Tropical Forcing Backs Mid December Pattern Change

 The stratosphere has began to warm gradually and will continue to do so over the next few weeks. This is being caused by numerous tropical influences. A major player, the -QBO phase means that warm air from the equator is being transported to the north pole. This means that we are in a pattern that should naturally want to warm the stratosphere, especially as we head into NDJ. In november the stratosphere as a whole has experienced record cold because of above normal solar activity which cools the stratosphere. The sun is quieting down now which will allow the stratosphere to begin warming. 2) Mountain torque values are on the rise in both east asia and north america which act as a trigger for a stratospheric warming event. Above normal torque found near 30N means that we can expect to see the stratosphere begin to warm significantly around the 10th of december. The extent of the warming is still unknown but in east QBO years this is common, usually occurring suddenly.

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Stratospheric temperature change is evident in the 240 hour forecast from the ecmwf:

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This would favour ridging across greenland, a trough over the bering sea, ridge in western canada and trough over the east.

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Remember the fact that modelling is beginning to pick up on this does not necessarily mean that this will occur. It is the observations that i want to look at in order to gain better support for a forecast. Now all of this tropical forcing supports a pattern change to colder weather over central and eastern canada. Now lets see what other indicators we can find too support this. Well we can see the trough recurving east of japan in this time period and AAM beginning to move below normal which supports changes in the pacific, particularly a shift to a -epo and possibly PNA. We need the persistent bering sea ridge to break down in order to bring the cold into the east.

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