2.2 Impactful Rains in California and Snow, Ice, and Cold in Oregon in Early 2017

Monday, 8 January 2018: 10:45 AM
Ballroom D (ACC) (Austin, Texas)
Lance F. Bosart, SUNY, Albany, NY; and T. Burg and A. M. Bentley

Record rainfall totals were observed in many parts of California’s Sierra Nevada Mountains in early 2017. After years of drought, precipitation accumulated to a record ~119 cm over the northern Sierras between 1 January and 1 March 2017—the most ever recorded in the region during this two-month period. Extensive river and stream flooding occurred, especially along the Feather River where concerns arose about the possible failure of the Oroville Dam and spillway. The California Department of Water Resources began releasing water from Lake Oroville on 13 January 2017 after record rainfall was observed over the Feather River watershed. After the water level on Lake Oroville reached capacity (275 m) on 11 February, water began to flow over the Oroville Dam emergency spillway for the first time in its nearly 50-year history, causing extensive erosion and raising concerns about about a possible spillway failure.

Farther north in Oregon, the problem was snow, ice, and cold. Portland averaged 0.8°C in January, their seventh coldest January on record. On 10–11 January, downtown Portland received ~30 cm of snow, the city’s biggest snowstorm in two decades, resulting in a “state of emergency” declaration by Mayor Ted Wheeler. Oregon Governor Kate Brown also declared a “state of emergency” as snow and ice accumulations disrupted civic life east and west of the Cascades. The snow and cold lingered too, consistent with NWS-Portland reporting that the first 17 days of January 2017 were the coldest the city had experienced since 1979.

We will show that the record January rains in California and the extended period of snow, ice, and cold in Oregon occurred in conjunction with a high-latitude omega block over the eastern North Pacific and low-latitude anticyclonic wave breaking (AWB) along the subtropical jet stream (STJ) over the central and eastern North Pacific. This high-latitude omega block was maintained by diabatically enhanced upstream ridge building associated with multiple cyclogenesis events over the western North Pacific. Repeated cold-air outbreaks on the eastern side of the high-latitude omega block resulted in persistent and widespread cold across over southwestern Canada and the Pacific Northwest. Low-latitude AWB along the STJ enabled moisture to be drawn northeastward from the tropical and subtropical eastern North Pacific via atmospheric rivers toward California.

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