After our European winter forecast from November 10 we had promised to keep a close eye on the month-to-month development of the winter of 2011-2012. December was mild and as we had forecast January indeed is mild and wet too. For those who like winter weather February will be the best month.
Compared to December and January February will likely be the cold month of the winter, with temperatures closer to the climate average.
Although different weather agencies are presently forecasting a frost period for the beginning of the month, we don’t think there is much of a chance winter will manage to get the continent in its grip. One of the reasons lies in the Atlantic Ocean, where the effects of the 2012 La Niña are gradually showing in sea surface temperature anomalies. More importantly however, this SST anomaly is likely to make an end to the exceptionally mild weather of December and January – and also likely to create drier and sunnier circumstances.
A slight La Niña influence is starting to show at this (January 23, NOAA) observed Atlantic sea surface temperature (SST) anomaly chart. This is likely to exert a stabilising influence on the above troposphere and could be of importance for NAO forecasts (which is shown below).
Although the image is geographically distorted there is an influx of cold water from the WHWP (temperature exchange between East Pacific and Caribbean/Gulf of Mexico, as high pressure blocks the trade winds – La Niña Pacific cold water anomaly is mirrored on Atlantic side) starting to show up in the North Atlantic.
This may lead to stabilising weather conditions over the North Atlantic and under the current SST anomaly distribution it would mean the Azores high will remain strong enough to not allow the North Atlantic Oscillation (NAO) to dip below a neutral phase. As the Azores high may even be inclined to move slightly to the north (to settle for latitude between the Azores and Ireland) chances are big that high pressure build-up over Scandinavia (which weather model keep showing on and off in the first half of the month) will form a bridge to the Southwest.
Despite high pressure developing over Scandinavia, the NAO index does not move to a negative phase – instead neutral at best, as shown here by NOAA’s NCEP climate model forecast.
At this NOAA GFS run the Azores high and Scandinavian high are connecting as soon as February 4. The bridge is too weak to block Atlantic depressions, so soon after the westerlies will once again pick up. The models are highly variable still though – so take note of the general pattern, but don’t regard this as a weather forecast for any specific date or location. [It has to be said the European meteorological models, like ECMWF and Met Office's UKMO show a much better fixed high over Scandinavia - so therefore a better chance of persisting cold easterlies during the first week of February, possibly some time beyond. Here too the uncertainty margins are high - and in most runs the general pattern still shows positive NAO - so high probability for the Azores high to move in at some stage (depending on location this could also reinforce cold weather scenarios).]
For West-European weather this means a declining chance for permanent cold easterlies and for all countries on the north flank of the high pressure ridge an alternation with Atlantic westerlies. In general (also because snow cover in East Europe is low and the Baltic Sea is open) this means temperatures may dip a little and frosty days seem likely – but the really cold winter weather in some forecasts is still highly unlikely.
The good news is weather on general will be more stable in many parts of West Europe, the exceptionally early spring growth of nature will be slowed down a bit, while many people can enjoy a drier weather type – with a good chance of seeing some winter sun.
© Rolf Schuttenhelm | www.bitsofscience.org