a partial rainshadowing effect from downslope winds coming of the Olympics. And of course, if you want to be completely dry, go to south-central Washington.
Valid 05:00 am PDT Sun, 23 Oct 2011 - 96hr Fcst - UW 12km 12z WRF-GFS 48-hour precip

North Seattle will actually see very little rain from this system. The shadowing will be felt less in south Seattle, but it will still be significant at certain times. 

After Monday, the weather is actually looking quite pleasant as a strong ridge of high pressure sets up off our coast. You can see the ridge moving the jet stream northward in the diagram below.

Valid 11:00 pm PDT Tue, 25 Oct 2011 - 162hr Fcst - UW 36km 12z WRF-GFS 300mb Isotachs

You better enjoy this nice weather next week, because inevitably, we will get stormy when November rolls around.

Have a good one,

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