A Detailed Synopsis Of The Cold Spell In Ireland

With IWO issuing a Severe Weather Alert for more wintry weather from Thursday, let's take a closer look at the synoptic situation that will generate the strong northerly outbreak during the week, writes iWeather Online synoptics forecaster Fergal Tierney. 
The situation at 12Z today shows a surface anticyclone sitting right over Ireland, connected to a large upper omega block in the north Atlantic. The ridge of this block lies between Biscay and Iceland, with the westerly trough extending south-eastwards from Baffin Bay. There is another cold trough over northern Canada, with thickness values below 500dm, and it is this trough that will generate the low that will affect us later this week.

The trough near Baffin Bay is in the process of cutting off, which will form a more zonal (westerly) upper jet over Canada and Greenland by Tuesday. The northern Canadian trough advects eastwards in this strong flow, and by late Tuesday is deepening a surface low pressure over northern Greenland. A following anticyclone extending across from northern Canada deepens and forms an extreme area of high pressure over central Greenland, possibly up to 1080hPa if the GFS is to be believed - not quite challenging the all-time record of 1,085.7hPa recorded in Tonstonsengel, Mongolia, on 19th December, 2001 - but remarkably high nonetheless.

The extreme pressure gradient between this high and the low to its northeast is what will drive the cold Arctic airmass southwards through the Greenland Sea on Wednesday, with hurricane-force winds possible between Jan Mayen and Iceland. 850hPa temperatures, which are at around 1,300m altitude in the area, are a good indicator of how deep this cold airmass is, and at this stage will be down as low as -30°C to the north of Iceland. 

As the airmass moves out over warmer waters it will start to modify, but the stronger the winds the less time the airmass has to modify over the 1000km sea track to Ireland. It is possible that we could see some lee cyclones form to the south of Iceland as the island interacts with these winds, and these lows could complicate things even further, effectively slowing down the progress of the coldest air, modifying it slightly more by the time it gets here. It looks though as if we will see -10°C temperatures easily survive the trip, moving in behind the cold front that will move southwards across the country throughout Thursday. This would be enough to turn rain to sleet and then snow in the numerous showers that will follow behind.
Another feature to watch out for is the possible formation of polar lows, also known as an Arctic Hurricanes. These are a common wintertime feature off the west and north of Norway, formed by the release of latent heat when an arctic airmass moves out over relatively warm seas. They are similar to tropical hurricanes in that they are warm core systems that get their energy from the sea, and can bring areas to a standstill with heavy blizzards. 

Upper conditions may be ideal for one of these to form and head our way Friday, but even if one doesn't, we should still see comma and cold-air-development systems form off to our north and move down over the country. These systems are more robust than the lake-effect showers we saw form over the Irish Sea in recent weeks, and could therefore affect a more sizeable portion of the country Friday and Saturday. By that stage it looks like we will also indeed have showers developing over the Irish Sea, therefore the scene looks set for some widespread snowfall by next weekend.

For more information on Polar Lows, see this very useful guide from the Austrian Met Office

One thing that differentiates this spell from the recent one is that once the coldest air reaches us, the supply gets cut off, so we wont have the constant stream of frigid air from the northeast like we had earlier in the month. The airmass will slowly modify as it circulates around us, but will still be cold enough to bring wintry precipitation to many. The GFS hints that a low may push up from the south next Sunday, bringing a band of precipitation with it, which could give organised snowfall as it rides over the cold airmass in place. This is a week away, however, and therefore subject to change, but it is one to keep an eye on during the week.

As with any weather systems in the north Atlantic, one small shift in one area can upset the whole applecart downstream, so stay tuned to Peter's daily forecast tomorrow morning and watch out for further updates as things become clearer in the next few days.

Other IWO updates on the upcoming cold spell HERE and HERE