# Only a slight chance of confusion

It is as a direct result of Model Output Statistics (MOS) that the nifty forecast feature known as probabilistic precipitation outlooks exists. Statements like 'a 70 percent chance of rain” now appear in forecasts around the globe. These were launched in the mid-1960s in the United States, where MOS was pioneered. As familiar as they seem now, probabilistic forecasts were quite the novelty when they debuted. MOS generates such numbers by calculating how often an expected weather scenario has produced a wet day in the past. Each facet of a given weather situation is fed into an equation that computes how likely it is that the overall weather picture will be a damp one. If the forecast is calling for a 30 percent chance of rain, this doesn't imply that 30 percent of the forecast area will see rain, or that it'll rain for 30 percent of the day – only that, for a given point in the forecast domain, there's a 3 in 10 chance it will rain sometime during the day. Nor do probabilities indicate how hard it might rain: a given forecast might show a low chance of a storm that could pour buckets should it materialize, or a high likelihood of drizzle that won't amount to much water. Human forecasters often bump these raw MOS numbers upward or downward based on a host of factors. You might still hear “a 100 percent chance of rain* here and there, although meteorologists now tend to avoid this invitation to embarrass themselves by simply saying “rain” if MOS decrees it's a certainty. In the US, when probabilities are below 20 percent but more than negligible, the number is typically omitted and the word “isolated1' (showers, snow flurries or whatever) is substituted.