December
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.
One can see when looking at the December 500hPa anomaly chart the overall pattern to be expected.
Troughing is expected to dominate initially over and then to the south of the UK and this will dig further south towards the end of the month. Overall the temperature of the month is likely to be around average - any colder spells will be tempered by the above average temperatures forecast for the start of the month.
Precipitation is likely to be above average for the south and below average for the north.
The one proviso here is that this likely pattern could be accelerated or delayed depending upon the success of the stratospheric wave break. Any significant wind events are likely to be early enough the month as the jet stream becomes invigorated prior to the stratospheric changes.
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