May in the West
Temperatures were below to well below normal throughout the entire West except for a small portion of eastern New Mexico. Many locations were 5-7 degrees F below normal, and in Billings MT was the second coolest May on record. After a decade of consecutive warm Mays extending from about 1999, this was the second very cool May in a row, with 2011 not quite as cool as 2010 in most locations.
Precipitation was above normal in parts of California, eastern Oregon and Washington and well above normal in portions of Utah, Wyoming and eastern Montana. The extreme southwest was very dry. It was the wettest May on record in Glasgow, Billings and Miles City, all dating back over 100 years. Both Miles City and Billings had only one month in their history wetter than this May.
Significant snowpack still remains in most of the Intermountain West with some locations over 1000% of normal. Snowpack in some locations, including the Sierra Nevada, continued to accumulate through and beyond the last day of the month, and is now among the largest on record for this time of year in many basins, with the exception of parts of Arizona and New Mexico. Yet another cool, cloudy and wet spring month has delayed runoff in many western rivers, with high potential for historic flooding along mountains streams and rivers throughout the entire region. Many areas in the northern Rockies and northern Plains are already flooding with the heavy rains that occurred in May, with releases on the Missouri River the highest since construction of its large upper mainstem reservoirs. Simultaneously, the winter-long effects of La Nina have left most of Arizona (especially its southeast), and nearly all of New Mexico (especially southern and plains areas) in the deepest category of drought.
Significant Events for May 2011
May 14-16: Heavy rain and flooding in Eastern WA and OR. Some locations in the eastern Cascades received nearly 4 inches (101 mm) of rain in a 24-hour period on the 14-15th causing local creeks and rivers to rise above flood stage. Numerous rockslides were also reported, closing some highways for a short period.
May 25: Tornadoes in California. Several reports were received from the Willows and Chico areas north of Sacramento of tornadoes on the ground, one photographed by a news helicopter. Some damage reported.
Late May: Flooding in the Intermountain West. Local flooding occurred in Montana, Wyoming, Idaho and Utah from a combination of heavy rain and snowmelt in the mountains. In Montana, up to 50,000 cfs of water is expected to be released from Fort Peck dam by July 1st, far in excess of the previous record (since 1937) release of 35,400 back in 1975. Much of Roundup, MT, remains underwater (May 30th) after the Musselshell River went above flood stage. Some 50 structures were inundated and US 87 south of town was closed to Billings. Numerous other Montana highways are closed due to water over the road or damage to bridges. In Idaho, work continued on shoring up levees along the Snake River, while in northern Utah flood warnings were in effect as the month ended, on the South Fork of the Ogden River and lower Weber River due to heavy rain and rapid snow melt.
May 28-29: Late Season Snowfall: A cold, late season storm produced up to a foot of snow during the Memorial Day weekend in the Sierra Nevada of California causing a brief closure of Interstate 80 over Donner Summit. Nearly two feet (60 cm) of snow fell in the mountains of eastern Nevada. The Leavitt Lake Snotel site at 9617 ft (2931 m) elevation in the central Sierra Nevada had 86.5 inches (220 cm) of water content contained within the 179 inches (15 ft, 455 cm) of snow remaining on the ground, compared with a long term average of about 50 inches (127 cm) of water content.