The West Coast, which has been suffering from a strong drought, is finally seeing some much needed relief as a steady stream of storms and rain pour into the coast from the Pacific. The rains began in Washington and Oregon and have now worked their way down the coast to southern California. The weather pattern responsible for all of the rain is known as the "Pineapple Express." The Pineapple Express is known as an atmospheric river. A large, slow-moving low pressure center off of the West Coast taps into tropical moisture originating around the Hawaiian Islands, which is then channeled northeast by the subtropical jet steam towards the West Coast. The coastal topography aids in squeezing out the moisture as air flows over the mountain ranges. Though the rains are welcome, they have also led to flooding and mudslides, and the powerful storm system has also brought with it high winds that have reached hurricane-force and caused additional damage.

With its combination of passive microwave and active radar sensors, TRMM can be used to calibrate rainfall estimates from other satellites to expand its coverage. The TRMM-based, near-real time Multi-satellite Precipitation Analysis (TMPA) at the NASA Goddard Space Flight Center is used to monitor rainfall over the global Tropics. TMPA rainfall totals are shown here for the period 5 to 12 December 2014 for the West Coast. The highest rainfall totals for the period extend from the San Francisco Bay area northward where upwards of from 140 mm (~5.5 inches, shown in dark yellow) to over 200 mm (~8 inches, shown in red) are estimated to have fallen over the Northern Coastal Range. Elsewhere, anywhere from 60 to over 120 mm (~2 to 5 inches, shown in green and yellow) of rain extend from the Cascades in Oregon down through the central Sierra Nevada.

So far, two persons have died as a result of the storm in Oregon, while in California strong winds, flooding and mudslides have caused widespread power outages and closed many roads and schools.