Summertime Rainfall Events in Eastern Washington and Oregon

Andrew M. Chiodi Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, and NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

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Nicholas A. Bond Joint Institute for the Study of the Atmosphere and Ocean, University of Washington, and NOAA/Pacific Marine Environmental Laboratory, Seattle, Washington

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Narasimhan K. Larkin Pacific Northwest Research Station, U.S. Forest Service, Portland, Oregon

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R. James Barbour Pacific Northwest Research Station, U.S. Forest Service, Portland, Oregon

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Abstract

The temporal and spatial characteristics of summertime rainfall events in the Pacific Northwest are examined in relation to the prevailing regional 500-hPa geopotential height conditions, with focus on the forested slopes of eastern Washington and northeastern Oregon, where the absence/occurrence of events largely determines the start and end of the wildland fire season. The Daily U.S. Unified Precipitation dataset is used for specifying rainfall events (period 1949–2008). Events are defined as one or more consecutive days of rainfall exceeding 0.25 in. (0.65 mm), and occur on average two to three times per summer (July–September) in the focus region, east of the Cascade Mountain crest, with a minimum in frequency in late July. A relatively high percentage of the events in the northern portion of the domain of interest, and over the higher terrain, is associated with anomalous midtropospheric southwesterly flow; a high percentage of the events in the southern and lower elevation portions of the domain is associated with southeasterly flow anomalies. Southeasterly flow events are much more likely to be accompanied by lightning and a more localized rainfall distribution than southwesterly events. Southwesterly events mainly account for the late-July frequency minimum and produce more widespread/heavier precipitation on average. The forests of eastern Washington and Oregon receive a mix of southeasterly and southwesterly events. Results suggest that identifying flow types by (skillful) extended-range 500-hPa forecasts may provide a useful basis for predicting the associated aspects of the rainfall event distribution.

Denotes Open Access content.

Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/WAF-D-16-0024.s1.

Joint Institute for the Study of the Atmosphere and Ocean Contribution Number 1888 and Pacific Marine Environmental Laboratory Contribution Number 3755.

Corresponding author address: Andrew M. Chiodi, JISAO/University of Washington, Box 354925, Seattle, WA 98195-4925. E-mail: andy.chiodi@noaa.gov

Abstract

The temporal and spatial characteristics of summertime rainfall events in the Pacific Northwest are examined in relation to the prevailing regional 500-hPa geopotential height conditions, with focus on the forested slopes of eastern Washington and northeastern Oregon, where the absence/occurrence of events largely determines the start and end of the wildland fire season. The Daily U.S. Unified Precipitation dataset is used for specifying rainfall events (period 1949–2008). Events are defined as one or more consecutive days of rainfall exceeding 0.25 in. (0.65 mm), and occur on average two to three times per summer (July–September) in the focus region, east of the Cascade Mountain crest, with a minimum in frequency in late July. A relatively high percentage of the events in the northern portion of the domain of interest, and over the higher terrain, is associated with anomalous midtropospheric southwesterly flow; a high percentage of the events in the southern and lower elevation portions of the domain is associated with southeasterly flow anomalies. Southeasterly flow events are much more likely to be accompanied by lightning and a more localized rainfall distribution than southwesterly events. Southwesterly events mainly account for the late-July frequency minimum and produce more widespread/heavier precipitation on average. The forests of eastern Washington and Oregon receive a mix of southeasterly and southwesterly events. Results suggest that identifying flow types by (skillful) extended-range 500-hPa forecasts may provide a useful basis for predicting the associated aspects of the rainfall event distribution.

Denotes Open Access content.

Supplemental information related to this paper is available at the Journals Online website: http://dx.doi.org/10.1175/WAF-D-16-0024.s1.

Joint Institute for the Study of the Atmosphere and Ocean Contribution Number 1888 and Pacific Marine Environmental Laboratory Contribution Number 3755.

Corresponding author address: Andrew M. Chiodi, JISAO/University of Washington, Box 354925, Seattle, WA 98195-4925. E-mail: andy.chiodi@noaa.gov

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