Ireland Set For Second Cold Spell Of Winter 2010-11

Use this mild spell and the open conditions (even when it turns a bit colder early next week, it won't be very wintry as such) to get yourselves well prepared, because I think a real blast of winter is coming back with a vengeance around the 16th or 17th, writes iWeather Online long range forecaster Peter O'Donnell. 

There is very real potential for a massive arctic outbreak with a cyclonic disturbance embedded. I would take that as the main theme and given the energy peak at 20-21 Dec, a plausible outcome would be for cold to sweep south, create a vast pool of sub-freezing air mass, then create a storm at the maritime boundary somewhere west of France that would try to move east along the southern boundary of the cold air mass at that time.

Snow (neige) risk for Dec 17. GEFS (c) Meteociel
This could spell snowstorm or even blizzard potential for the UK and Ireland around that time frame, and as for any push back of mild air afterwards, that's likely down to the GFS weather model tendency to try to return to zonal default. The signal is so persistent and so strong (and so well-timed with the retrograde index peak on 20 Dec from my research) that I have to say, word to the wise, there could be a very disruptive period ahead from about Thursday 16th on.  People should use these mild and open days while they have them especially if they are in a location or situation that is prone to travel disruptions.

Just to give iWeather Online readers some extra thoughts on this, the weather models at day 8 and beyond can only be taken as general indicators. Some of the details especially the location and intensity of actual storm systems, often come into focus only by about day 4-5. This is why I try to blend my research with the model output in forecasting, because on a statistical basis, the research is actually more likely to verify in gross terms than the model output at day 8 and beyond. 

Now there's quite a difference in the ECM and GFS weather models at present, but they share the common theme of massive retrogression (that being the westward shift of high pressure next week) and large-scale cold outflow from the arctic towards Europe. If I blend that general foundation and take some of the shaping of each model, then factor in my research data, I see every possibility of winter storm conditions developing some time during the period and most likely around 20-21 Dec but certainly within the larger time frame of 18-23 Dec. 
Upper air temps (850hPAs) on ECM model for Sunday 19 Dec (c) Meteociel
18z GFS model output for Christmas Eve (c) Meteociel
As to any milder push back around Christmas, that so far is only based on the inevitable tendency of the GFS to start returning to default values -- you can pretty much sense as a weather watcher that the massive cold outbreak on the GFS 18z run is being slowly dissolved into some artificial zero-anomaly grid, nature wouldn't just bring south all that potent cold air and have it decay over a five day period, it's much more likely to lead to active frontal boundary dramatics of some kind, especially with the major energy peak so conveniently timed. 

More details to follow in this morning's long range outlook here on Irish Weather Online.