Western Regional Climate Center

PROVIDING CLIMATE SERVICES SINCE 1986

May in the West

May 2015

The West saw a transition to a more active atmospheric pattern this month. Several slow-moving low-pressure systems passed through the region bringing widespread above normal precipitation to the Great Basin, Desert Southwest, and the Central and Southern Rocky Mountain states. Below normal temperatures accompanied the storminess.

Trajectories of low pressure systems across the southern Great Basin favored easterly winds directed at the eastern slope of the Sierra Nevada, facilitating well above normal precipitation in this area. Parts of eight western states experienced their record wettest May. Bodie, California, noted its wettest May on record with 6.06 in (154 mm), 866% of normal, surpassing by 79 % the previous record of 3.38 in (86 mm) in 1995. Bodie also recorded its wettest May day on record, 1.67 in (42 mm) on the 8th; records began in 1895. Over two feet of late season snow fell in the high Sierra Nevada during the month, providing a small addition to the nearly non-existent snowpack. Slow-moving systems brought precipitation and thunderstorms to other parts of the Great Basin and Southwest as well. Tonopah, Nevada, and Rome, Oregon, each saw their wettest May on record at 2.44 in (62 mm) and 3.48 in (88 mm), respectively. Records began in 1902 at Tonopah and 1949 in Rome. In the eastern Great Basin, Cedar City, Utah, recorded its 2nd wettest May in a 68-year record at 3.12 in (79 mm), 405% of normal. In the Southwest, Phoenix, Arizona, observed its wettest May in an 83-year record at 1.17 in (30 mm). Precipitation in these areas increased soil moisture, reduced irrigation demands, and supported vegetation growth. However, it did not reduce the persistent hydrologic drought in California, Nevada, and southern Oregon. Above normal rainfall was observed in central Washington as well, where Yakima logged 1.8 in (46 mm), 310% of normal, the 4th wettest May since records began in 1946. In the Rocky Mountain states, Clayton, New Mexico received 6.81 in (173 mm), 347% of normal and the 2nd wettest May since records began in 1896. In Colorado, Colorado Springs logged 8.13 in (207 mm) precipitation, 400% of normal and the wettest May in a 68-year record. This precipitation helped to ameliorate drought conditions across a large area of the Colorado-Utah-Wyoming borders, and along the eastern part of the Colorado-New Mexico border.

Precipitation was below normal along the Pacific Coast west of the Sierra and Cascades from Los Angeles to the Canadian border and along the northern tier of the West. In northern California, Arcata received 0.1 in (3 mm), 4% of normal rainfall, tied with 2008 for driest May since records began in 1945. Further north, Quillayute, Washington, observed 0.67 in (17 mm) precipitation, only 13% of normal, the driest May in a 50-year record.

The Pacific Northwest saw above normal temperatures this month. In central Washington, the average May temperature at Yakima was was 64.9 F (18.3 C), 7.8 F (4.3 C) above normal and the 2nd warmest May in a record that began in 1946. In contrast, the rest of the West was generally 1-4 F (0.5-2 C) cooler than normal, with some departures reaching -4 to -6 F (2-3 C) in the Southwest. Douglas, Arizona, reported an average 64.9 F (18.3 C) for the month, 5 F (2.7 C) below normal and the second coldest May in a 68-year record. In Colorado, Boulder observed its 5th coolest May in a 123-year record at an average 52.4 F (11.3 C).

May marked the end of Alaska’s main snow season and end of the least snowy season on record for Anchorage, where only 25.1 in (63 cm) of snow fell September through May. The previous lowest snowfall was 30.4 in (77 cm) in 1957-58 and normal is 74.5 in (189 cm). Several locations throughout Alaska observed their warmest May on record this month including Juneau in the southeast (54.1 F/12.3 C) and Kotzebue (40.3 F/4.6 C) in the northwestern part of the state. Records for Juneau began in 1936 and for Kotzebue in 1897. Further south, as Hawaii moved into its dry season this month, precipitation was variable across the state. Kahului received 2.23 in (57 mm), 300% of normal and the 8th wettest May since records began in 1905. In contrast, Honolulu recorded only 0.2 in (5 mm), 32% of normal.

Significant Events for May 2015

May (all month): Severe drought persists in California/Nevada/Oregon: Despite above normal May precipitation, drought conditions remain in these states. Impacts include low spring runoff threatening fisheries and water resources, curtailment of water deliveries for agriculture, mandatory water restrictions in California, and ecological impacts on forests and wildlife.

May 15: Washington Governor declares statewide drought emergency: Due to low snowpack, wildfire hazards, crop losses, and threats to wildlife, Washington Governor Jay Inslee declared a statewide drought emergency.

May 30-31: Hurricane Andres forms in eastern Pacific: Andres was the 5th major hurricane to form in the Pacific Basin in May since reliable records began in 1970.

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