El Nino is not the only force influencing our mild fall and winter. Another important signal, the North Atlantic Oscillation (NAO), has been in the positive mode for months, also supporting a mild weather regime for our area. The NAO is tied to a large, semi-permanent low-pressure area near Iceland. In its positive phase, a strong Atlantic jet stream brings wet weather to Northern Europe and blocks Arctic air from dropping into the Northern Plains. The negative phase allows Arctic air to drop southward into Europe and North America. There are signs from the medium-range weather models that the NAO could switch to the negative phase in a few weeks. This is far from certain as the NAO is difficult to forecast, but if it does switch there would be a good chance of much colder weather in time for Christmas.
