While most people think of winter starting on December 21st or 22nd, also known as astronomical winter, Thursday December 1st is the start of meteorological winter. Meteorological Winter in the northern hemisphere includes the 3 coldest months of the year: December, January, and February. The coldest temperatures of winter normally occur a few weeks after the solstice, much like what we saw last January 9th with our big snowfall. So with that, let's look at the next week or two to see what we can expect.
The upper level flow stays very blocky over the next week and the models don’t handle blocking regimes very well. But having said that, the long range models remain in pretty good agreement regarding the evolution of the next upper level low which is forecast to cut-off in the southwestern US and then move toward the Great Lakes over the weekend. As this is happening, a cold front will push southeast from the central part of the country while an area of high pressure shifts east to the Atlantic. This will bring the possibility of an overrunning event on Monday or Tuesday, which means a chance of rain for us. Despite the tricky pattern this far out, days 6 and 7of the 12z runs of the GFS and ECMWF have an almost identical pattern in place with only a slight difference in timing which is pretty unusual this far out. But NO snow is forecast for this next system though!
With a near zonal southwest flow in place over the south and eastern US and a ridge of high pressure stuck over the Gulf of Mexico, we’re staring at a typical La Nina pattern which is not good news for cold and snow if that's what you like! What we want to see is blocking over Greenland so that cold air gets forced south instead of east. Right now, the NAO (North Atlantic Oscillation) is in a positive state which allows cold air to slide eastward instead of being blocked and forced southward. But the models are showing the NAO going negative sometime between the 7th and 15th. We would be best served by either a -NAO west based or a PNA pattern with some kind of blocking in western Canada or eastern near Baffin Island, or a split flow which is most likely in El Nino's. The models are also depicting the Polar Vortex moving from it's current location in Siberia to Canada which would also help to bring colder air our way.
So we’ll just sit back and watch and wait for the next potential surprise!
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