The West Coast of North America experiences extreme precipitation from Atmospheric Rivers (AR) that form over the Pacific Ocean. This is the most studied of AR regions, and 3-day rain totals associated with landfalling ARs can be as high as those related to landfalling tropical cyclones (Ralph and Dettinger, 2012). The impact of these ARs extends beyond the coastal regions of California, Oregon, Washington, and British Columbia; and have been documented in Arizona (Neiman et al., 2013) and in Nevada and Idaho (Dettinger, 2004).
The source for the West Coast landfalling ARs include two primary areas: the far western Pacific (west of the date line), and the Eastern Pacific. Those forming in the Eastern Pacfic are commonly referred to as “pineapple express” events. The event of Feb 6-9th 2014 in the San Francisco North Bay was of this type. The RSS MW images shown here from Saturday, Feb 8th 2014, show the river of atmospheric water vapor extending from Hawaii toward California. In these images, higher water vapor values are in yellow, orange and red (values greater than 40 mm). More than 6 inches fell Saturday in Sonoma County, with over 20 inches in Marin County on Mt. Tamalpais.
The distribution and intensity of precipitation over land is directly related to the transport of water vapor from over the ocean. Moderate to heavy precipitation does not have enough moisture available in the local vicinity to support it, so these events require a supply of moisture transported from other locations (Trenberth et al. 2003). If the pathways of moisture transport remain open and active for an extended period of time, flooding can occur. If the normal routes of transport become blocked by high pressure, drought can occur. California has been blocked by high pressure for much of the winter and we are in one of the worst droughts on record. However, another atmospheric river is dumping near the Orgeon/California border this week with rain expected in Sonoma county again this weekend. Much more rain is needed to mitigate the already dry conditions.
ENSO is known to modulate ARs, in particular, the pineapple express events (Dettinger, 2004; Knippertz and Wernli, 2010), producing the most vigorous ARs in neutral and near-neutral conditions, and suppressing the events in La Niña winters. Bao et al. (2006) found that a weakened subtropical ridge was favorable for ARs, and this is the same set-up that occurs for the neutral ENSO case.
The critical role that water vapor transport plays is recognized and discussed in the U.S. Geological Survey’s (USGS) ARkStorm scenario for the Multi Hazards Demonstration Project (Porter et al., 2011). This hypothetical superstorm was realistically simulated using the Weather Research and Forecasting model, and was based on the 1861-1862 series of winter storms in California. This series of storms lasted 45 days and turned the Sacramento valley into an inland sea. The flooding was so severe, it forced the state capital to be temporarily moved from Sacramento to San Francisco, and required then Governor Leland Stanford to take a rowboat to his inauguration. There is no reason to believe that an event of this magnitude could not occur again.
Understanding the spatial and temporal patterns of water vapor transport and their variability is vitally important to society. The satellite microwave data processed by RSS can help to increase our understanding.