An aid for forecasting summer maximum temperatures at Seattle, Washington

An objective forecast system for predicting summertime maximum temperatures for the following day in selected situations is described. The study is limited to days when temperatures are above normal. Seattle data are used for developing the system, but the parameters are effective in forecasting maximum temperatures for all cities in the Puget Sound Lowland and Willamette Valley from Seattle southward to Eugene, Oregon. One predictor consists of the larger of the pressure differences between Seattle and Astoria or Seattle and North Bend, Oregon. The other predictor is the difference between 24-hour pressure changes at North Bend, Oregon, and Omak, Washington. It is anticipated that as the numerical surface prognostic charts become more accurate, pressure differences required for these predictors may be obtained from them. At that time, a 48-hour forecast as welI as a 24-hour forecast wiII be feasible. Verification statistics from an independent data sample indicate a 2.5 degree average temperature error for this objective system as compared to a 4.6 degree error for Seattle WSFO forecasts and 5.9 for the NMC (Klein-Lewis) product. A forecast of persistence would have given a 5.7 degree error. Edgar C. Johnson. Includes bibliographical references (page 12).