Two separate stories unfolding at the moment.
First one to do with how the Atlantic trough shapes up late this week, which is where the ECM 00z made a notable jump over to GFS with consolidation of most of the positive vorticity into one strong system that moves east, as opposed to two competing systems with the western one later developing into a broad low to our west in the style of the UKM 00z.
Second to do with the MJO signal which has been trending stronger in the modelling. GFS has been responding strongly to that, predicting a rapid response leading to a cut-off high attempt in the vicinity of Iceland. It will be interesting to see how the ensembles behave in the coming few days, because the implication of an active MJO crossing phase 6 is that the high should not sink southeast across the UK. That is, providing the MJO does actually have sufficient amplitude - this underpins the chances, not guarantees, of cold weather that Catacol so brilliantly overviewed earlier today.
The first story might yet prove to be one that follows the UKM/GEM 00z pathway but it's very feasible to get a high north of the UK from that position too - the key difference being much higher UK temps until any cold feed from the E or NE establishes (again, if the MJO forcing proves sufficient).
The Scandinavian High chances will hit peak as we approach NYD. The GFS 06z is a bit west-shifted compared to the composite but I don't think it's an outlandish run given the signals at hand.
Should the MJO forcing prove sub-par, we probably would end up with some manner of high pressure across the UK after all, making the current ensemble signal look rather skilful. In fact, it would be a similar sequence of events to what set up an unusually dry January this year, except the zonal flow looks considerably weaker this time around so there's more chance of the high being in a chilly rather than mild position relative to the UK. http://community.netweather.tv/topic/98132-model-output-discussion-the-run-up-to-christmas/?do=findComment&comment=4773981
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James Peacock (@peacockreports) / Twitter
