The 5-9 February 1996 flooding event over the Pacific Northwest: Sensitivity studies and evaluation of the MM5 precipitation forecasts

This paper describes the flooding event of 5-9 February 1996 in which a series of landfalling Pacific storms brought 30-70 cm of rain to many mountain sites over southwest Washington and northwest Oregon. This event was simulated at 36-, 12-, 4-, and 1.33-km horizontal resolution using the Pennsylvania State University-National Center for Atmospheric Research mesoscale model (MM5). The model precipitation was verified with over 300 rain gauges in western Washington and Oregon as well as WSR-88D radar data from Portland, Oregon. There was a significant improvement in the precipitation forecast skill as the grid spacing was decreased from 36 to 4 km; however, the 12- and 4-km resolutions had excessive precipitation shadowing in the lee of barriers. Although increasing resolution from 4 to 1.33 km did not produce a significant improvement in precipitation skill for the entire domain, the 1.33-km domain had more precipitation in the immediate lee of the Cascades, and thus verified better in those regions. Additional simulations explored the effects of changing vertical resolution. Enhancing the number of levels from 29 to 38 increased the model precipitation over the windward slopes of the Cascades by 10%-30%; further increasing the number of levels to 57 decreased windward precipitation to the 29-level amounts. The leeside precipitation varied by 20%-80% for the different vertical resolutions as a result of variations in mountain wave structure over the Cascades. Five different MM5 microphysical schemes were compared for a 24-h period during the February 1996 flooding event. The warm rain scheme dumped too much precipitation along the windward slopes, underlining the importance of ice microphysics during the cool season. The most sophisticated scheme (i.e., graupel scheme) also did not provide the best forecast. An additional microphysical sensitivity run, in which the snow fall speed was reduced by approximately 20%, improved model skill by advecting more precipitation advecting to the lee of the Cascades. Overall, these microphysical results suggest that further improvements to microphysical schemes are needed in order to more accurately predict precipitation.