Orographic effects of the 1997 Pineapple Express storms in Northern California

The December 1996 - January 1997 ‘Pineapple Express’ storms in Northern California caused hundreds of landslides and produced severe flooding throughout mountainous regions of Northern California. The spatial variability of intense rainfall may exert significant control over the location and density of landslides and, hence, over the erosion of mountainous regions. We have performed a series of high-resolution MM5 simulations (4 km and 1.3 km resolution) of this event to gain insight into the physical processes responsible for the spatial distribution of intense precipitation during the December 26, 1996-January 3, 1997 period. The model-derived precipitation field closely matches rain gauge observations and successfully reproduces the observed loci of intense precipitation in the northern Sierra Nevada and in coastal regions of near the California-Oregon border. Preliminary results suggest that orography played a central role in the localization of intense rainfall through blocking of low-level winds by the Sierra Nevada that directed moist air northwest along the mountain front thereby focusing precipitation in the northern Sierras.