UPDATE (5.30PM 14 Dec) Cold Spell In Ireland

Wexford. Pic by Joe Whelan
(UPDATE 5.30pm, Tuesday 14 Dec 2010): The main thing to focus on at this point is the rapid onset of the cold spell and the sudden nature of the frontal passage on Thursday. This is likely to set off some very squally mixed showers turning rapidly from rain to hail then sleet and snow as it moves south at a very fast pace.

I will make an effort to time that for iWeather Online readers on Wednesday afternoon or evening, but at present, it looks like this front will hit Ulster between 0600 north to 0900 south, then should move through Connacht and north Leinster between 0900 and noon, and be clearing the south coast around 1500h.

If you have travel plans that places you and this front in the same place at the same time, expect some slow going or perhaps the need for a well-timed meal break while it blows through. The further north you are, the more likely it will be that instead of just blowing through and clearing afterwards, the front will be followed fairly quickly by further snow (except along the immediate west coast, everything behind this front should be snow or hail).

There is potential (80% confidence) for accumulations of 10-20 cms in some parts of Donegal, Mayo, Roscommon, Leitrim and Ulster, from Thursday to Friday. This would be in advance of any more widespread snow that could develop with low pressure systems circling around on the weekend into Monday. I don't want to be too specific yet on that period as the details will be all-important, but could say that the current maps show potential for several inches of snow in many areas over that later time period.
Snow risk chart for Saturday. (c) Meteociel
I've noticed an expected tendency for the longer-term maps now available to begin a delaying process for any potential warming trends mid-week. I guess it's almost inevitable that one or two days between the weekend and New Years will be milder than most, although there is some chance of a complete "locking in" of a cold northeast flow pattern too.

The thing everyone should keep in mind here, I would conclude, is that even today's relatively reliable weather models are being challenged here by the very large-scale changes underway, bringing the polar vortex which usually stays up around Svalbard or Greenland, right down across Britain and Ireland by Friday night.

Analogues for this are scarce, so the models are working off a very limited data base in terms of modelling how the atmosphere will respond with this frigid air spilling out over the 10-12 deg North Atlantic where storms will then develop along a boundary. Just slight changes in the track or intensity of those features still 4-6 days away, would mean large differences in snowfall forecasts for any part of Ireland or the U.K. ... but in the more general sense, I think we can say that snow is likely on a fairly widespread basis given this pattern development.

The fact that my research energy peak for 20-21 December is right in that time frame has me concerned that a major snowfall event is very possible, and of course it could begin as soon as the event starts to take shape rather than waiting until those dates.

UPDATE FROM iWeather Online forecaster Peter O'Donnell (issued at 3pm Tue 14 Dec 2010): After reviewing the most recent GFS weather model output, the only real change to previous forecasts is to upgrade the severity of the outbreak on Friday with a fast-moving reinforcement of the Thursday initial blast. This may give a mechanism for snow showers to reach the east and south at times, rather than keeping most of the initial two days snow potential confined to the west and north.

Beyond that, there is enhanced potential for snowfalls of 5-20 cms through the weekend and Monday.

Our readers in all parts of Ulster, Connacht and parts of West Munster, as well as now north and west Leinster, should be on alert for rapid onset of wintry conditions mid-day Thursday with several waves of heavy snow possible in some areas on strong NW winds. 

Don't be caught napping by the direction here, this air is super-cold and can withstand the Atlantic passage. There are sure to be reports of rain and hail at sea level where this sweeps in, but 90% of residents of the affected region are not at sea level, and even those who are may have to drive places.

I envisage a snowfall map by late Friday with 5-10 cms over a large part of inland Connacht, most of Ulster, and pockets of north Leinster, as well as some inland parts of Clare, Kerry and west Cork. Further east in Munster, it may be more hit and miss but these will be powerful streamers with potential to bring some snow almost across the country, and there will also be troughs embedded to promote some convection. Amounts of 2-5 cms could fall almost anywhere. 

For Dublin, the most likely amount by late Friday would be 1-3 cms coming from a few passing snow showers. Some places will get no covering but maybe fewer than we were speculating earlier.

Friday night and early Saturday could be bitterly cold with clearing skies following this fresh snow. Then outbreaks of snow appear likely through the weekend and these could even include normally temperate south coast regions because a lot of the emphasis will be on bands coming in from the southeast in association with low pressure circling around the west and later south coasts. But the whole country including the NI portions of Ulster are likely to be significantly affected with considerable snow potential extending across Britain also.

I would say the risk of disruptive amounts of snow is increasing with these model runs trending towards a snowy pattern. Will update my perspective after the 12z runs have come out.

Further updates will be issued throughout the day. Watch for updates on Twitter and Facebook and visit the IWO chatboard. For the weather forecast anywhere in Ireland and the UK visit our local weather page.