Heavy Precipitation in the Oregon Coast Range: A Numerical Modeling Sensitivity Study

An atmospheric river event aided by the complex orography of the Coast Range of northwest Oregon produced particularly heavy rainfall over the Wilson River Basin on 17 November 2015. The Wilson River Basin is a relatively small basin covering an area slightly over 500 km2. Topography of the basin ranges from peaks around 850 meters MSL to sea level at the river’s mouth near Tillamook, Oregon. The basin precipitation to peak flow lag-time at Tillamook is approximately six hours.

While operational models predicted moderate-to-heavy rainfall associated with the atmospheric river, QPFs at Lees Camp (located around 450 meters MSL in the northern Oregon Coast Range) were well below the observed 00Z 18 November accumulated 6-hour and 24-hour totals of 81 mm and 180 mm, respectively.  The official Wilson River forecasts produced by the National Weather Service and issued at 12 UTC 17 November predicted steady volumes over the next 24 hours. In reality though, a major flood occurred at 05 UTC 18 November, undoubtedly aided by the excessive rainfall during the 20-02 UTC period (102 mm at Lees Camp) preceding the 05 UTC peak flow.

In addition to the moisture and dynamics associated with the strong atmospheric river, the orographic enhancement of precipitation (below the resolvable scale of operational numerical weather prediction) contributed to the excessive Coast Range precipitation. This study will use the Weather Research and Forecasting (WRF) Model at sufficiently high resolution to better resolve the Coast Range orography (~ 1 km grid spacing).  The experiment consists of very short term (3-6 hours) WRF sensitivity runs, initialized using only the observed 00 UTC 18 November raob at Salem, Oregon, and perturbations thereof. These simulations are designed to assess the sensitivity of basin rainfall to prevailing wind direction and speed.

The results presented will show the sensitivity of precipitation rates to variations in wind direction and speed over the Wilson River Basin. Application of the results will aid predictive decision support services during atmospheric rivers, by establishing sensitivity and possible “sweet spots” for excessive rain in the Wilson River Basin based on predicted wind speed and direction. Although initially focusing on the Wilson River Basin, if successful, the methodology could be extended to other critical basins along the Coast Range.