http://www.netweather.tv/index.cgi?acti ... 6316;sess=
December is likely to start with a similar pattern that is being experienced during November but with some subtle differences. The Scandinavian block is set to continue to dominate during the first part of the month though is likely to retreat slightly more into Russia. However, the jet stream is likely to veer further south as the month progresses elongating and stretching the North Atlantic polar field. This will then slowly allow heights to build further north to the northwest of the UK until eventually a block is formed in this area. This will create an environment for any wave breaking into the stratosphere to create a SSW. At that point towards the end of the month, the UK will be at the mercy of polar air and a cold spell will develop. The length and timing of the cold spell will be determined by the extent of the stratosphere polar vortex collapse but at this point it is expected to last well into January. A negative NAO will eventually become established.
January is likely to start with the continued cold theme (or become cold if the stratospheric wave break hasn’t occurred yet). The risk of significant snowfall is most likely during these weeks of winter. The early winter Scandinavian block will fade as a more west based negative NAO becomes established. The cold will slowly fade in intensity as often in these situations, the block that prevented the westerly winds from reaching the UK moves west, allowing milder Atlantic air to break through. There still exists the possibility of further cold intrusions during this time before the stratospheric vortex reorganizes and influences the troposphere again.
February is probably the most difficult month to forecast. At this point there are indications that the Scandinavian block is likely to become re-established. If the stratospheric polar vortex is yet to reform in the lower part of the stratosphere, then the possibility remains that a further cold outbreak is possible as cold continental air will never be too far away. In fact, if it does then the possibility remains that any cold spell in this month could deliver the coldest conditions of the winter.