It’s the time of year when we begin to wonder if we will see a white Christmas! While Forecasting 19 days out is nearly impossible, a general pattern favorable for certain weather scenarios can usually be determined.
Let’s begin with the pattern of the jet stream.
The GFS keeps hinting at a meridional pattern closer to Christmas. A meridional pattern allows colder air from the arctic move south into the lower 48. While the placement of the trough is not necessarily set in stone, there is likely to be some troughing during this time period. There is support for this idea from the AO, NAO, and PNA, which we will get into later.
Now take a look at the GFS model sea level pressure and precipitation map for the same time. The GFS is showing a possibility for precipitation to move in from the gulf somewhere between the 22nd and 23rd of December. This is entirely realistic if the above mentioned meridional pattern pans out. The question then becomes: can we get enough cold air into the region at the same time as the precipitation?
Now on to the regional teleconnection outlook:
A teleconnection is a linkage between changes in atmospheric circulations occurring in widely separated parts of the globe. The North Atlantic Oscillation (NAO) has the most influence on Winter in the eastern U.S. Typically a negative NAO index means a meridional pattern in the eastern U.S. and corresponds to colder and snowier conditions. The NAO is forecasted to go neutral to negative around December 16th. This is good news for bringing in the cold air that we would need for snow.
The Arctic Oscillation (AO) is a pattern that reflects the pressure anomalies in the Arctic. The NAO can be viewed as a manifestation of the AO. A negative AO index allows cold air to move south into the Midwestern U.S and eastern U.S. It too is forecasted to move negative over the coming weeks.
The Pacific/North American (PNA) teleconnection also plays a large role in eastern U.S. winters. When the PNA is positive, troughing can occur in the eastern U.S., leading to below normal temperatures. The PNA looks to remain positive the next few weeks.
So what does this mean?
The GFS showed a meridional pattern and precipitation moving into the region a few days before Christmas. I believe the GFS idea is entirely possible because of the phase of the 3 important regional teleconnections. The AO and NAO both look to be negative or at least neutral during this time period and the PNA is forecasted to remain positive. With all 3 teleconnections in phase with one another, trough in the eastern U.S. is a definite possibility. However, while an active pattern during the week of Christmas has a lot of support, this by no means guarantees us a white Christmas.
Unfortunately for all the snow lovers out there, history tells us Bowling Green doesn’t have a real good shot at seeing a white Christmas. I believe this year has a better than average shot of seeing some sort of frozen precipitation in and around the Christmas time frame. But average for Bowling Green is only 5 – 10 %. This year I would bump us up to the 10 – 20% category. As you can see, still not a good chance. But we at least have some systems to entertain our hopes over the weeks closing in on Christmas. For more Christmas snow statistics check out this earlier blog from wxornotbg.