Determining divergence and associated rate of precipitation using the 3-hourly tendency and pressure fields

Quantitative precipitation forecasts are transmitted to the field forecasters by the National Meteorological Center once every 6 hr; however, occasions may arise when the meteorological situation may have changed and the meteorologists would like to revalue the predicted amount. Other papers have been written relating the rate of precipitation to vertical motion through the field of divergence; however, a considerable time is consumed before the precipitation amount can be computed. The technique shown here is simple. After making a few assumptions, the rate of precipitation can be determined quickly at one locality from parameters taken directly from the surface weather chart or the hourly teletype sequence. This is accomplished through the use of Showalter's rate of precipitation by substituting for vertical velocity the divergence determined by the horizontal change in gradient of the three-hourly pressure tendency and the horizontal change of the pressure gradient plus a Coriolis-density term.