Storm Patterns Conducive to Extreme Precipitation in the Willamette River Watershed of Oregon

Located in northwest Oregon, the Willamette River watershed is the largest watershed in the state covering more than 11,500 square miles. Like much of the region, the watershed gets the majority of its precipitation during the winter season. This study identifies and characterizes the storm patterns associated with extreme precipitation days over the watershed in order to advance our understanding of the meteorological processes that drive extreme precipitation and the impact they have. We implement the self-organizing map (SOM) approach, applied to daily integrated water vapor transport (IVT) on days with extreme precipitation as a way to highlight the range of storm types. The results of this study show that of all the storm types that lead to extreme precipitation days, the most frequent patterns were not often associated with an atmospheric river (AR) despite ARs being known as a primary driver of extreme precipitation over the region. However, the most intense storm types, with the strongest IVT and highest precipitation totals, were associated with ARs. Analysis is repeated for three sections of the watershed, upper, middle, and lower, revealing slightly different patterns of IVT for each section. Results from this study may provide additional insight for weather forecasting by highlighting the storm types and their physical characteristics that most commonly lead to extreme precipitation across the watershed. This will not only aid in the management of the Willamette River watershed but also contribute to a better understanding of the drivers of extreme precipitation along the coast of Oregon.