This previous rainfall may have slightly reduced the FFG from the values listed in Table 1, perhaps by 0.2 inches. If the 1 hour FFG was reduced to 1.5 inches, the additional runoff would help to explain the enhanced flooding in Pitcairn. Remember that ABR is a WSR-88D radar estimate. The forecaster needs to be assured that the radar is estimating the rainfall with some accuracy. Figure 11 shows a plot of the radar estimate for the two rain gages in Fig. 5.
Solid line is radar estimate.
Detailed WSR-88D data in the form of the DHR product can be used to successfully calculate the ABR data in these small watersheds. The ABR analysis must be carried out in short time steps of 5-6 minutes, so that the details of the temporal distribution of ABR can be found. The short time duration rainfall analysis also allows the computation of ABR rate, which can alert the forecaster to developing flash floods well before flooding begins. The National Weather Service in Pittsburgh uses an alert threshold of 1 in hr -1 ABR rates, printing all streams that reach this threshold every 6 minutes. Notice in Fig.