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United States Department of Agriculture

Agricultural Research Service

Title: The Northwest Flood of February 1996 in Southwestern Idaho

Authors
item Slaughter, Charles
item Huber, A - RETIRED
item Johnson, Greg - NRCS
item Cooley, Keith
item Hanson, Clayton

Submitted to: American Institute of Hydrology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: October 6, 1997
Publication Date: N/A

Interpretive Summary: Long-term research can aid understanding extreme hydrologic events. Reynolds Creek Experimental Watershed (RCEW), southwest Idaho, has over 35 years of research and hydro- meteorological record. Major winter floods of the interior Pacific Northwest, including the December 1964 storm which produced the flood of record for RCEW, typically result from strong jet streams transporting warm, moist air from the Pacific to the interior contributing high rainfall rates on frozen soils or snow. The February 1996 Pacific Northwest flood, termed "as big as the flood of 1964," narrowly missed southwestern Idaho. Mountains of the interior Pacific Northwest were "primed" with average or higher snowpacks and extensive frozen soils. The Blue Mountains and western and northern Idaho experienced major flooding, while RCEW had high within-bank flows. Rapid warming produced virtually complete snow melt up to 1600 m elevation in RCEW, causing a sharp flow increase peaking on February 8; the missing ingredient for RCEW was precipitation. A minor southeast shift of the storm track could have contributed over 200 mm of warm precipitation to the snowpack, which would have resulted in record high streamflows in southwestern Idaho. Historically about half of all major hydrologic events in RCEW have occurred in January and February, and have been either rain-on-snow or accelerated snowmelt phenomena. Understanding this risk is useful in emergency planning.

Technical Abstract: Long-term research can aid understanding extreme hydrologic events. Reynolds Creek Experimental Watershed (RCEW), southwest Idaho, has over 35 years of research and hydro- meteorological record. Major winter floods of the interior Pacific Northwest, including the December 1964 storm which produced the flood of record for RCEW, typically result from strong jet streams transporting warm, moist air from the Pacific to the interior contributing high rainfall rates on frozen soils or snow. The February 1996 Pacific Northwest flood, termed "as big as the flood of 1964," narrowly missed southwestern Idaho. Mountains of the interior Pacific Northwest were "primed" with average or higher snowpacks and extensive frozen soils. The Blue Mountains and western and northern Idaho experienced major flooding, while RCEW had high within-bank flows. Rapid warming produced virtually complete snow melt up to 1600 m elevation in RCEW, causing a sharp flow increase peaking on February 8; the missing ingredient for RCEW was precipitation. A minor southeast shift of the storm track could have contributed over 200 mm of warm precipitation to the snowpack, which would have resulted in record high streamflows in southwestern Idaho. Historically about half of all major hydrologic events in RCEW have occurred in January and February, and have been either rain-on-snow or accelerated snowmelt phenomena. Understanding this risk is useful in emergency planning.

Last Modified: 10/21/2014
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