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How did Met Éireann get caught by surprise with Friday morning’s snowfall?

‘Cold pool’ in the upper atmosphere made weather very unstable and unpredictable, forecaster says

A motorist clears snow from his windscreen on the N11 in Dublin on Friday. Photograph: Nick Bradshaw/The Irish Times
A motorist clears snow from his windscreen on the N11 in Dublin on Friday. Photograph: Nick Bradshaw/The Irish Times
Ronan McGreevy's picture
Ronan McGreevy
Fri Mar 01 2024 - 15:55

Not many people went to bed on Thursday night expecting to wake up to a winter wonderland – including the national forecaster.

How did Met Éireann get it wrong? Forecaster Brandon Creagh said the meteorological service had issued a yellow weather warning for eastern counties at 4.48pm on Thursday covering the counties of Dublin, Louth, Meath and Wicklow from 3am on Friday to 3am on Saturday.

The forecast warned of “persistent rain with a mixture of sleet and snow at times and potentially hazardous driving conditions”. Met Éireann also warned road engineers in the respective counties about it.

Unfortunately, not many people were paying attention. At 1am the yellow rain warning was upgraded to a snow-ice warning for eastern counties, but by that stage most people were asleep, hence the surprise on Friday morning.

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A low-pressure system developed to the northwest of Ireland on Thursday and brought a band of showers with it. It looked from the weather models as if it would fall mostly as rain and sleet with some snow at higher levels.

By Met Éireann’s calculations snow was forecast for elevations above 200m in the path of the system, but not at lower elevations.

“What made it complicated was a feature in the upper atmosphere called a cold pool, which makes the air very unstable,” Mr Creagh explained.

“It allowed the precipitation to be a lot greater than the model had anticipated. Just because something happened doesn’t make it entirely predictable.”

The property which governs whether precipitation falls in rain, sleet or snow is dependent on air temperature. Sleet is partially frozen rain, snow occurs when the temperature drops below freezing.

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“A degree or two in different parts of the atmosphere can make all the difference,” he said. “That’s why we issued a warning for rain, sleet and snow. It turned out to be more snowfall than we would have hoped for.”

To a certain extent Met Éireann is damned if it does and damned if it doesn’t issue snow and ice warnings. As forecaster Gerry Murphy told RTÉ, sometimes Met Éireann gets the blame when snow and ice warnings end up being rain instead.