It would be foolish to base a forecast on any particular model run solution. For long range forecasting in particular, when accuracy greatly falls away week at 5 days or beyond, we look for trends, consistently over one particular solution and cross model agreement.

Let me just say there’s consistency and agreement that the big positive centred over southern Ontario and delivering a mild December pattern for many, isn’t always going to remain in that position.

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For days now, I have looked at the 5-10 and 10-15 day solutions off ECMWF, EPS Control, GFS operational and ensemble and they all point to the same thing. For that ridge to build north over the Davis Straits which signals a colder pattern for a large swathe of the Lower 48 during the second half of December.

I suspect a colder trend with more trough than ridge begins to develop within the 5-10 day period and that means a large scale shift towards Dec 17 onwards.

This is not a forecast yet but likelihood is growing on a colder pattern. Remember that PV has been strengthening and there’s a strong polar air mass up there now. Don’t underestimate what may come down this winter…

GFS ensemble

5-10 day

10-15 day

GFS operational

5-10 day

10-15 day

EPS Control

5-10 day

10-15 day

GFS ensemble trending negative with both AO and NAO.

Video will be available later.

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