December and meteorological winter starts mild and wet with the Atlantic the rule. The tail end of November finished with a rare SSW but being of the reflective type and supportive of North American cold outbreaks, the result for Europe is a stronger jet and reinforced low pressure dominance.
While the MJO is strongly in phase 7, this typically supports a blocky setup into the arctic but what’s going on within the stratosphere means the SSW effects are cancelling out the MJO. However, I and models suggest change beyond mid-month.
As the stratospheric PV regroups and returns to it’s polar position, I believe the MJO’s phase 7 influence will begin to show.
If all is equal (never is of course), this is what phase 7 to start Dec should look but constructive interference is preventing this outcome for now. (see Paul Roundy graphic)
Another perspective with La Nina
Here’s week 1 at 500mb and surface temp anomaly from a polar perspective.
Note GOA ridge and Hudson Bay trough. Cross-polar connection allowing Siberian air to cross the top directly into US while flat flow linking Canada with Ireland. Potential for some deep lows.
While I have niggles that other factors could prevent change, I am going with the mid month shift in which momentum begins to slow allowing the effects of the MJO is take over the 2nd half of December, check out the week 2, 3 and 4 500mb pattern as per GFS ensemble and not the break up of the strong, flat zonal flow over the Atlantic.
Granted the above does show high pressure over the UK extending north but you can clearly see a flip in the PNA (Pacific North America pattern) from + to – and there is a lot of uncertainty and temp charts aren’t showing cold.
When looking at the 50mb level or lower stratosphere, I like to use this level as I believe we get a slightly better reflection underneath. Note on 3rd Dec it supports that week 1 500mb setup.
With further warming of the stratosphere and hopefully better influence from the MJO, Christmas shows the warmth at 50mb much more spread over the pole including Greenland into N Atl, suggesting much less flow and more blocking and potential cold.
Month as a whole.
GFS ensemble
ECMWF
The Verdict
I suspect average to above average temps/precip for UK, Ireland, W Europe as we are likely milder through at least the first half, Colder Christmas/New year possible.
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