Agnes, the first large-scale sprawling storm system of the 2023-2024 fall and winter storm season in the British Isles, is about to lash Ireland and Britain on Wednesday and Thursday. Forecasters expect strong winds, rough surf and a slug of heavy rainfall as the storm sweeps inland from the Atlantic Ocean.
Ireland is forecast to be hardest hit. Orange warnings, signifying “dangerous/disruptive” conditions, are in effect for its southern areas.
Conditions won’t be quite as intense in Britain, but the Met Office expects Agnes will “bring a spell of strong and disruptive winds through Wednesday afternoon into early Thursday.” Yellow warnings - one step down from orange - have been hoisted for northern and central parts of Britain, where travel could be affected and there’s “a chance of the odd road closure.”
While Agnes isn’t an overly potent storm, at least by the standards of what Britain is used to in winter, it marks the start of a turbulent period that continues into the spring as frigid air spilling south from the Arctic meets milder air over the Atlantic Ocean. This seasonal clash spawns storms. Agnes’s anticipated arrival will come just days after the official start of fall.
Agnes will bring winds gusting to at least 45 mph over much of Ireland. Southern Ireland - which will catch the brunt of the storm - could see gusts of 60 to 75 mph along the coast and areas of flooding.
“It will be wet and very windy with outbreaks of heavy rain extending nationwide with the likelihood of some spot flooding,” wrote Met Eireann, Ireland’s meteorological service. “Becoming extremely windy or stormy for a time in the east and south with the potential for severe and damaging gusts.”
Orange warnings for wind and rain are in effect for Cork, Kerry and Waterford in Ireland’s south.
The storm won’t be quite as intense in Britain but gusts of at least 30 to 45 mph are expected in most areas. Zones along Britain’s north and west coasts and its hilly and mountainous terrain could see gusts reach 50 to 75 mph.
“The strongest winds are expected to affect Northern Ireland, southwest Scotland, west and northwest Wales, Cumbria and Lancashire where some places inland may see gusts of 60 mph and 65-75 mph over hills and around coasts,” wrote Met Office Chief Meteorologist Matthew Lehnert.
There will also be rough surf near the coast, and waves offshore could tower some 50 feet high.
There’s no official start or end to the storm season in the British Isles but storms are most common between fall and spring when the jet stream - which marks the divide between polar and tropical air - dips farther south over the Atlantic.
Some of the region’s storms are similar to powerful nor’easters that sock New England with hurricane-force winds. In fact, some are every bit as strong as a Category 1 or 2 hurricane.
On rare occasions, violent channels of wind called “sting jets” can even tear through the British Isles on the south side of sprawling storm systems, bringing gusts over 100 mph and widespread damage.
Agnes will bring impacts not unlike a tropical storm over Ireland and parts of Britain, but is forming in a fundamentally different way. Rather than being fueled by ocean heat, it is developing from temperature contrasts over the North Atlantic set up by the jet stream.
Agnes is forming from a disturbance or “short wave” riding along a dip in the jet stream that is carrying chilly air toward the equator. Milder air is colliding with this chilly air from the south and east, energizing the storm.
Because it is still early in the fall, Agnes will have a warming effect over most of Europe. The winds from the south ahead of the storm will result in temperatures 9 to 18 degrees (5 to 10 degrees Celsius) above normal through the end of the week.
The cold front associated with Agnes, carrying the chill sourced from the Arctic, will sweep across only far-northwest Europe. Some chilly air will reach Ireland and Britain in the storm’s wake Friday, but the rest of Europe will remain rather mild.
Later this fall and into the winter, when there is more cold air for similar storms to draw from, they will drag much more cold air southward over Europe compared to Agnes.