Meteorology Lesson: Rex Block

By: Shaun Tanner , 9:13 PM GMT on October 01, 2012

It is October again, in case you didn't know. With the entrance of October comes the beginning of "offshore season" for the West Coast. This is the time of year when weather patterns set up such that winds often shift from onshore (from the ocean to the land), to offshore (from the land to the ocean). Some might wonder why they care. It is true, you might not care. But, being the dork that I am, I DO CARE.

I have one main reason to care about offshore season in California...FIRES. Here in the San Francisco Bay Area, offshore winds can be very dangerous. Remember, offshore winds come from the land and move toward the water. Thus, for the Bay Area, offshore winds move over the East Bay hills and descend in elevation toward the San Francisco Bay. This sinking air warms significantly, creating a fire danger headache. Sure enough, the infamous Oakland firestorm of 1991 began on October 20, 1991, and was fanned by warm, offshore flow.

This type of wind is not limited to the San Francisco Bay Area, however. The famous Santa Ana winds in Southern California are also created by offshore wind pushing through the narrow canyons east of Los Angeles and San Diego. This wind is also descending in elevation from the mountains to the ocean, thus warming. And correspondingly, the fire danger in Southern California has skyrocketed to extreme recently due to the very warm temperatures that the region have been receiving.

This leads me to something I would like to explain to you...the Rex Block. Trust me, it is not as scary as it sounds.

You see, the atmosphere behaves exactly like a river. The air subscribes to the same dynamics and laws as a mountain river meandering through a forest. Thus, if you ever get confused about the dynamics of the atmosphere, all you have to do is think to yourself, "what would a river do?".

If you ever have time to sit down and watch a lazy river, it can be quite fascinating (I know I am a dork). Within the river, embedded in the basic downriver flow, you will see swirling eddies that make their way downstream. Some of these swirls are long-lived, lasting several seconds before being swallowed up by the main flow within the river. Some of the swirls make their way toward the edge of the river, where they sit in place for awhile, spinning independently from the main flow. These rogue swirls will remain at the edge of the flow until another, larger swirl picks it up and ushers it back to the main flow.

Guess what? This happens in the atmosphere as well. When it happens, it is called a blocking pattern. During these patterns, the atmosphere will remain very stagnant. Want another surprise? A blocking pattern is currently set up off the West Coast of the United States.

I realize that the above image looks like some sort of psychology test to a lot of you. However, take a minute to look at the image and orient yourself to what you are looking at. See California near the middle of the image? The image you are seeing is what the atmosphere looks like if you were to go up to the 500 millibar level (half way up into the atmosphere). Meteorologists like to use this level of the atmosphere because it is away from the effects of the ground (the ground causes problems). Of note are the greens, yellows, and reds. For my stream analogy, these colors represent the main flow of the atmosphere that tracks from the middle of the Pacific Ocean, into British Columbia, then through Canada and southward into Louisiana.

Do you see the lonely swirl off the coast of Southern California? That is the rogue swirl that would be equivalent to a swirl along the edge of the stream independent of the main flow. Specifically, this is called a Rex Block weather pattern. This blocking pattern will remain stagnant off the West Coast until another swirl to the west will sweep it back into the main flow and everything will then carry on from there.

Sure enough, if the forecast holds true, this lonely swirl will meander northward over the next couple of days before a low pressure system in the Pacific Ocean picks it up and moves it inland over the western half of the country.

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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6. WunderAlertBot
5:29 PM GMT on October 29, 2012
shauntanner has created a new entry.
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5. weatherbro
1:15 AM GMT on October 06, 2012
i hope that low breaks down by late next week
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4. sp34n119w
5:43 PM GMT on October 05, 2012
Pretty picture. I'm missing how the discussion of offshore winds (caused by high pressure to the east of Cali) leads to explaining the Rex Block - how are the two related? Does the Rex Block affect west coast weather in any way? Or is it just an interesting eddy in the atmospheric river? I mean, that is interesting. Just wondering what it does.
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3. kabloie
5:45 PM GMT on October 03, 2012
Another MET term demystified. Thanks!
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2. Kingschapel1
4:16 PM GMT on October 03, 2012
Very interesting! Thank you.

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1. Patrap
10:31 PM GMT on October 01, 2012

Oop's, er, wrong Rex seem's.

: )
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Wunderground Meteorologist Shaun Tanner

About shauntanner

Shaun Tanner has been a meteorologist at Weather Underground since 2004.

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