On the weather menu: another taste of spring, another risk of strong thunderstorms and/or heavy rain, and another cold wave or more.

Now is the winter of our content. Does not quite have the same ring as the famous line from Shakespeare in Richard the III. But the difference between last winter and this winter is truly “A Tale of Two Cities”, or a tale of two quite different winters in the same city here in Atlanta. This winter has so far had little of the snow and cold made famous by the writings of Charles Dickens and made the first half of last winter tough in the south. Last year a White Christmas in Atlanta, this year Christmas was preceded by 6 tornadoes in the metro region.

It is a great lesson in the vagaries of Mother Nature. This winter, like last winter the South Pacific Ocean is in a condition known as La Nina with colder than normal surface water. But the previous La Nina was strong this one weak. Its mirror opposite El Nino is when South Pacific waters are warmer than normal.

These patterns come and go at 3 to 7 year intervals and impact the global jet stream and thus average weather for months at a time. However, it is just one factor controlling the weather and climate. Therefore, no two El Nino or La Nina seasons are ever exactly the same. In making long-range forecasts for a season ahead, they tell us how the odds are tilted but do not guarantee the usual weather will occur.

Last winter a negative AO or Arctic Oscillation and Greenland Blocking (GB) accompanied La Nina for an 8-12 week period with a -AO/NAO. No repeat thus far this winter. The AO measures the magnitude of cold air in the Arctic. When this value is positive the coldest air is locked up near the Arctic Circle leaving the US with normal to above normal temperatures. But when it goes negative the polar air is released and drives south into parts of America.

The Greenland Block (negative NAO or North Atlantic Oscillation) is a bulge in the jet stream near Greenland and it forces the jet stream to dip south driving cold air into the eastern US and often providing energy for winter storms. This combination is what caused the cold and snow of last winter for an unprecedented La Nina pattern. It’s absence thus far this year made November and December mostly mild. “Blocking” in the jet stream allows cold air to “lock-in” or block-in for long periods. In the absence of a -AO and blocking -NAO GB cold snaps come and go. Thus far this winter La Nina impacts have been muted by the MJO recycling into warm phases accompanied by a +WPO/+EPO in the West and North Pacific Ocean which are also warmer influences with a "firehose" Pacific Sub-Polar Jet Stream.

The La Nina favors a mild winter on average going forward. But there are numerous signals pointing to a significant pattern change that will open the window of opportunity for more sustained cold if not cold and snow as we move deeper into January. At least the pattern will be MORE FAVORABLE for this to occur with one shot of cold air as early as next weekend, and more to follow for the second half of January and early February.

This is because a sudden stratospheric warming SSW event has been observed over the Arctic and West Canada and forecast by the models, along with a change in the forecast AO and NAO/Greenland Blocking. The QBO is now more easterly and the MJO is forecast to move toward more winter like phases in the weeks ahead. At the same time low pressure over AK is transitioning to High Pressure with a resultant jet stream connection to Siberia underneath the SSW with a block around 70 degrees North 70 degrees East helping to change the PNA to a ridge west trough east configuration. When such pattern changes occur as a result of all these signals they typically last from 4 to 6 weeks before breaking down. But what is interesting is how these jet stream induced weather regimes can flip on a dime. The rubber band theory of weather holds… the longer you stretch an extreme the more likely it will snap to the other extreme.

When, how strong, and for how long this occurs will determine if Old Man Winter awakes from his slumber for more than a few days at a time as we’ve seen so far. It probably will since we usually see at least one abrupt pattern change each winter. That does not mean we repeat last winter, it just means we don’t coast through the whole season on easy street. Obviously snow and ice are not a certainty even when it turns bitter cold as we’ve already witnessed.