Extreme precipitation events in the western United States related to tropical forcing

Three-day accumulations of precipitation for 2.5 degrees long x 2.0 degrees lat areas along the west coast of the United States are used to rank preci pitation events. Extreme precipitation events (those above the 90th percent ile) occur at all phases of the El Nino-Southern Oscillation (ENSO) cycle, but the largest fraction of these events (for the West Coast as a whole) oc cur during neutral winters just prior to the onset of El Nino. In the tropi cal Pacific these winters are characterized by enhanced activity on intrase asonal (roughly 20-60 day) timescales and by relatively small sea surface t emperature anomalies compared to ENSO winters. For these winters, lagged co mposites are used to document a coherent relationship between the location of extreme precipitation events along the west Coast and the location of en hanced tropical convection on intraseasonal timescales. The evolution of th e atmospheric circulation patterns associated with the extreme precipitatio n events is described and a physical mechanism relating tropical intraseaso nal oscillations, the "pineapple express," and the extreme precipitation ev ents is proposed and illustrated.