Long Range Lookout: Stormier Pattern On The Way

The pattern for the next while is looking much stormier in the near future.

Above is a 4 panel forecast of two teleconnections, the North Atlantic Oscillation (NAO) on the bottom half and the Pacific North American (PNA) index on the top half. The left half forecasts are from the ESRL/PSD, a physics-based branch of the government's weather services, and the right half forecasts are from the NCEP, which gives us the GFS model. If you didn't catch that, here's a breakdown:

Top left: ESRL/PSD PNA Forecast
Top right: NCEP PNA Forecast
Bottom left: ESRL/PSD NAO Forecast
Bottom right: NCEP NAO Forecast

The Pacific North American index involves what the atmosphere does in the northeast Pacific and the western coast of North America. When we see a stormy pattern in place over these regions, we call such a pattern a negative PNA, due to the below normal height anomalies in this region. In a similar sense, when high pressure dominates that same region, we call that a positive PNA. A negative PNA will bend the jet stream to give the storms to the Plains and the Deep South regions, frequently initiating high pressure system formations over the Central US. A Positive PNA will bring about an opposite response to high pressure (HP) over the West, and will have the stormy pattern evolve over the East US. These forecasts call for a deep negative PNA, which typically is not that favorable for cold in the East. We'll discuss that prospect later, but keep in mind that negative PNA.
The North Atlantic Oscillation involves the presence of a high pressure system over Greenland (negative NAO) or the presence of a low pressure system over Greenland (positive NAO). In the negative NAO, the jet stream will buckle into the Northeast to allow storms and cold to thrive in that region. The positive NAO denies this region any of these benefits. The ESRL and NCEP are varying on their forecasts, but I have a feeling that the NAO will stay negative, so HP systems caused by the -PNA may be deterred slightly by the -NAO.

This is the Arctic Oscillation (AO) forecast. In its negative phase, you will see high pressure forming over the North Pole, displacing cold air and letting it flow south into North America. The positive phase of the AO brings about a very strong polar vortex (low pressure system that sits on the North Pole), which locks up the cold air in north Canada. Multiple models and ensembles are predicting that the AO will go positive for a while before dropping negative during the second to last week of December, finally trying to push positive in the days leading up to New Year's. There seems to be good agreement among the models, so cold shots are likely to be shooed away in the positive phase and enhanced in the negative phase.

All in all, a negative PNA, weak negative NAO and rollercoaster AO support a much stormier pattern but not a huge potential for a cooldown. Unless we see a sturdy -NAO and -AO, don't expect to see any sustained cold.

Andrew