Wunderground.com-5/3/13- Dr. Jeff Masters
A highly unusual jet stream pattern is bringing a bizarre combination of heavy May snows, flooding, extreme fire danger, and well below average severe thunderstorm activity to the U.S. A strong "blocking" high pressure system has set up over Greenland, blocking the normal west-to-east progression of weather systems. A truly unusual situation has developed where the blocking high has forced a low pressure system near Greenland to move southwestwards to a point just off the New England coast. The blocking high has also forced an unusually sharp southwards dip in the jet stream over the Central U.S., where all-time May snowfall and cold temperature records are being set. This loop in the jet stream will get cut off from the main flow of the jet over the weekend, forming a "cutoff" low that will drift over the Southeast U.S., bringing cold, flooding rains of 2 - 4" over a wide swath of the Southeast. But over the Western U.S., an unusually sharp ridge of high pressure has set up, bringing record high temperatures, a strong Santa Ana wind event, and dangerous fire weather. The Santa Ana wind event has entered its second day over Southern California, where a clockwise flow of air has brought offshore winds, record high temperatures in the 90s, powerful winds gusting from 40 - 75 mph, and relative humidities less than 5%. Three destructive fires have erupted since Wednesday. The largest of these fires is called the Springs fire, and has burned 10,000 acres near Camarillo, California, about 50 miles west-northwest of Los Angeles. According to Cal Fire, the blaze was 10% contained at 6:30 am PDT May 3. The hourly observations from Thursday, May 2 at Camarillo show the onset of the Santa Ana winds impressively. The temperature jumped from 54° to 81° between 7 am and 8am, and the wind went from calm to sustained 35 mph, gusting to 43 mph, by 9 am. The temperature Thursday afternoon topped out at 98°--a new record high for the date--and the humidity dropped to a desiccating 4%. The Santa Ana wind event will not be quite as strong Friday, but will still be powerful enough to keep firefighters from gaining the upper hand on the blaze. The situation will improve dramatically over the weekend, when a low pressure system will bring in air 10 - 15° cooler, onshore winds, and rain.
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