These unpredictable details like AO are why CPC climate forecasters provide the odds, and they are never 100% over the U.S. For example, this fall, forecasters indicated there would be a 60% chance of increased precipitation over southern California during the winter due to the knowledge there would be a strong, potentially historic El Niño; reliable probability forecast means that, for similar outlooks over a long record, 6 out of 10 years should end up wetter and 4 out of 10 years should be near average or drier (click further to learn how to interpret and grade probabilities).
These unpredictable details, like the AO, are why CPC climate forecasters provide the odds (probabilities) for certain impacts, and they are never 100% over the U.S. For example, this fall, forecasters indicated there would be a 60% chance of increased precipitation over southern California during the winter due to the knowledge there would be a strong, potentially historic El Niño. A reliable probability forecast means that, for similar outlooks over a long record, 6 out of 10 years should end up wetter and 4 out of 10 years should be near average or drier (click further to learn how to interpret and grade probabilities).