Why is the situation on Thursday and Friday on a knife-edge?
Low-pressure Emma is coming up from Iberia; it is going to take a left turn east at some point. If it turns 200 km earlier than current predictions it will primarily affect the south. Whereas if it takes that left turn 200km later it affects everybody. The models have still not yet quite converged on when it'll turn and therefore how this system will impact Ireland later in the week.
So, with this uncertainty, what should people do?
People need to remain vigilant and return to Met Éireann for the latest updates over the coming days. While it is certain that it will feel bitterly cold, the high impact snow details remain uncertain at this point.
What complicating factors exist in coming days?
Certain things are locked in at this stage. The cold easterly flow and the fact that nights will be very cold and days struggle to raise above freezing at least through the early weekend are pretty certain. Over the next couple of days the location, duration and intensity of sea-effect snow bands are going to be key for the eastern half of the country. There could be very sharp north-south gradients in snow amounts. Later in the week, how far Emma encroaches and how much snow she drops are the key. The recurving of storms to the east is uncommon which means its unclear how well the models used to forecast the weather can handle this type of situation.
What if there is no snow outside my window?
Particularly over the next couple of days in the east of the country, this may not be a great indicator of more general conditions. Drive a few kilometres north or south and the situation may be different. The snow bands will set up parallel to the wind so there will be sharp contrasts potentially north to south over small distances.
What about other impacts such as coastal flooding?
Although there will be a degree of wind piling of water onto the east coast, the high atmospheric pressure combined with the fact that we are not near spring tides means that this is not a great concern presently.
Is the time of year relevant in terms of impacts?
Yes, it’ll be hard to sustain an event like this for the duration of historical winters such as 1947 or 1962/63. It is simply too late in the seasonal cycle for this to last as long or be as severe as these historical events. And that is before accounting for the impacts of climate change.
The Met Office forecasts seem more detailed. Why?
The Met Office and Met Éireann have developed over the years slightly distinct warning criteria and communication strategies. But both are driven by very similar underlying data and guidance. I’m not sure it would be correct to infer that more detail is necessarily better. You can be more detailed and wrong. Snow, in particular, is very hard to forecast with any confidence more than a day or two out.
Furthermore, the comparison is a little unfair in terms of size and resources. The Met Office (where I used to work) is funded by a population 10 times the size of Ireland and is resourced commensurately. It has almost 2,000 staff and a substantial R&D budget. It would be unrealistic, not to mention unnecessary, to expect Met Éireann to be similarly resourced. Met Éireann, as evidenced by Ophelia, does a fantastic job and is an institution we should rightly be proud of and trust to deliver the very best weather forecast guidance for Ireland, making use of the best model guidance combined with decades of local knowledge.
Peter Thorne is Professor of Physical Geography at Maynooth University