Posted by: Dr. Jeff Masters, 4:11 PM GMT on July 03, 2013
The jet steam is exhibiting unusual behavior over the U.S., a pattern we've seen become increasingly common in summertime over the past decade. There's a sharp trough of low pressure over the Central U.S., and equally sharp ridges of high pressure over the Western U.S. and East Coast. Since the jet acts as the boundary between cool, Canadian air to the north and warm, subtropical air to the south, this means that hot extremes are penetrating unusually far to the north under the ridges of high pressure, and cold extremes are extending unusually far to the south under the trough of low pressure. The ridge over the Western U.S., though slowly weakening, is still exceptionally intense. This ridge, which on Sunday brought Earth its highest temperatures in a century (129°F or 54°C in Death Valley, California), was responsible for more record-breaking heat on Tuesday. July 2. Most notably, Redding, California hit 116°, just 2° short of their all-time record. Death Valley had a low of 104°, the second hottest night on record since 1920 (the hottest was just last summer!) Numerous daily high temperature records were set in Arizona, California, Nevada, Utah, Montana, Oregon, and Washington. It was the opposite story in the Central U.S., where the southwards-plunging jet stream allowed record cold air to invade Texas. Waco, Texas, hit 58°F this morning (July 3), the coldest temperature ever measured in July in the city. Numerous airports in Texas, Nebraska, Arkansas, Louisiana, Kansas, and Missouri set new daily record low temperatures this morning. And over the Eastern U.S., the northward-pointing branch of the jet stream is creating a potentially dangerous flooding situation, by pulling a moisture-laden flow of tropical air from the Gulf of Mexico over the Florida Panhandle north-northeastward into the Appalachians. Up to five inches of rain is expected over this region over the next few days, and wunderground's severe weather map is showing flash flood warnings for locations in Florida, Georgia, South Carolina, and North Carolina.
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This week's extreme jet stream pattern is the third time in the past five weeks that we've seen a highly amplified ridge-trough pattern that has led to extreme weather.
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As I discussed in a March 2013 post, "Are atmospheric flow patterns favorable for summer extreme weather increasing?", research published this year by scientists at the Potsdam Institute for Climate Impact Research (PIK) in German found that extreme summertime jet stream patterns had become twice as common during 2001 - 2012 compared to the previous 22 years.
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When the jet stream goes into one of these extreme configurations, it freezes in its tracks for weeks, resulting in an extended period of extreme heat or flooding, depending upon where the high-amplitude part of the jet stream lies. The scientists found that because human-caused global warming is causing the Arctic to heat up more than twice as rapidly as the rest of the planet, a unique resonance pattern capable of causing this behavior was resulting. According to an email I received from German climate scientist Stefan Rahmstorf, one of the co-authors of the study, unusually extreme jet stream amplitudes likely played a role in the May - June Central European flooding event.
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