Extreme cold snap will be a shock after prolonged warm spell

So which is more significant – the icy blast taking hold now or the exceptionally mild spell which preceded it?

With buds forming on trees, bulbs sprouting from the earth and the odd bumblebee still in circulation, the answer comes firmly down on the side of the latter and the explanation lies in climate change.

But just taking a short-term view and looking at the last few weeks compared to the next few days, the contrast is pronounced and is leading to lots of questions around the cause.

At its simplest, this is winter getting wintry after a very warm autumn meant a drop in temperatures was overdue.

But it is arriving with added bite that will see the thermometer readings drop several places below normal when normal is a cold December.

Matthew Martin of Met Éireann says the weather system that has kept true winter at bay has essentially been reversed.

“Over the last month or so it’s been driven by an Atlantic-dominated regime so we’ve had low pressure to the north of Ireland and high pressure to the south,” said Mr Martin.

“That has let westerly winds push bands of rain in from the Atlantic along with mild air. But what’s happening now is it has turned around so the high pressure is in the north and the low pressure is now in the south.

“That’s allowing the wind come from the east and north-east so that’s a total weather pattern change.

“We’ve had the typical mild south-west winds and now they’re being replaced by very cold east, north-easterly air.”

That kind of flip in weather patterns happens at least once or twice in a winter season so it’s not unusual, he added, but the severity of the incoming cold is notable.

Mr Martin says it is expected to be four to five degrees below the December average with night-time temperatures dropping to minus five or minus six degrees, beginning tomorrow night.

If it falls to -6C, that will be the coldest night of the year so far, outdoing anything last January, February or March had to offer.

“It is going to be noteworthy regardless of whether we had the mild November,” he said.

“But it will be made feel more of a shock to the system with it being so mild of late.”

Concerns about conditions on roads and for people on low-incomes given the cost of home-heating are well-founded, but for those who are able to get out and about safely and have a warm home to return to, the conditions will not necessarily be unpleasant.

Some sleet and snow might fall but the recent spells of heavy rain will give way to much drier conditions.

“There will be pleasant winter sunshine after cold frosty mornings,” said Mr Martin.

“Certainly away from the north and north-west, Thursday and Friday should be pleasant during the daytime in much of the country – but with that caution again that the frost will set in quickly after dark.”

Some UK forecasters spotted signs of the coming change as far back as September, tentatively debating whether a “beast from the east” type of scenario might be building up.

So far, there’s no sign of a resurrection of the 2018 big freeze, but Met Éireann is reluctant to predict the forecast with any guarantee of accuracy beyond early next week when it is expected to still be cold, though with temperatures a little higher.

And while our neighbours are preparing for significant falls of snow, especially in Scotland, we are unlikely to need the shovels this time.

“Being that bit closer to the northern latitudes, Scotland gets the cold air first and then by the time it moves down over the sea it moderates itself a little bit. The sea temperatures are very warm at the moment,” Mr Martin said.

By very warm, he means 13C which is just a degree or two below summer temperatures.

So it will be warmer in than out – with the caution that “warm” is a relative concept.