May in the West
May featured several closed low-pressure systems moving slowly across the West, similar to the pattern observed in April. The path and orientation of these lows favored above normal precipitation along, variously, the western edge of the Great Basin, Desert Southwest, central Rockies, and northern Montana. Above normal temperatures were observed in the Pacific Northwest, while below normal temperatures occurred throughout the Southwest in association with the low pressure systems and increased cloudiness.
The procession of closed low-pressure systems kept precipitation persistent through the month, with many western locations affected by two or more of the disturbances. Some of the highest May totals accumulated between the 5th and 10th, from a moisture-laden system plodding eastward across the Southwest. San Diego, California, is typically quite dry in May but recorded 0.44 in (11 mm), 366% of normal and the 12th wettest in a record that began in 1939. The Reno, Nevada, total of 1.16 in (29 mm), 237% of normal and 14th wettest in an 80-year record, fell over just 6 days. Elsewhere in the Southwest, Flagstaff, Arizona, recorded its 11th wettest May at 1.97 in (50 mm), 249% of normal. In western New Mexico, Gallup observed 1.27 in (32 mm), 178% of normal. This was Gallup’s 5th wettest May since records began in 1973. Several locations in the northern Intermountain West saw a wet start to their rainy season. Kalispell, Montana, logged 3.12 in (79 mm) of precipitation, 158% of normal and the 13th wettest May since records began in 1896. In central Wyoming, Lander recorded 5.03 in (128 mm), 229% of normal and 9th wettest since records began in 1946. This total includes Lander’s 2nd wettest day on record; an impressive 3.35 in (85 mm) fell on May 7th, just shy of the all-time daily record of 3.48 in (88 mm) set on June 11, 1947. Some of this month’s storms added to mountain snowpack, boosting values to well above normal in the central Great Basin and central Rockies. Improvement of drought conditions was seen in many areas of the Great Basin, northern California, western Wyoming, and portions of northern New Mexico. In contrast, drier than normal conditions dominated coastal regions in May. Seattle, Washington, recorded 0.94 in (24 mm), 48% of normal. North Bend, Oregon, observed 0.61 in (15 mm), 18% of normal and the 6th driest May since records began in 1902. San Francisco logged 0.15 in (4 mm) for the month, 21% of normal.
The first 1-2 weeks of the month saw strong high pressure over the Pacific Northwest, elevating temperatures more than 10 F (5.6 C) above normal and resulting in an overall warmer than normal May for the region. Seattle, Washington, recorded an average 59.8 F (15.4 C), 3.8 F (2.1 C) above normal and the 4th warmest May on record. In central Washington, Yakima also had its 4th warmest May on record at 63.1 F (17.2 C), 6 F (3.3 C) above normal. Airport records for Seattle began in 1945 and Yakima in 1946. Further south, temperatures at Portland, Oregon, averaged to 62.6 F (17 C) for the month, 3.9 F (2.2 C) above normal, the 3rd warmest May in a 79-year record.
Low-pressure systems brought cool air masses and cloudiness to the Southwest, keeping monthly temperatures 1-4 F (0.5-2 C) below normal. In southwestern Nevada, Pahrump recorded an average 67.2 F (19.5 C), 2.8 F (1.5 C) below normal, the 20th coolest May since records began in 1914. Further south, Yuma, Arizona, recorded an average 78.1 F (25.6 C) for the month, 3.2 F (1.8 C) below normal. In northeast New Mexico, temperatures at Clayton averaged to 59.4 F (15.2 C), 2.1 F (1.2 C) below normal.
Many locations in Alaska observed their warmest May on record, some on the heels of their warmest April. Among these locations were Kotzebue, on the northwest coast, which recorded an average 40.6 F (4.8 C), 8.7 F (4.8 C) above normal, and King Salmon, in the southwest, which averaged 49.0 F (9.4 C), 4.8 F (2.7 C) above normal. Records for Kotzebue began in 1897 and King Salmon in 1917. Further south, several locations on Oahu, Maui, and Big Island saw above normal precipitation. Kahului, Maui, observed its 5th wettest May in a 112-year record at 3.05 in (77 mm), 401% of normal. Honolulu, Oahu, recorded 3.17 in (81 mm) 511% of normal and the 8th wettest May since records began in 1940. Temperatures were above normal throughout much of the state; Hilo, on the Big Island, recorded an average temperature of 76.2 (24.6 F), 2.5 F (1.4 C) above normal, Hilo’s 3rd warmest May since records began in 1949.
Significant Events for May 2016
May 7: Flash flooding in southern California: Heavy rains caused minor flooding and mudslides in areas of inland southern California, disrupting traffic. The Grapevine portion of Interstate 5 was temporarily closed due to flash flooding.
May 7-9: Flooding in Wyoming: Heavy rainfall caused flooding in several areas of Wyoming, damaging homes and roads. In the western part of the state, the Little Wind River hit its third highest crest of record. In southeast Wyoming, the Laramie River rose to flood stage, causing minor damage.
Mid-May: Fires in Arizona: Lightning ignited several fires in Arizona, including the Juniper Fire (~12,000 acres/4856 hectares) east of Phoenix and the Mormon Fire (~6,000 acres/2428 hectares) in northern Arizona.