The NBM has calibrated 6, 12, and 24-hour precipitation amounts. This means that four, 6-hour precips do not necessarily equal the 24- hour precip for that same time period. In order to be as accurate as possible with accumulated precipitation, we sum the largest calibrated precipitation. Because of this, accumulated precipitation can decrease at times. An example of our summing procedure is below.
06 hour acc. precip = 06 hour precip
12 hour acc. precip = 12 hour precip
18 hour acc. precip = 12 hour + 6 hour precip
24 hour acc. precip = 24 hour precip
30 hour acc. precip = 24 hour + 6 hour precip
In addition to this, don’t expect the deterministic NBM to match the NBM PQPF 50th percentile. This is in part because each uses different data. Additionally, the deterministic NBM is tuned to minimize MAE while PQPF data is not.