Operational omission and misuse of numerical precipitation probability expressions

Weather forecasts available from the U.S. National Weather Service were examined to assess how often and in what manner expressions of numerical precipitation probability were used. Of the forecasts that mentioned precipitation, it was found that numerical precipitation probabilities were not issued in 42% of the cases. Omitting the precipitation probability withholds potentially useful information and can be a disservice to users who require the probabilistic guidance to make optimal decisions. Moreover, examination of the individual forecasts that did include a numerical precipitation probability revealed several frequently used, but poorly conveyed, numerical probability expressions in which the probability was embedded in the body of the worded forecast. Mixing the numerical probability into the body of the worded prediction can confuse the user as to which event the probability actually refers and can possibly result in a serious misinterpretation of the forecast. In order to reduce interpretation errors, it is recommended that the numerical probability and the explicit statement of the corresponding event be kept separate from the worded forecast