Monday, February 22, 2010

Meteorological constipation...

I think we may be hearing a lot about "blocked" weather systems in the future.
Atmospheric blocking occurs between 20-40 times each year and usually lasts between 8-11 days, Lupo said. Although they are one of the rarest weather events, blocking can trigger dangerous conditions, such as a 2003 European heat wave that caused 40,000 deaths. Blocking usually results when a powerful, high-pressure area gets stuck in one place and, because they cover a large area, fronts behind them are blocked. Lupo believes that heat sources, such as radiation, condensation, and surface heating and cooling, have a significant role in a blocking's onset and duration. Therefore, planetary warming could increase the frequency and impact of atmospheric blocking.
"It is anticipated that in a warmer world, blocking events will be more numerous, weaker and longer-lived," Lupo said. "This could result in an environment with more storms. We also anticipate the variability of weather patterns will change dramatically over some parts of the world, such as North America, Europe and Asia, but not in others." [More]

The first time I heard this concept (I think these are the same phenomena) was from USFR meteorologist Mike Hoffman after the incredible rains in Georgia last year.  He described it as storm systems that get cut off from the jetstream and simply sit over the same spot for days.

Cut-off low pressure systems are counterclockwise swirls in the atmosphere that become entirely detached from the jet stream, which is the fast-moving river of air high in the atmosphere that steers weather systems.
If a strong pocket of winds is moving through the jet stream, that energy can sometimes swing southward and then back around so rapidly that it makes a complete circle, cutting the circulation off from the jet stream.
It's a little like a roller coaster making a huge vertical loop, the kind that you're upside down for a second or two. The roller coaster has to have enough speed to move upward and turn over in that loop.
Another factor in a cut-off low is that the jet stream itself may be displaced northward from the circulation, leaving the circulation to spin by itself without any nudging from the strong river of winds aloft.
As a result, cut-off lows are not pushed along very rapidly as most weather systems are this time of year. Cut-off lows meander erratically, producing several days of similar weather for the locations they affect. [More]

It makes you wonder if the old "if you don't like the weather, wait five minutes" gag will die out in the Midwest.

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