Monday, December 3, 2012

Watching the Satellite

Monday, December 3, 2012
10:36 A.M.

10:00 am PST Mon 03 Dec 2012 - UW West Coast 4km water vapor satellite

Good morning everybody, I hope you are all having a nice first week of December. It was pretty rainy last night, but we'll be heading into some calmer weather soon.

However, we've got a storm to go through first.

Sometimes, there is a large discrepancy between what the models initialize the current state of the atmosphere to be and what the atmosphere actually looks like on satellite. One example of this was the snow event back in January 2012 (which was probably my finest moment as a forecaster because I totally went against the models and ended up being correct). But looking at this satellite image compared to the initialization, you can see some pretty big differences. Let's first look at what the models say what the atmosphere should be like right now. This is 6 hours after the initialization because the UW doesn't have this model map for the initialization.

Valid 10:00 am PST Mon, 03 Dec 2012 - 6hr Fcst - UW 12z WRF-GFS 36km outgoing longwave radiation (similar to infrared satellite)

Take a look at that swath of clouds offshore. That's the low pressure wave that's expected to develop and slam into our area Tuesday morning. Now, let's take a look at the actual infrared satellite.

10:00 am PST Mon 03 Dec 2012 - UW West Coast 4km NWS infrared satellite

As you can see, the maps generally look the same, but there are some important differences. In the actual infrared photo, there is more of a "hook" at the upper left portion of the storm. This is called a bent-back occlusion and it is a classic feature of strong cyclones. This occlusion is not very pronounced in the infrared satellite.

For an even better comparison of images, take a look at the water vapor satellite picture.

10:00 am PST Mon 03 Dec 2012 - UW West Coast 4km NWS water vapor satellite

There is a very pronounced "dry slot" here, which is another indication of a strong cyclone. The low pressure center is typically located within the dry slot. 

Do I think we are going to see a major windstorm out of this? No. The cyclone is not developing very quickly and the jet stream supporting it is not particularly strong. However, I do think this is something the NWS service should keep an eye on. In their 9:15 forecast discussion, they mentioned issuing a wind advisory for the coast and north interior later today. They may have to upgrade that to a high wind warning, at least for the coast, and they may also have to issue a wind advisory for the Puget Sound lowlands if this storm comes in stronger than expected. It won't be a megastorm by any means, but it just goes to show that models aren't perfect representations of the atmosphere at a given time, and these satellite images show that.

Off to calculus, send your prayers my way. :)

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