Fires, Floods, and Heavy Snow: an Extreme May Weather Situation

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.


Figure 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)


Figure 2. A bad day to be on the Channel Islands. Offshore winds blow smoke from the Springs fire, burning about 50 miles west of Los Angeles on May 2, 2013, over the Channel Islands, where the smoke mixed with brighter marine stratocumulus clouds. Image credit: NASA.

A early start to the California wildfire season
According to the National Interagency Fire Center, as of April 26, the U.S. had the fewest fires and the lowest acreage burned by wildfires in 2013 for any year-to-date period over the past ten years. But in just the past two days, the acreage burned for 2013 has jumped by 20%. Severe drought conditions across much of the Western U.S. are likely to help fuel an early and severe wildfire season during 2013, they said in their latest monthly outlook, issued May 1. Fire season is expected to be in full swing during May--a full month earlier than usual--in Southern California, California's Sacramento Valley and adjacent lower foothills, and South Central Oregon. In California, “precipitation pretty much shut off at the beginning of the year,” NIFC wildfire analyst Jeremy Sullens said during a conference call with reporters. “Since they’re not expecting a lot more precipitation for the remainder of the summer, conditions are going to worsen as we go into the hotter part of the year.” Significant fire potential will increase to above normal during May in southeastern Arizona, much of western New Mexico, and northern Virginia. Late-season snows across the Rocky Mountains in Wyoming and Colorado have dumped enough moisture to delay wildfire season until its usual June start. However, these storms have largely missed southern Colorado, where the wildfire risks remain elevated.


Figure 3. Predicted May fire activity from the National Interagency Fire Center.

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.


Figure 4. The water content of the snowpack in the Southern Sierra Mountains of California, from San Joaquin through Kern and Owens, was 9% of average for the date on May 2, 2013 (and 7% of the average for April 1.) The snowpack is usually not this thin until early July. Statewide, the snowpack was 17% of average for the date. Image credit: California Department of Water Resources.

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".)

Here are the latest peak snowfall totals by state as of 11am EDT May 3:

Buckhorn Mtn., CO: 28.2"
Near Warren AFB, WY: 22.3"
Blooming Prairie, MN: 18"
Rice Lake, WI: 17"
Bessemer, MI: 13"
Chariton, IA: 12"
Dalton, NE: 8.5"
Beresford, SD: 6"
Warrensburg, MO: 6"
Traer, KS: 5.3"
Maysville, AR: 3"
West Siloam Springs, OK: 1.5"

And here is a partial list of cities that have set all-time low temperature records for the month of May:

Shreveport, LA: 41° on May 3 (previous May record: 42°) Records for the city go back to 1874.
Abilene, TX: 33° on May 3 (tied with 33° on May 4, 1907.)
Denver, CO: 19° on May 2 (tied with 19° on May 3, 1872)
Fort Benton, MT: 14° on May 2 (tied with 14° on May 10, 1946)
Denton, MT: 9° on May 2 (previous May record: 13° set on May 1, 2005.) Denton's records began in 1948.

Wunderground's weather historian Christopher C. Burt plans to fully document the records from this week's epic storm in his post coming on Saturday.


Figure 5. The jet stream pattern for Winter Storm Achilles showed a very high-amplitude trough over the Midwest U.S., which allowed record-breaking cold air to flow southwards out of Canada.

Nor'easter for Northeast Florida
Adding to Mother Nature's unusually varied bag of tricks for the U.S. today is a very wet and windy Nor'easter centered just east of Melbourne, which is lashing the east coast of Florida with heavy rains and rough surf. The storm brought 8.29" of rain to St. Augustine, Florida in the 24 hours ending at 8 am EDT May 3, and a daily record deluge of 1.7" of rain to Orlando, 3.1" to Fort Lauderdale, and 4.14" to Key West on May 2. The storm also spawned an EF-0 tornado with 70 mph winds that touched down in Boca Raton on May 2, causing minor damage. Radar loops out of Melbourne, Florida show disorganized heavy rain bands with a bit of rotation just offshore, and satellite loops show disorganized heavy thunderstorms extend from Cape Canaveral to Georgia. Development into a subtropical depression is unlikely due to very high wind shear of 40 knots, but this system will hang around through Saturday, generating heavy rains of up to three inches along the northeast coast of Florida. Gale warnings and flood watches are posted along much of the East Central and Northeast coasts of Florida.

Jeff Masters