Improving the Understanding and Prediction of Heavy Rain in Land-falling Pacific Winter Storms: The CALJET and PACJET Experiments

Each winter extratropical storms develop over the relatively data sparse eastern Pacific ocean and then strike the U. S. west coast as they make land-fall. Some of these storms create hurricane-force winds and large rainfall accumulations in response to strong orographic forcing of steep coastal mountains. Flooding associated with such storms is the phenomenon that emergency managers in the region must deal with most often, and these events lead to most of the Presidentially Declared Natural Disasters in the region. When averaged over the last 30 years, land-falling Pacific winter storms have been responsible for fatalities and damage comparable to those of earthquakes in the western coastal states, and in the last few years annual losses have occurred at double or even triple this rate.

In response to the importance of this problem, and because of the development of new observing systems and strategies, efforts have been mounted recently to explore how to improve the 0-24 h prediction of these storms as they approach the coast. Field experiments have been integral to this, including the California Land-falling Jets experiment (CALJET) in 1997/98 and the Pacific Land-falling Jets experiment (PACJET) in 2000/01. These efforts have developed partly in response to priorities set by the USWRP and by the National Weather Service that focus on either quantitative precipitation forecasting, studies related to optimizing the observing system for weather prediction, or generally improving west coast forecasts.

The approach in CALJET and PACJET is based on exploring atmospheric physical processes that cause flooding and high winds in these storms, testing of new tools and observing strategies to improve monitoring and forecasting, and the development of new forecasting techniques. It is an approach that integrates new ideas that emerge from research with testing of these ideas in an operational environment.

This presentation will provide an overview of this effort to improve prediction of heavy precipitation in land-falling Pacific winter storms. This will include recommendations made by researchers, forecasters, and forecast users received at planning workshops, results of physical process studies in CALJET, and assessments of the coastal and offshore observing system for prediction of heavy rain. Finally, a summary of the PACJET experiment, which will be underway during this conference, will be given.