A Study on the Methods of Mid-Range Forecast for Plum Rains, Forecasting Group, Shanghai Meteoroligical Bureau.
NATIONAL AIR INTELLIGENCE CENTER WRIGHT-PATTERSON AFB OH
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An investigation was made for the mid-range forecast of the heavy to torrential rain at the beginning and during the period of the plum rains by using synoptic circulation patterns and numerical statistics. It was found that the beginning of the rainy season is closely related with the seasonal northward displacement of the ridge line of the western North Pacific sub-tropical high between 125 deg 140 deg East at 500 mb. The plum rains occur when the 500 mb ridge line is generally stable between 19 deg 24 deg North and when the subtropical high ridge line basically extends to 26 deg N or more. The forecast of early plum rains should emphasize the movement and variation of the subtropical high while forecasts of normal period or late plum rains should emphasize adjustments and movements of westerly short-wave troughs and ridges. It should be noted that the time of the occurrence of the heavy to torrential rain synoptic process during the plum rainy season in Shanghai is related to the region in which the interaction between the cold and warm air masses takes place, the characteristics of variations in their intensities as well as the variations of meteorological parameters at the station.