The Beast from the East -- V -- The Pest from the West

 
It really is a case of the Beast from the East versus The Pest from the West. Will the Atlantic prevail or will the colder airmass stretching down from Scandinavia over Britain extend its reach far enough over Ireland to push back the rain to snow frontal boundary? 

The answer at present is that nobody knows for sure as subtle changes in model output would present a radically different and potentially disruptive scenario for many. At present, the odds favour any wintry precipitation being confined to Ulster and eastern parts of Leinster on Friday, particularly high ground here.

Today, the UK Met Office has issued a Yellow Weather Alert for snow in Northern Ireland on Friday. It said: "As the weather breaks down, there is the potential for a spell of snow to spread from the southwest. As winds strengthen, blizzard conditions are a possibility, especially over high ground. Accumulations of 2-5 cm are possible quite widely and in some locations, especially higher ground, 15 cm are possible."

UKMO Yellow Alert for Friday, 18 January 2013

Snow risk chart for Friday (click to enlarge). Image c/o netweather.tv

Met √Čireann is less bullish in their forecast for Friday although the agency is indicating a turn to much colder and potentially snowy conditions this weekend. Met √Čireann say: "The boundary between the cold air to the east and the milder air in the near Atlantic will stay close to Ireland for the coming week. Weather fronts will move in from the Atlantic from time to time, with precipitation mostly falling as rain. However, during the weekend, winds are likely to turn easterly, increasing the risk of snow and severe frost at night."

As for The Meteo Times, we consider that there is an increasing threat of more widespread wintry weather later on Sunday. High ground in the east and North are favored for any snowfall with the southwest of the country least at risk. It is a fluid situation and as ever, snowfall on this island is ridiculously difficulty to forecast with a high degree of accuracy beyond 24 hours, let alone 120 hours.

According to TMT Senior Forecaster Peter O'Donnell: "A major storm system is currently indicated on several models and the details are uncertain, but the most likely outcome would be heavy rainfalls and strong east winds with temperatures in the 3-6 C range, also with heavy snow on higher summits. It would not take much to change this outlook to mixed precipitation at low elevations or even all snow in some northern or central districts. The track of the low is close to the south coast on a trajectory from WNW to ESE, which would only need to shift 100 miles south to bring in enough cold air for a snowstorm."

Commenting on the long range trends appearing, Peter said: "I would rate the chances of significant snow and cold before mid-February at about 80 to 90 per cent -- the GFS 12z run looks colder the further out it goes to 16d and precision at that time scale is poor so with only slight changes we could start to see some definite, nowhere near marginal synoptics. I expect the eventual outcome will be 2-4 weeks of cold on a scale similar to late 2010 and other epic periods in the past. Just seems to be close to inevitable as the Atlantic jet is clearly starting to sink towards a much lower track than it took most of December. Key will be when higher pressures in the 1035-50 mb range start to appear in western Russia as this will intensify the battleground scenario already in place and give the cold air the upper hand. Meanwhile even by Sunday or Monday we could be looking at snow if some models are right, awaiting the 12z ECM with interest as it showed a lot of potential although just on the rain side of the tipping point (for Sunday) on the 00z run. So far other model trend is south of its track. Would point to events around 24-27 Jan as the breaking point for this pattern and a stronger cold air mass forming in Scandinavia. Would not rule out a 2-3 day mild spell as a necessity for pattern reload and to clear the Atlantic of residual warmth."

There are a number of possible scenarios being presented for Sunday. With 6 days to go, the system may end up missing Ireland completely keeping us in the current cool setup. However, the general consensus amongst models at presents is that we are looking at a 'Slider Low' where high pressure over Europe prevents an Atlantic originating system from proceeding on its normal west to east track over Ireland.. The nature, extent and longevity of any precipitation over Ireland depends on where the system tracks i.e. a) further south than its current position would drag in colder air from the east resulting in a heightened risk of snowfall or b) further north would lead to a mainly rain event.

A 'Slider Low' is explained in this illustration from British-based forecaster Matt Hugo.
The below GEFS (American weather model) ensembles certainly keeps our weather cool to cold and underlines the potential for wintry outbreaks.  Temp 850hpa relates to the air temperature at 1500m above the surface and is a useful gauge of the temperature at ground level.

GEFS chart. Image meteociel.fr

It really is battleground scenario, one that has in the past produced some of the country's most memorable and disruptive snowfalls. 

The winters of 2009-10 and 2010-11 skewed our understanding of what winter weather is or should be be about in this country. Cold lovers were spoiled and since then will not accept anything less than 18-inches of Irish Sea streamer-produced snow. This setup was exceptional, however.  In reality,  both  winters ended being relatively mild outside of the severe wintry blasts that lasted no longer than 3-4 weeks. 

We have had cold winters in the past (1947, 1963, 1987, 2010 for example) but in the main Ireland's position on the delivery end of the gulfstream means that mild and unsettled is 'standard winter fare' for this country.  The vast majority of potential snow fall scenarios in this country are marginal and almost always extremely difficult to forecast for the aforementioned reason.

If you are a fan of cold, snowy weather then it is best to be pessimistic about your chances of seeing snow. If you hate the snow and the disruption and hardship it often brings, then you are more often than not going to be relieved by the final outcome. Very often when it comes to snow, it is the forecaster that is most surprised. This weekend will be no exception but we will do our best to keep you posted on the most likely outcome. In the meantime, keep an eye on our forecast (linked below) and the other links provided.

--- MARK DUNPHY, TMT

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