The Landfall and Inland Penetration of a Flood-Producing Atmospheric River in Arizona. Part 1: Observed Synoptic-Scale, Orographic, and Hydrometeorological Characteristics

Oceanic extratropical cyclones typically contain enhanced water vapor fluxes in the warm sector, especially during the baroclinically active cool season. A subset of those storms that tap into the tropical water vapor reservoir and/or are accompanied by strongly confluent/convergent flow ahead of the polar cold front concentrate those fluxes into long (>~2000 km), narrow (<~1000 km) plumes. These plumes, referred to as atmospheric rivers (ARs), are situated in the lower troposphere within a broader region of generally poleward heat transport in the warm sector. ARs are a dominant mechanism for generating intense wintertime precipitation along the west coast of continents in the midlatitudes, including the U.S. West Coast. While studies over the past 10 years have extensively explored the impact of ARs on the temperature and precipitation in, and west of, California's Sierra Nevada and the Pacific Northwest's Cascade Mountains, their influence on the weather across the intermountain west remains an open question. Part 1 of this presentation utilizes the 1/2°-resolution Climate Forecast System Reanalysis (CFSR) gridded dataset from the National Centers for Environmental Prediction in conjunction with satellite imagery, rawinsonde soundings, a fortuitously positioned 449-MHz wind profiler and GPS receiver, and operational hydrometeorological observing networks (i.e., precipitation gauges, snow pillows, and stream gauges) to explore the dynamics and inland impacts of a landfalling, flood-producing AR across Arizona in late January 2010. Plan-view, cross-section, and back-trajectory analyses from the CFSR quantify the synoptic and mesoscale forcing that led to widespread precipitation across the state, and reveal that the AR formed in the lower midlatitudes over the northeastern Pacific Ocean via frontogenetic processes without tapping into the adjacent tropical water vapor reservoir to the south. The wind profiler, GPS, and rawinsonde observations document strong orographic forcing in a moist-neutral environment that led to extreme, orographically enhanced precipitation focused in the center of the state. High snow levels during the heaviest precipitation contributed to region-wide flooding, while the high-altitude snowpack increased substantially. In short, the dynamical evolution of the transient AR that impacted Arizona in late January 2010, and the resulting heavy orographic precipitation, are comparable to landfalling ARs along the west coasts of midlatitude continents but quite different from a quasi-stationary AR that penetrated northward into the central U.S. in early May 2010 which contributed to the growth of deep, training convection and deadly flash flooding. The follow-on talk will utilize high-resolution WRF simulations of this storm to investigate water-vapor pathways directed from the Pacific coast into the interior, and to determine the role of key mountain ranges in redirecting, or blocking, the incoming water vapor. This research has the potential to better inform decisions about dam safety and flood hydrology across the semi-arid and arid intermountain west and inform where additional observations may be most useful to improve short-term forecasts.