MJO to Trigger Atmospheric River Category 2 Event in California Discussion: A strengthening Madden Julian oscillation episode this week will activate an atmospheric river of high wind and water vapor into California causing a 1-2 day period of high wind, heavy rain and heavy mountain snow. The event for California is projected as a category 2 atmospheric river event which according to definition is considered “somewhat hazardous” due to borderline precipitation amount to trigger flooding. The event will be most focused on the San Francisco Bay area (heavy rain and high wind) and Sierra Nevada (heavy snow). Up to 4 in. of rain is expected in a 24-hour period with 3 feet of snow in that timeframe over the Sierra Nevada. Heavy rain and mountain snow is less widespread but locally intense in the coastal region from Los Angeles to San Diego. The event takes place on Feb. 13-14. The atmospheric river event is caused by a strengthening MJO shifting across the eastern equatorial Pacific this week into next weekend (phase_7). All models agree that the MJO will shift east into the tropical Atlantic next week (phase_8). During phase_7 the West U.S. is typically cold and stormy while the East is warm. MJO phase_8 usually drags the western cold into the East U.S. Fig. 1: An atmospheric river of high wind and water vapor caused by a strengthening Madden Julian oscillation in the east-central equatorial Pacific forecast for Feb. 13th. Fig. 2: The ECMWF 24-hour precipitation forecast for Feb. 13-14 identifies an atmospheric river category 2 episode for California. Fig. 3: Atmospheric river(s) into the U.S. West Coast are caused by intense MJO episodes. The MJO 14-day forecast indicates an amplified intensity signal shifting from phase_7 (East Pacific) to phase_8 (Atlantic).