Commercial weather forecasters need to be accurate, but they also need to be perceived as being accurate, and sometimes the latter trumps the former.
For instance, the for-profit weather forecasters rarely predict exactly a 50% chance of rain, which might seem wishy-washy and indecisive to customers. Instead, they’ll flip a coin and round up to 60, or down to 40, even though this makes the forecasts both less accurate and less honest.
Forecasters also exaggerate small chances of rain, such as reporting 20% when they predict 5%.
People notice one type of mistake — the failure to predict rain — more than another kind, false alarms. If it rains when it isn’t supposed to, they curse the weatherman for ruining their picnic, whereas an unexpectedly sunny day is taken as a serendipitous bonus.