The Meteorology of Large Wildfires over Western Oregon and Washington

Most wildfires over the western U.S. occur in climatologically dry regions, such as over the Desert Southwest or over the interior of the Pacific Northwest east of the Cascade crest. However, infrequent, sometimes catastrophic, fires have burned west of the Cascade Crest in Washington and Oregon. Such fires are infrequent because the coastal regions of Washington and Oregon experience moderate to heavy precipitation during the cool season, and cool, moist marine air spreads inland year-round over the coastal Pacific Northwest west of the Cascade Range. When they do occur, wildfires west of the Cascade Crest can be large and devastating, driven by the huge fuel reservoirs in the dense forests of western Oregon and Washington. Westside fires generally occur during August and September at the end of the dry summers of the region and can disrupt the lives of millions of people in the populated corridor stretching from Salem and Portland to Tacoma, Seattle and Bellingham.

This talk will address fundamental questions regarding the westside fires of the region:

• What are the synoptic and mesoscale conditions associated with the large westside fires in the Pacific Northwest?
• Are strong downslope winds, and particularly strong easterly flow, required for major fires within the normally moist forests of the western slopes of the Cascades and coastal mountains?
• What is the seasonality of such fires?
• Are anomalously dry, warm summers a precondition for such westside wildfires? Are there other climatological precursors important, such as conditions during the previous winter or the prior several years?

A major innovation of the research to be presented will be the simulation of all major westside wildfires during the past 120 years, running the WRF model driven by the 20^th Century Reanalysis. Initial results of this work suggest that all major westside wildfires are associated with strong easterly or northeasterly flow.