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  1. Depends on the probability distribution, but assuming a normal distribution, only roughly 2.5% of events would lie beyond 2 sd - in this case 2.5% of daily zonal wind speeds would be lower than the forecast on those days.
  2. Interesting. live outside at the BBC right now just looks pretty wet.
  3. Tomorrow not uncertain in the slightest, just a question of amounts, will there be a rain (snow) shadow effect from the Peak District with the easterly wind?
  4. Should be OK, but you can check the road cameras later eg http://trafficcameras.uk/roads/ - shows possibly a dusting on the edges of the carriageway on parts of M61 so care required obviously.
  5. Some milder air modified by the Irish Sea this morning - the showers in Warrington are a mixture of snow flakes but predominantly ice grains. Liverpool airport reporting 3/2°C temp/dp. Rostherne 1/0°C light rain showers.
  6. Not just this, the 'descending coupling' is more than the Baldwin & Dunkerton downwelling of zonal wind anomalies. As Shaw & Perlwitz note in The Life Cycle of Northern Hemisphere Downward Wave Coupling between the Stratosphere and Troposphere (2013) - http://journals.ametsoc.org/doi/pdf/10.1175/JCLI-D-12-00251.1 This paper highlights many of the other studies into the trop/strat links via hydrostatic adjustment and potential vorticity anomalies which affect the trop-strat and vice versa over the course of days rather than just weeks or seasonally.
  7. Lol, they are coupled! Geopotential wave 1 from troposphere is creating a prominent stratospheric Aleutian high pressure, displacing the stratosphere vortex. In return, it appears that there may be wave reflection back to the troposphere linking the the stratospheric high with troposphere ridge on Pacific coast of North America - watch the Hannah Attard animation of 10mb and 500mb geopotential heights here - http://www.atmos.albany.edu/student/hattard/realtime/dt_500/DT_500.php
  8. At its most simple, as geopotential height is proportional to virtual temperature, perpetual positive GpH in the lower strat/trop reflects anomalous warmth and lowering upper strat heights or thicknesses shows cooling. Critically important - upper height changes can only indirectly affect heights at lower levels or the surface through ageostrophic circulations. To quote Steenburgh & Holton from "On the Interpretation of Geopotential Height Tendency Equations" (1993) https://atmos.washington.edu/~gcg-dlh/JR_site/papers/1993_3.pdf This is worth absorbing and it follows that time/geopotential height cross-sections may not be the best indicators of trop/strat linkage. It all depends on what exactly is the nature of, and what is referred to as troposphere-stratosphere coupling. A common indicator is by annular mode i.e Arctic oscillation and stratospheric NAM, as used in the well known charts from Baldwin and Dunkerton to show the downwelling of zonal wind anomalies following SSW. If limited to this it could be argued that the recently moderately strong higher vortex is decoupled from the lower strat / trop - Snowking above referred to a 'distinct lack of coupling'. However, the wave/mean flow interaction which we are seeing at the moment weakening the strat vortex is obviously also a form of coupling! Peter Haynes summed up coupling well in SPARC newsletter 25 (bold my emphasis) - http://www.atmosp.physics.utoronto.ca/SPARC/News25/coupling.html There has been much research in the 12 years since this was published, advancing many of the ideas which are described therein, but a definitive description of coupling processes still doesn't appear to be complete.
  9. It's the same run, this graph is at 65°N, the other is 60°N
  10. Interesting, though maybe a couple of days out of date - About-turn...
  11. GFS/GEFS gradually backing off the magnitude of the vortex disruption, confirming previous opinion Overcooks things early doors, but important to note that it forecasts the trends in the first place and they don't just jump out of the woodwork at the last minute.
  12. Highly likely that the Alaskan ridge is a manifestation of the stratospheric anticyclone i.e downwelling.
  13. Maybe, however it turns out with only 4 SSW in that time, totalling 19 days duration, that it would be impossible for all December days to have had reversed zonal winds.