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Wayne E. Sangster

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Wayne E. Sangster

Abstract

A coordinate system is presented in which the lowest coordinate surface represents the topography of the earth. Higher coordinate surfaces reflect this topography in diminishing degrees and become isobaric at and above the 500-mb level. A method of representing the field of the horizontal pressure force in the nonisobaric coordinate surfaces, using geostrophic stream and potential functions, is described. Geostrophic winds computed from these functions are compared with those obtained from the customary sea-level charts and with observed winds.

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Wayne E. Sangster

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On conventional surface analyses, sea level isobars allow a forecaster to compute the horizontal pressure-gradient force at sea level, which for much of the world is fairly close to the earth's surface. However, over elevated terrain the procedures invoked in reducing the observed station pressures to sea level can give results that are markedly different from those obtained directly from surface data, as shown in an earlier paper by the author. This paper represents an extension of that work. A revised variable reference atmosphere is developed and used as an aid in computing the horizontal pressure-gradient force on a smoothed terrain approximating the earth's surface. Surface observations are input to the procedure, and surface geostrophic winds, or alternatively, stream and potential functions of the surface geostrophic wind, are output.

The methods described herein are currently in operational use at the National Meteorological Center. Charts are shown for summer situations over the Great Plains when and where they have especial utility, as well as for a winter case where the reduction-to-sea level process led to very strong gradients over elevated terrain on sea level pressure charts.

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Lawrence A. Hughes and Wayne E. Sangster

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Using screening regression procedures, an attempt has been made to standardize probability of precipitation Brier scores for difficulty. Climatological factors affecting the difficulty of forecasting used are: precipitation frequency, time persistence and small amount frequency. Standardizing equations were derived for three-month seasons from seven years of data. Four-term regression equations were developed for each season and lead time. Local forecaster improvement over guidance scores varied inversely as the Model Output Statistics (MOS) scores, indicating that poor machine forecasts are easier to improve upon than good machine forecasts.

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Lawrence A. Hughes and Wayne E. Sangster

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Two methods are discussed for combining the routine forecasts of the 12 h probability of precipitation made by the National Weather Service, for use when longer period probabilities are desired but cannot be created independently. Both apply a year's forecasts from 28 forecast offices to basic equations of probability to adjust for the obvious dependence of the precipitation events among the forecast periods. Both methods suggest that warm season precipitation events are more independent than cold season ones, as would be expected. One method gave unrealistic results for probability combinations outside the range of those actually used. The other method applied realistic constraints to eliminate this undesirable feature. The largest deviations from probabilities for independent events occurred when combining probabilities of 60%, but the deviations wore only about 5% in the warm season and 10% in the cold season. Tables and an equation for combining probabilities are given.

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