LOCAL CLIMATIC GUIDANCE FOR PROBABILISTIC QUANTITATIVE PRECIPITATION FORECASTING

The predictand of a probabilistic quantitative precipitation forecast (PQPF) for a river basin has two parts: (i) the basin average precipit ation amount accumulated during a fixed period and (ii) the temporal d isaggregation of the total amount into subperiods. To assist field for ecasters in the preparation of well-calibrated (reliable) and informat ive PQPFs, local climatic guidance (LCG) was developed. LCG provides c limatic statistics of the predictand for a particular river basin, mon th, and period (e.g., 24-h period beginning at 1200 UTC and divided in to four 6-h subperiods). These statistics can be conditioned on inform ation entered by the forecaster such as the probability of precipitati on occurrence and various hypotheses regarding the precipitation amoun t and timing. This article describes two probability models of the pre dictand, details guidance products, and illustrates them for the Lower Monongahela River basin in Pennsylvania. The first model provides mar ginal climatic statistics of the predictand on an ''average'' day of t he month. The second model conditions the statistics on the timing of precipitation within the diurnal cycle. The resultant characterization of the precipitation process allows the forecaster to decompose the c omplex assessment of a multivariate PQPF into a sequence of feasible j udgmental tasks.