The theme of a shift to colder conditions (higher chance of blocking via SSW; greater incidence N or E'ly flow) is still the preferred likelihood later into winter and you'll find this represented and explained in the latest 3-month outlook for contingency planners from UKMO (just published a moment ago). Tekst van een BBC-weerman
UKMO contingency planning:
http://www.metoffice.gov.uk/media/pdf/m/5/A3_plots-temp-DJF_v1.pdf
A strong, mature El Niño event continues in the tropical Pacific Ocean. Seasonal prediction systems suggest it will strengthen slightly before the end of the year.
This El Niño is comparable in strength to the 1997-98 and 1982-83 events and is highly likely to rank among the three strongest on record.
El Niño is already creating wide-ranging weather impacts across the globe. The influence on UK weather, however, is more subtle. El Niño moderately increases the probability of the positive phase of the North Atlantic Oscillation (NAO) in
early winter. At this time of year, the positive phase of the NAO is associated with milder- and wetter-than-average conditions, whilst the negative phase is associated with colder- and drier-than-average conditions. In late winter El Niño
increases the probability of sudden stratospheric warming events occurring.
These events disrupt the stratospheric polar vortex and, more often than not, bring cold weather to the UK.
The Quasi-Biennial Oscillation (QBO), an oscillation of the equatorial winds in the stratosphere, remains in a westerly phase. The QBO influences winter
conditions over Western Europe by modulating the strength of the stratospheric polar vortex and thereby the phase of the NAO at the surface. The westerly phase of the QBO tends to favour a stronger stratospheric polar vortex, particularly in early winter, leading to a higher likelihood of a positive phase of the NAO.
During December, the factors described above suggest an increased likelihood of positive NAO, which is consistently supported by predictions from the Met
Office seasonal prediction system along with systems from other global forecast centres. The left-hand graph in figure T2 shows a clear shift towards milder
conditions. This does not preclude temporary incursions of colder weather, but the chance of a prolonged spell of cold weather taking hold in December is low compared to normal.
Through the first half of the 3-month period, milder-than-average conditions are more likely than colder-than average. However later in the winter, particularly into February, several seasonal forecasting systems, including the Met Office
system, are in good agreement in suggesting a shift towards more blocked weather patterns; these patterns increase the chance of cold northerly or
easterly winds affecting the UK. Therefore, the right-hand graph of figure T2 does not tell the whole story and in late winter the probability of
colder-than-average conditions is actually higher than normal. Thus we consider the greatest risk of cold weather impacts, such as snow, to be in late winter.
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