Observations from polar-orbiting and GOES satellites and research aircraft over the eastern Pacific Ocean during the California Landfalling Jets (CALJET) Experiment in the winter of 1997/98 are used to document narrow plumes of water vapor that occur regularly within landfalling winter storms. Cross-plume statistical characteristics of vertically integrated water vapor (IWV) from polar-orbiting SSM/I satellite data collected over the full winter season will be shown. An objective selection criteria identified the presence of IWV plumes with a core region 2 cm that occurred on 46 of the 137 days examined that winter. Using 312 baselines, each 1500 km long, crossing these plumes it was found that on average 70% of the water vapor was contained in a narrow region ~400 km wide. Similar results were obtained for the more moisture-laden plumes with maxima exceeding 3 cm and 4 cm. SSM/I statistical characteristics of cloud liquid water and rainrate across these plumes will also be presented, as will the GOES cross-plume statistics of IWV, cloud-top temperature, and cloud-top pressure.

Although the satellite data are useful in quantifying the IWV distribution, they are unable to quantify the horizontal water vapor fluxes. This requires coincident wind and water vapor measurements collected from dropsondes released by a NOAA P-3 aircraft during a representative storm. Dropsonde observations in a landfalling storm on 25-26 January 1998 clearly document the narrow lateral extent of the IWV plume in a frontal cloud band. These observations also document the horizontal and vertical structure of horizontal water vapor fluxes from the surface up to 500 mb. The observations show the total horizontal flux concentrated in the warm sector of the storm below 600 mb, including a maximum of >80 × 10⁵ kg s^-1 in the region of the low-level jet at ~900 mb. Initial estimates reveal that ~65% of the total flux below 600 mb was located in a 400 km wide zone. It is important to note that a significant portion of the region of strong flux was elevated roughly 1-2 km above the surface.

These data combine to uniquely quantify the narrow nature of these plumes (also referred to as "atmospheric rivers") that contain and transport most of the atmospheric water vapor during oceanic winter storms. Since atmospheric rivers typically include the region of the prefrontal low-level jet, these rivers play a critical role in determining the spatial distribution of precipitation when these storms strike the West Coast. They also play a major role in quantitative precipitation forecasting, in the global water cycle, and in linking the weather and climate scales.