After some relatively brief flirts with snow and ice during December and mid-January, Ireland could finally be on course for a period of much colder weather from this weekend and into the early days of next week.
The model output tonight is pretty mouthwatering if you like snow but things are far from set in stone. A continuation of the current trend in output for the next 48 hours would justify a dusting off of the snow sled and the ordering in of extra heating oil. What is certain tonight is that colder weather is on the way this weekend and what is being suggested by forecast models (as of tonight) is the coldest spell of weather so far this winter.
They simply call such a scenario 'winter' in other European countries, but recent winters in Ireland (with the exception of February 2018) have failed to deliver deep cold to this far flung part of the continent. On this occasion however, computer models do point to a polar continental air mass becoming established over Ireland in the relatively reliable timeframe of 4-5 days times. In such a setup, cold air is drawn in from the Eurasian landmass, bringing the cold and wintry conditions that have given rise to the 'Beast from the East' moniker.
Since early January, headline writers across Ireland and the UK have made numerous mentions of the ‘Beast from the East’ and how it could be set to return to our shores for the first time since the epic snowfalls of February 2018. These predictions came after the Sudden Stratospheric Warming (SSW) event of early January. However, as iWeather Online indicated on 15th January, a SSW does not always mean we will be shovelling snow off our driveways two weeks later. The swift jump in temperatures in the stratosphere over polar regions can often take many weeks to impact our weather. In fact, history tells us that less than half of all SSWs lead to an outbreak of severe cold weather here.
So, is the 'Beast from the East' on the way?
If current computer model output for Sunday into the early days of next week verifies, then we would be looking at subzero temperatures at night and daytime temperatures struggling to get above freezing. The caveat is that computer models have chopped and changed since the Sudden Stratospheric Warming (SSW) event such is the unpredictable nature of such events and their consequent impact on the weather in the northern hemisphere. Further changes in the next 48 hours towards a less cold solution are possible. However, the trend is your friend in the forecasting world and right now, the output is trending towards a period of significantly cold weather from this weekend.
Will it snow in my back garden?
Forecasting snow totals at this range would be a fool’s errand but in a north-easterly to easterly setup east and north Ulster, Leinster, and east Munster would be favoured. Tonight's model output would give rise to streamer activity which that brought heavy snow to the eastern half of Ireland in particular during 2010 as this evening’s ECM snow depth chart for Thursday 11th February suggests (please treat this chart as a visualisation of the current output rather than a nailed on outcome). Streamers are lines of showers that form over a body of water and align themselves with the wind-flow, bringing a constant "stream" of showers over a certain area. They are also known as Sea or Lake Effect showers, as they are a common wintertime occurrence near the Great Lakes region of the USA. In Ireland they generally form over the Irish Sea and affect north and east-facing coastal regions of Leinster and eastern Ulster.
How long will the cold spell last?
Current indications are that the cold spell will begin on Saturday and persist until midweek next week. Tonight's output gives little to suggest that a prolonged spell of cold weather is on the horizon. With North America lined up for its coldest outbreak of the winter, a similar cold outbreak in Ireland and the UK would go some way to dispelling the myth of both regions incapable of experiencing concurrent significant cold spells.
Model analysis
The 850hPA upper air temperatures in the latest GFS run from Thursday through Tuesday show an East to Northeasterly setup over Ireland. All precipitation would be snow in such a setup.
This evening's ECM is slower than the GFS in bringing about a transition to significantly colder weather this weekend, but it gets there eventually! It's a much colder ECM run that this morning's output. The trend is certainly pointing towards the coldest spell of winter 2020-21 so far.
The Canadian GEM model is less progressive with the cold and suggests a less severe period of weather.
The UKMO has been bullish in its output in relation to a cold outcome and remains so.
What the Mets Say
Met Éireann in its latest national forecast says, "Turning cooler as a northerly airflow moves in over the country. Scattered showers persisting through Friday, although sunny spells will occur also. Current indications suggest it will be drier on Saturday."
The UK Met Office is forecasting that high pressure looks to gradually build to the north of the UK from Saturday, which will introduce cold and dry weather, with widespread overnight frosts, though wintry showers could still feed in from the east coast at times.
From mid-month to the end of February, the UK Met Office says "similar conditions look most likely with high pressure remaining to the north of the UK, bringing overall colder than average conditions and below average precipitation amounts. Throughout this period the risk of wintry hazards continues to be greater than normal with snowfall possible for all areas, but most likely in eastern areas."
iWeather Online's daily long range weather forecast contains further details. Click here to view.
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USEFUL LINKS:
Sea Effect Showers ("Streamers") Explained
How Snow Is Formed
The Isle Of Man Shadow
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Satellite & Radar