WEATHER ALERT More Snow And Ice On The Way

Haggardstown, Dundalk Co.Louth. Pic by Fergal McCabe

The small low pressure system that has formed in the lee of Iceland today and is heading southeastwards towards the northwest of Ireland overnight is throwing up a few surprises, writes iWeather Online's Fergal Tierney.  

Take the UK Met Office FAX charts, which are generated by the forecasters in Exeter. Their 00Z run showed the frontal system crossing the country from the north overnight, clearing the south coast by Monday night.
00Z Fax Chart (c) UK Met Office
Twelve hours later and their 12Z run shows a very different picture, with a wave depression forming along the Connaught coast, pivoting the frontal zone and stalling it over the north of the country. 
12Z Fax Chart (c) UK Met Office

This will serve to keep most of the country in the current freezing fog conditions, with the precipitation only slow-moving southwards during Tuesday. Current theta-w forecasts show northern and eastern areas with the highest chance of snow, but I don't think we'll see it at all levels like last week, maybe only above 100-150 metres. 

Below that should be sleet and cold rain - still enough to add to the treacherous conditions as it falls on frozen roads and pavements. But the slow-moving nature of this front could give more sizeable accumulations, with 5-10cms possible on hills over the next 36 hours. People on higher ground take heed.

Wednesday shows another lee cyclone forming and taking a similar route. With the general upper pattern starting to shift eastwards at that stage, though, it remains to be seen exactly where this low will end up. 

I would imagine it will take a slightly more easterly route than the models are showing now, which would introduce a slightly milder polar maritime airmass, bringing temperatures up a notch or two during the day. With the High then settling in over the country, there could be a lot of inversion cloudiness on its northern and western sectors, which would crucially keep night-time temperatures from falling too low in these areas. This would all aid in the thawing process, and really start to take us out of the situation we're currently in. 

Depending on the exact loaction of the high pressure centre, certain areas to its southeast could remain cloud-free, so it's certainly one to watch over the next couple of days.

REPORT by Fergal Tierney, iWeather Online snyoptics forecaster.

Meanwhile, iWeather Online Long Range Forecaster Peter O'Donnell added: "A frontal trough moving in from the northwest will arrive over Donegal about 0300h and move through Ulster before sunrise, then will continue south all day tomorrow ... expect some outbreaks of snow with this mainly to the east of a line from Mayo to Lough Derg to Waterford, but at higher elevations to the west of that also."

"At lower elevations in the southwest showers are likely to be hail, sleet and some rain but mixing at times with snow ... in the north and east where the snow is heavier, some amounts of 3-7 cms are possible. This will hold temperatures down to about -1 to +2 C in that zone and perhaps 3-5 C in the southwest. Tuesday will stay quite cold with some further snow possible especially near the Irish Sea, and Wednesday is looking quite chilly for Ulster and Leinster now as the track of a slightly milder infusion from the west appears to be only as far inland as Monday's somewhat indistinct rain-snow divide that I mentioned above."

"In other words, the models are responding to the usual principle that cold air is hard to shift without massive upper support and that just isn't really part of the equation ... although I still think it will turn a bit milder end of the week and next weekend, this may be also a rather brief and subtle warming that perhaps touches normal values for a day before falling back in the next cold wave to follow"

Keep an eye on our Twitter account and Facebook page for further details.  Peter O'Donnell will also be providing a long range forecast tomorrow morning, 06 December 2010.