LIVE BLOG Cold Spell 25 Nov 2010

10.45PM - Snow reported in Fintown area of Co Donegal

9.20PM - Snow reported near Gorey, Co Wexford

9.01PM - Snow falling near Hacketstown, Co Carlow. Pic from Snowman10

8.34PM - Snowing in Lacken (200m asl) just outside of Blessington, Co Wicklow

7.47PM - iWeather Online forecaster Peter O'Donnell says: There is good model consensus now on a snowfall event for Tuesday, with all four (or five) of the major models showing a storm tracking northeast from near northwest Spain Monday night to some position in northern France. The GFS is most aggressive in bringing this storm north into southeast England and the North Sea. The GEM model has almost the same evolution but not quite as far north and seems the strongest of all in terms of snowfall potential for Ireland. The UKMO and ECM models are more prone to stalling the low in France as they maintain the signal of blocking high pressure to the north. With that evolution, snowfall depends mainly on the strength of the easterly flow across Britain and Ireland.

Obviously too early to be definitive, but snowfalls in the 10-15 cm range would be plausible with any of these models or the model consensus at this stage, with the heaviest falls inland eastern and southern counties.

I continue to think Friday and the weekend will bring scattered outbreaks of 3-5 cms of snow, with the north favoured more than other parts of the country at first, but then later the east and central counties well exposed to incoming snow streamers from the Irish Sea. Winds will tend to veer from NNE towards ENE during the period which should be the main reason for a shift further south in the heavier accumulating snowfall bands.

I also think that snow streamers on the weekend will have vigorous convection due to the extreme difference in air mass temperature and sea temperature (still reported at 12 C at the M2 buoy). Air mass temperatures streaming off the U.K. land mass will be something like -1/-5 for temp dp values. This air will warm rapidly to about 5/2 near the surface but the profile above 200m will only be slightly modified. This will lead to vigorous convection and our many weather watchers in Meath, Dublin and Wicklow as well as some neighbouring counties will no doubt be reporting heavy falls of snow, hail and snow pellets as well as possibly a bit of rain at times, and lightning seems quite likely with this (especially later Saturday and Sunday).

I seem to be going backwards here because now I'm talking about almost a nowcast. As picked up in our forecast, there is a weak warm sector being pulled into the northerly circulation from the near Atlantic and this is temporarily raising temperatures especially in the west. This is a response to the developing wave dropping southwest from Norway, and this warm sector should be pushed off the south coast late tonight, so expect the current rain showers to start mixing more with sleet, hail and snow as the night goes on, but also, more to the east where the warming is less noticeable. A fairly strong cold front will develop tonight around the Hebrides and sweep south across Ulster during the morning. This should mark the onset of the coldest weather and the more organized bands of accumulating snow.

5.44PM - RE: Potential heavy snowfall next Tuesday.  iWeather Online forecaster Peter O'Donnell says: The 12z GEM (CMC) falls in line with the 12z GFS on the strong low in northern France by Tuesday. In fact, the low on that model is even stronger at 967 mbs. All eyes on the other major models now to see how much consensus there is at this early stage. A low this strong would have the b-word in play for large parts of the U.K. and Ireland. But must stress this development is far from a certain outcome at this stage. It would be almost unprecedented too, coming from a source west of Iberia. The storm of 31 Dec 78 into 1 Jan 79 came from more of a westerly source but the setups at storm time would be similar.

Before that stage is reached, would generally suggest that snowfall through the weekend will be patchy and a case where some places get a nice coating, others get very little and there are various mixtures in play as well, including some hail which in some cases falls as snow pellets -- this can be counted as snow if you like, but you don't have to shovel it away, just watch it bounce!

5.28PM - Parts of Ulster and Leinster could see snowfall overnight as a band of rain and sleet pushes down across the country from the north.  Western parts of the country will not be as cold as elsewhere. However, temperatures will drop to between 0c and -4c by dawn with some icy patches forming on roads as a result of overnight rain, sleet and snow.

5.04PM - Fergal Tierney writes: Regarding the question of why it snowed at sea level in Wicklow this morning, I think it may be due to the fact even the coast was affected by an offshore flow off the cold land, the opposite to a sea breeze. This layer of cold air must have been deep enough to prevent melting.
The showers were being steered by upper winds, roughly 850-700hPa, which were just slightly east of due north, which would steer them ever so slightly onshore. But with the backing wind in the cold low-level layer, this would have stopped melting.
This northerly flow turns more easterly over the weekend and will therefore advect a marine layer over eastern coastal regions, as opposed to this cold land layer. This will make a big difference with melt layers, and I'd be extremely surprised to see any snow inthe same location come Saturday or Sunday.

4.30PM - Sunset in West Clare

3.18PM - There is potential for a countrywide significant snow event on late Tuesday and Wednesday. A low pressure system will form in the mid Atlantic and track along the Portuguese coast before deepening over the Bay of Biscay. It will track northwards over western France before veering NE towards southern England and Ireland.  Models show the system retreating eastwards as a blocking high over Iceland tracks south-westwards somewhat and another low pressure system in the Atlantic deepens and moves eastwards.

Some very small alterations to the above set-up could bring some significant snowfall to Ireland. At present, much of southern England and the East and south East of Ireland will see some snow from this system.  The following would result in the system tracking closer to or over Ireland: the blocking high over Iceland holding or retreating NE somewhat towards Greenland OR the blocking high over Scandinavia shifting eastwards.

2.24PM - Met Eireann Weather Warning Issued: From late Friday (26th November) through the weekend, snow showers affecting north Connacht, Donegal and eastern counties of Leinster will give accumulations of 3 to 5 centimetres at times below 250 metres. These accumulations will occur mainly during night time, with occasional thawing likely by day. Generally throughout the country, there will be sharp or severe ground frosts at night, leading to ice formation on damp and untreated surfaces.

1.21PM - Rain/sleet front approximately 75-100 miles NW of Donegal, pushing south across the country this evening and overnight. Snow likely on high ground, approximately 300m ASL.

12.53PM - UK Outlook for Friday 10 Dec 2010 to Friday 24 Dec 2010: The cold conditions are likely to continue. Precipitation amounts should be average or slightly below, giving the risk of sleet and snow at times in many areas. Sunshine amounts are likely to be above average, although some southeastern parts may see more in the way of cloud. Temperatures are likely to continue well below average, with widespread frosts, sometimes severe. There is some chance of it turning less cold, particularly for southern and southwestern parts, but still with the risk of further rain, sleet and snow.

12.10PM - It remins icy on the N20 Adare/Limerick Rd. Gardai are asking motorists take extra care especially through sheltered areas as there have been a number of collisions, according to AA Ireland.

11.52AM - View of Kippure mountain on the Dublin/Wicklow border.  The scene will turn incresingly wintry over the next 24-48hrs. Pic Ian Carruthers.

11.20AM - Heavy Snow In Yorkshire:

11.15AM - The east and north of the country will see most of any showery activity come the weekend. Monday and more particularly, Tuesday and Wednesday bring the threat of heavy and prolonged showers for the rest of the country. Leinster and East Munster, along with parts of Ulster presently are at most risk of seeing snow accumulating over the next five days. The cold spell also looks like sticking around for at least another 10 days.

10.50AM - Snow has fallen in parts of Wicklow and there is frosty conditions in Tullamore, Navan & slippery conditions in Westmeath around Mullingar & Athlone.

10.35AM - Road temperatures are beginning to improve although they are still quite low on the N2 Monaghan at -3°C, the N81 in Baltinglass & on -2°C on the M7 Roscrea.

10.27AM - Snow fell overnight in the Fintown area of Co Donegal, the Wicklow mountains and parts of Co Tryone.

10.15AM - Snow reported just outside Wicklow town

09.51AM - AA Roadwatch is reporting dangerous on many roads this morning after temperatures dipped to minus 4c in places last night.  Road temperatures dropped in some areas to as low as -4.3°C on the N2 Monaghan, -3.5°C on the N81 in Baltinglass, Co Wicklow, and -3.7°C on N7 Roscrea, Co Tipperary.  MORE

08.01AM - Dawn, Donegal Town. Pic Diarmuid Drumm