UPDATE Upcoming Cold Spell In Ireland And UK

iWeather Online forecaster Peter O'Donnell follows up his weather alert from early this morning with an update on the upcoming cold spell.  
(A Detailed Synopsis Of The Cold Spell In Ireland HERE)

The basic outlook remains very wintry with very cold air arriving rapidly on Thursday,  and settling in for several if not many days.  Meanwhile, a frontal battleground  (Atlantic fronts meeting very cold air over Ireland) develops to the south of Ireland. At this stage, you cannot ask for much more than that if you're a snow fancier. The details are bound to be murky for several days yet.

I noticed the Bergen Ocean Model (BOM) model getting a boost earlier, it happens that the GEM (Cdn) model is also showing a winter storm at day 6. The GFS (see upper air temperatures for Christmas Eve below) is moderately positive for snow, the ECM has the strangest looking evolution that would be good for snow if it shifted perhaps more realistically to a southwest track for the energy, and the UKMO has the best setup for continuing cold.
GFS, Christmas Eve (upper 850hPAs). (c) Meteociel
Having in the back of my mind this energy peak for 20-21 Dec, I believe this could all fall into place with the cold air locked in, then a storm tries to push back from a position near 50N 20W, somewhat similar to last year's full moon end of December storm (the dates shift forward by ten days a year with lunar analogues). If you recall, that storm pushed mild air into the south, gave a heavy rainfall, then backed off and the precip turned to sleet and then snow before much colder air returned. This time around, we seem to have deeper cold air in place before the event. This is why I figure it could turn into a snowstorm if there are similar dynamics.

Dealing with the more certain forecast first, though, we need to stress that cold and locally heavy snow will arrive very rapidly Thursday, and wintry weather will become a problem for the north as early as mid-day Thursday -- will be considering an alert for the Monday morning forecast here, as we have plenty of time, but on the current model consensus, could see 5-10 cm snowfalls in parts of Ulster and inland Connacht on Thursday. This is basically a Greenland express coming down the line on 30-50 mph winds (the remnant of 50-70 mph winds further north around Jan Mayen). 
ECM forecast for Friday next. (C) Meteciel.fr
The speed of development is truly staggering with this -- far to the north, conditions change within 24 hours from almost calm to hurricane force winds as the gradient suddenly increases. The Greenland high reaches values into the 1075 mb range with a 980 mb low not that far away west of Svalbard. This is a huge pressure differential and will accelerate the southward charge of arctic air.

The thing that looks wonky to me on the models is how this low transfers west so effortlessly when the evolution seems to call for low pressure formation in the North Sea. I'm wondering if the missing link is that pressures may rise faster from northwest Russia across Norway late in the week and clamp down on the whole complex of low pressure -- watch for that tendency in later model runs. 

Read Peter's earlier update HERE and his daily long range forecast HERE.

iWeather Online's Long Range Forecast for Ireland will be posted on this website before 9am tomorrow, Monday.