Although runoff and streamflow have generally decreased in the Northwest over the past century (Luce and Holden, 2009), instances of flooding – particularly in rain-dominated and ‘transition’ basins – have increased over the same period (Hamlet and Lettenmaier, 2007), and are often driven by exceptional precipitation events. Impacts are basin specific and dependent on antecedent conditions, though higher intensity storms generally result in increased flooding and runoff risk, and such events can disrupt systems ranging from ecological (e.g. Knapp et al., 2008), to agricultural (e.g. Rosenzweig et al., 2000), to urban (e.g. Rosenberg et al., 2010). Further, the synchronicity of precipitation across a basin or region has the potential to enhance the impacts of an extreme event.
III. Background and Previous Work
A. Hydroclimate of the Northwest
Synoptic scale storm tracks from the Pacific provide the majority of the region’s precipitation between November and March, while relatively dry conditions persist between late spring and early fall. However, significant spatial contrasts exist, largely as a function of the Cascade mountain range (Figure 3 – Map of MAP for the PNW from PRISM and a secondary map that shows the percent of MAP falling in winter – showing east/west contrast).
i. The Pacific Northwest
The region to the west of the Cascades, as well as the western slopes of the range, experience substantial amounts of rainfall, with the majority falling in the late fall and winter months. Valleys in the Pacific Northwest (PNW) average 75cm rainfall annually. The western slopes of the Cascades receive annual precipitation averages close to 200cm (often falling as snow at higher elevations in winter), while the Coast Range and Olympic mountains see mean annual precipitation values of roughly 300cm and 500cm, respectively (CSES, University of Washington). This variability across the sub-region exists as a result of orographic effects, as well as latitudinal effects. The El Nino Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO), which modify the strength and latitude of the jet stream, can also influence precipitation variability across the PNW and that variability may be further amplified during periods of ENSO-PDO synchronicity (e.g. Gershunov and Barnett, 1998).
ii. The Inland Northwest
The region to the east of the Cascades is typified by generally sunnier and drier conditions as compared to the PNW. Annual average precipitation across the Inland Northwest (INW) is less than 50cm, with a greater percentage of that precipitation coming during the late spring and early summer months. Lower average annual precipitation amounts exist even within the higher elevation mountainous areas of northern Idaho, northeastern Oregon, and western Montana, with annual averages of only 150-250cm as compared to the 200-500cm MAP in the mountain ranges west of the Cascade crest (CESE, UW). The sub-regional variability of MAP within the INW...