Dr. Jeff Masters / Wunderground: 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.

Water a precious commodity in California in 2013

As of May 2, California’s Sierra Mountain snowpack was just 17% of average for the date, with a water content more typical of what is seen in early July. That’s bad news for a state that relies on a steady stream of meltwater to keep reservoirs filled during the summer. The poor 2012 – 2013 snow season comes on the heels of a poor 2011 – 2012 snow season, as well. But thanks to good water years leading up to 2011 – 2012, two key reservoirs are above 80% capacity: Lake Oroville, the main reservoir for the State Water Project (86%), and Lake Shasta, the main reservoir for farmers in California’s Central Valley (83%.) So, California will likely weather the dry conditions of the summer of 2013–but the snows of the winter of 2013 – 2014 had better be plentiful, or the state could be looking at a serious water shortage in 2014.

Amazing May snowstorm smashes more records

A rare and historic May snowstorm continues to set all-time snow and cold records for the month of May. Winter Storm Achilles brought Arkansas its first May snowfall in recorded history this morning, and four other states have set unofficial new May snowfall records for a 2-day storm: 18″ in Blooming Prairie Minnesota (previous record of 15″); 17″ in Rice Lake, Wisconsin (previous record, 15.4″); 12″ in Chariton, Iowa (previous record: 8″), and 6″ in Warrensburg, MO (previous record: 4.5″.)

Image 1: Residents look on as a back fire set by firefighters consumes the hillside behind their homes as a wildfire burns on May 2, 2013 in Newbury Park, Calif. (Photo by Kevork Djansezian/Getty Images)