Monday, March 26, 2012

New Quarter, Old Weather

Monday, March 26, 2012
10:34 A.M.

UW Quad: 3/22/2012

UW Quad: 3/24/2012

Both of these photos were taken from

It's a new quarter at UW. It's springtime in the northern hemisphere. The cherry blossons in the Quad are blooming. And, as of 10:47 A.M., the Dow Jones Industrial Average is up precisely 129.00 points. Everything is happy in the Pacific Northwest.

Except the weather.

We had a very nice stretch of weather over the weekend. Saturday and Sunday featured light winds, temperatures well into the upper 50s, and a few high clouds overhead. I hope you savored these days, though, because they'll be the last warm days we will see for a while.

You see, springtime in the Pacific Northwest is unpredictable. The sun can rise high in the sky and give you a high of 50 after a brief snow shower in the morning. I remember one spring day in 2007 when I saw heavy graupel, almost got struck by lightning (don't even get me started on this one), and saw a weak tornado form and touch down right before my very eyes. If you take a look at the ensemble chart below, you'll see just how hard it is to predict springtime weather in the Pacific Northwest. Comment courtesy of Greg Carstens.

This chart shows the different GFS ensemble members, and the operational, ensemble mean, and control models are highlighted. The climatology for the season is highlighted in red. As you can see, we have some pretty good agreement up to March 29th, with a few periods of light precipitation and near normal temperatures. Take a look at the upper-level 500mb vorticity chart of the 12z WRF-GFS initialization at 5 A.M. this morning.

Valid 05:00 am PDT Mon, 26 Mar 2012 - UW 36km 12z WRF-GFS 500mb vorticity, heights

If you look at the height lines over our area at this point, you'll see that there is a trough offshore and a ridge over our area. There may be a thermal trough down in Oregon as well. But the main thing here is that the flow over our area is light, and we are not seeing much in the way of weather. We do not have a super strong ridge overhead, and we do not have a strong zonal flow, which is generally indicative of stormy weather here.

But all of this is going to change.

Take a look at the same chart a mere 36 hours later. The trough has shifted towards us and is directing cooler air into our region.

Valid 05:00 pm PDT Tue, 27 Mar 2012 - 36hr Fcst

But it gets even worse. The dreaded strong zonal flow will begin to impact our area, bringing with it rain and windy conditions, particularly for the coast. Take a look at the chart 72 hours in advance.

Valid 05:00 am PDT Thu, 29 Mar 2012 - 72hr Fcst - UW 36km 12z WRF-GFS 500mb vorticity, heights

I'm not going to give you the details right now because if you remember that ensemble chart above, there are lots of variations between the models. I also gotta find out where my math class is. But one thing is clear. We will remain cool, wet, and windy down here in the lowlands for the coming week, particularly for the latter half of it.

As for the mountains? Expect 2-4 feet of snow by Saturday. Don't put away those skis yet. :)


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