Hi all. Ahead of a detailed briefing we're expecting via Hadley Centre, let me just emphasise some key thinking just given to us from Exeter. I've simplified this for those of you who are uncertain of the technicalities and exactly how the ongoing forecast expectations are being communicated (from us at the BBC). In many respects this reiterates earlier points on the forum, but also offers some idea of where we'll be heading in terms of the public forecasts you'll increasingly see/hear/read into the near-term:
http://forum.netweather.tv/topic/75444-winter-model-discussion-cold-hunting-12z-030113/page__st__420#entry2463736
The 10-15d period remains highly uncertain but the probability of a much colder, blocked set-up increases with time. Ridge amplification into the UK's longitude appears as early as next weekend in some output (UKMO-GM and NCEP) whilst closer to d9-10 in EC.
By mid next week - as the polar vortex undergoes splitting and disintegrates - we expect an increasingly easterly flow to extend across northern Eurasia
Surface response lag to SSW is variable, inevitably, and can be anticipated as from 1-3 weeks
There's no clarity presently, in Exeter's view, as to whether current deterministic and EPS NWP output - across all sources - has actually caught the signal as yet. This remains highly evident from the chaotic spread in ensembles. However, the cold, blocked story in the latest EC32 is likely significant and not least as it's changed from previous issue, as might be expected given the incipient SSW.
Increasing likelihood of a cold easterly outbreak for the UK essentially grows from day 10 onwards. Whilst ultimate mesoscale implications here are currently unclear, this quite evidently means an increased prospect of much colder weather with attendant frost/snow etc. Note also stronger signal for heavy ppn at times into S/SW and the PFJ drives southwards.
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