Updated Forecast, Heavy Rain on Monday?

Tuesday, September 21, 2011
1:08 P.M.

I’m writing this from the Seattle Public Library since I am not at home right now. I’m actually waiting for ID from the Department of Licensing downtown, which will take a while.

Anyway, the models have not changed their thinking much today. That big storm from that powerful wave I was talking about is still heading up north to Alaska. However, the models have eased off on what little rain there was in Seattle. There will be some afternoon light rain far north, but we may escape without seeing any measurable rain in Seattle. There is a chance for some rain, but if there is any, it will either be just a trace or a few hundredths of an inch. Nothing to worry about.

Of course, we will still see some additional cloud cover, and this will keep our temperatures from getting too hot today and tomorrow. Temperatures could actually get fairly mild today, as that big storm is bringing up a lot of air from the south, but Thursday will be cooler. Once this storm passes through, Friday and Saturday will be nice! I don’t think we will get into the 80s but we should be mostly sunny both days.

Clouds ahead of the next system come in on Sunday, and we should see some rain, particularly in the afternoon. This system will be pretty weak though.

I was particularly looking at the Monday system, because it looks like it could give us a fairly heavy dosage of rain for this time of the year. Take a look at the picture below, it shows 3-hour precipitation and gives a loose approximation of what the storm might look like.

Valid 08:00 am PDT Mon, 26 Sep 2011 – 123hr Fcst – UW 36km 12z WRF-GFS 3-Hour Precipitation

It definitely looks like a pretty large storm from here! And the 24-hour precipitation amounts ending at 5 A.M. on Tuesday are impressive as well.

Valid 05:00 am PDT Tue, 27 Sep 2011 – 144hr Fcst – UW 12km 12z WRF-GFS 24-Hour Precipitation

That is some heavy rain for this time of year! Compared to last night’s 00z run, the rain in the Olympics is a little lighter (though still significant), while the rain in the Cascades and lowlands is heavier. The eastern valleys of the Cascades will receive very little precipitation from this storm. This is because the mid level winds (850 millibars) are fairly fast, thus promoting orographic effects. This means that mountains will see more precipitation, and places behind the mountains (like Eastern Washington or Sequim) will see very little rain.
 

Valid 11:00 am PDT Mon, 26 Sep 2011 – 126hr Fcst – UW 12km 12z WRF-GFS 850mb temp, winds, heights
 
I realize that above map is hard to read, but you can see that tight pressure gradient and those barbs pointing from a southwesterly direction. Those barbs translate into the winds at that level. I forget the actual system of finding out how much wind those barbs mean, but I generally go by the ground rule that barbs with more stuff on them translate to higher winds. Regardless, there is a tight pressure gradient, and that definitely shows that there will be strong winds aloft.
 
Bottom line: look out for Monday, as we could see some hefty rain totals for this time of the year. Beyond that, we will be drier, but it will start to feel like fall and we will not be quite as warm. In the meantime, enjoy some nice sun and warm temperatures!
 
Thanks for reading,
Charlie
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