Matt Hugo. Engels meteoroloog.
Since my last post the outlook has most certainly shifted in a mannor that, overall was underestimated and that is clearly towards a more cyclonic spell. Therefore, I wanted to try and see if we could figure out why such a change in quite a short-space of time. I think the two variables that have influenced the outlook has been the MJO, initially in the 'wrong' phases but an understimation for more of a strat and trop connection, something which I didn't expected so quickly - Some may agree or disagree with this analysis and it is difficult to say for sure, but, as ever, 9 times out of 10 if the AO and NAO change state and the N Atlantic becomes far more mobile and cyclonic then it is usually down to influences from the strat on the trop. It is also worth remembering how prolonged the cold weather was during the first half of December too, as some years we may had this cyclonic pattern in place since late November, that hasn't been the case.
A few pieces of evidence that relate to the above are as follows;
The MJO, while not obvious on RMM plots did move through the I/O and into the Maritimes with reasonable amplitude, at this time of year phases 2-3-4 are not great for the UK and it is interesting to see how looking ahead some of the composite plots match this evolution. When compared with the up-coming short-term anomalies as well through into early January from the latest EC Monthly, the key area of low pressure over the N Atlantic, coupled with the zone of high pressure over the NE USA across the sub-tropics and into Europe and Russian match that MJO retrogression plot well.
The other underestimation, which I certainly got wrong, was the short-term and quick development of a greater influence from the strat on the trop, despite variable AAM regimes, I'll touch on those shortly - As we all know the strat influences on the trop can often dominate all others, especially under certain circumstances and, IMO, there has definitely been some downwelling of stronger W'lies from the strat to the trop and just the below few plots highlights this. The key feature here is the highlighted section for zonal winds at 100hPa through Nov and into Dec, note how into early Dec the zonal winds were at record low levels, not just at 100hPa either. However, note how that has reversed over the last week, rising above the climate average, while uWinds show descent as well.
The frustrating aspect to all of this is that this has occured when the long-predicted rise in AAM has taken place again with some highlly impressive plots making an appearance over the last week, with E'ly AAM anomalies dominating the mid-latitudes, while AAM transports continue to show favourable patterns as well - Therefore, we seem to have entered into a period whereby there is an overriding factor in play as I would put money on suggesting that if this were early December again, with lower zonal winds and less influence from the strat, then we would have had likely seen a return to amplification. AAM will now ease away looking ahead.
Where we go from here is problematic - If we continue to see greater influences from the strat onto the trop then the long-predicted seasonal data suggesting a +ve NAO regime through mid to late winter may well dominate. An SSW is and does remain a distinct possibility, but, as ever that is often influenced by the troposphere and nobody knows for sure, it doesn't look like happening now before the end of January. Can the MJO getting through into phases 5 and 6, perhaps 7 looking ahead alter the pattern? - The simple answer is it could, but we need to be mindful of the strat and trop connection as a continued overriding factor.
One thing that is for sure no significant winter synoptics are on the way within the next week, at least, perhaps 10-14 days. Beyond and all is still the play for, those now writing off January and suggesting Feb is our 'only hope' are mistaken, it doesn't work like that, so caution on jumping to Feb and Mar already on the 27th/28th December.
For now it is more standard fair synoptics for the British Isles, with often mild, wet and windy weather the dominant feature - Good for those who don't like cold, bad for those we have flooding concerns as some increasingly large rainfall totals are on the way, as ever always a positive to a negative and vice versa. If someone was putting me on the spot for potential changes then I would suggest somewhere between the 7th and 14th January for amplification, but with low confidence. For the cold, winter fans out there the key thing to hope for is that AAM doesn't fall away significantly and a more coherent strat and trop coupling arrives through January as that would write things off for some time.
Enjoy the rest of the Christmas and New Year period.
Matt.
https://community.netweather.tv/topic/98132-model-output-discussion-christmas-week-and-beyond/?do=findComment&comment=4778371
Vertalen kun eventueel je met deepl.