Unstable Air, A Convergence Zone, And An AMAZING Satellite Picture

Thursday, January 28, 2016
9:10 pm

Image taken at approximately 12:00 pm  1/28/2016
Credit: NASA Terra MODIS Satellite

Apart from an extremely rainy morning for many folks, today ended up being a pretty nice day. Our atmospheric river that had been giving us so much rain, wind, and warm temperatures finally sagged to our south as cold front, and in its wake came cool, unstable, and rather moist air. Although pressure gradients relaxed somewhat, things were definitely still gusty, and the amazing satellite picture above shows that. Let me explain.

I got this picture from NASA’s MODIS satellite imagery page. MODIS stands for “moderate-resolution imaging spectroradiometer,” and it is a very expensive instrument installed on two NASA satellites: the “Terra” and the “Aqua.” These two satellites are called “polar orbiting satellites” and orbit the Earth around the poles. Most meteorology satellites orbit the equator in sync with the Earth’s rotation so they can take shots of the entire Earth at any given time, but polar orbiters have to take a single, continuous image as they circle around the poles. It takes around 99 minutes for these satellites to orbit the poles, and each one can only take one picture of a certain region per day. However, these polar orbiters can come much closer to the Earth than the ones over the equator, and when they pass over right as an awesome weather event is occurring, you can get some sensational pictures.

The cold front has just passed Western Washington, and is now over the Cascades. If you look really carefully, you can see the actual cold front as a very thin discontinuity in the cloud cover centered over the Cascades and curving very slightly as you head north. Ahead of this front, winds were primarily from the south, but behind it, winds were from the west. This buoy near Destruction Island shows a very sharp change in pressure, wind speed, and direction following the passage of this morning’s front.

Credit: National Data Buoy Center
Credit: National Data Buoy Center

With strong, westerly winds and cool, unstable air in the wake of the front, air flows off the Pacific, and, after encountering terrain, is forced to rise, with clouds and precipitation resulting. On the other hand, places on the leeward side of terrain in westerly flow, like the Willamette Valley and the Southeastern portion of Vancouver Island, remain relatively cloud-free due to air sinking as it comes off terrain.

However, there is one place that is directly behind a mountain range and is NOT cloud-free. And that’s our trusty Puget Sound convergence zone. The zone forms because winds, like all of us in life, would rather avoid obstacles than fight an uphill battle. Therefore, they split around the Olympics and “converge” right over Puget Sound! This time, they do rise, forming clouds, precipitation, and even the occasional thunderstorm in the process.

Even though the zone looks pretty intense on that satellite picture, it didn’t really get going until later in the afternoon. Remember, that satellite picture was taken right after the front passed Western Washington, and even though it is extremely impressive that the convergence zone formed that quickly, it really ramped up as the afternoon went on. Take a look.

Credit: University of Washington

This radar image was taken at noon, which is close to when the satellite picture above was taken. You can see some weak convergence, with showers over Vancouver Island. This image is from the Camano Island radar, so there is blockage by the Olympics.

Credit: University of Washington

Three hours later, the zone is much more intense. There are even some reds in there!

Credit: University of Washington

By 4 pm, it has moved over northern Seattle. Red indicates very heavy rain.

Let me leave you with one thing before I sign off for the night. I’ll repost the satellite picture here for convenience.

Image taken at approximately 12:00 pm 1/28/2016
Credit: NASA Terra MODIS Satellite

To the north and south of the convergence zone, the skies are blue as can be. The reason is because after the air has finished rising, it diverges and sinks on either side, creating bluebird skies in the process. It’s great for basking in the sunshine while your neighbor’s house gets struck on lightning, and it sure makes for some great satellite pictures.

Thanks for reading!
Charlie

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