I must admit, I've been struggling to pin down what could be driving those two long-range models to produce such an extreme +NAO signal for the upcoming winter months.
- Lag from solar maximum favouring +NAO? Nope - we're past that.
- Westerly QBO? Perhaps weakly in the lowest layers of the stratosphere during the 1st half of winter - not nearly enough to suggest a big +NAO.
- La Nina? Nope - there have been hints of one trying to get going lately, but the models aren't seeing it being successful. In fact most, including GloSea for example, foresee a warm-neutral situation with the warmth focused in the Central Pacific. Like a very weak 'Modoki El Nino'. That if anything supports a -NAO!
- Low Arctic Sea Ice? Not likely - the influence on atmospheric patterns is debatable, but of the proposed linkages that are out there, the type of response is some form of increased high latitude blocking. Not a +NAO.
- North Atlantic SST anomaly pattern? Probably not - we've had a warm tripole pattern for most of the past few months, which has shown some correspondence to a -NAO instead. Neither of the models show much change to this for the winter.
So yeah... these model projections are very perplexing. Based on the above factors, I'd expect at worst a 'mixed' winter with some wintry episodes of appreciable nature for most parts of the UK (with the usual struggles in far S and SW), and at best something truly memorable for its quantity and coverage of cold, snowy weather.
One thing that does stand out to me as a possible 'reasoning' is a strong persistence of weather patterns from what's taking shape across the N. Atlantic to Europe sector this month and for that matter, has been quite common this year as a whole.
Maybe some manner of feedback mechanism is foreseen to sustain unusually high European heights, but it seems a bit of a weak approach to me, so I'm not convinced it can just be that.
If anyone can solve this riddle, I shall bow to them (albeit not likely in person 😛 ).
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