Storm Darragh to bring gales and heavy rain on Saturday

After a wet and windy end to the week for many, with more heavy rain and strong winds sweeping east this evening,  Storm Darragh arrives from the Atlantic Friday night into Saturday morning. The fourth named storm of the season is set to bring heavy rain and gale to severe gale-force winds amid amber warnings for the west issued by the Met Office this morning.

More heavy rain moving east this afternoon and evening

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Storm Darragh is still way to the west over the NW Atlantic Thursday afternoon. But the low will deepen over the next 36 hours, as it tracks east over the North Atlantic - guided by a strong westerly jet stream. The low deepens as a result of a tightening thermal gradient generated by a sharpening shortwave trough moving out of eastern Canada injecting colder arctic air along the boundary between polar air and tropical maritime air that runs parallel to the westerly jet stream. The low also moving under the developmental area of the jet stream where air divergence aloft is maximised, causing air to converge near the surface, lowering pressure.

The Storm Darragh low will not see a huge drop in central pressure. At noon today, the T+00 fax chart had a central pressure of 1001 hPa, midnight tonight the central pressure expected to be around 992 hPa, then at midnight Friday night, centre of the low is expected to be over N. Ireland or Irish Sea around 980/81 hPa. So the low is perhaps not a ‘weather bomb’ - which is associated with a pressure drop of 24 hPa in 24 hours. This low sees a drop of 12 hPa in 24 hours. The central pressure of Darragh perhaps falling to as low 978 hPa as it crosses northern England. But as low then turns southeast, the low will start to fill as it moves towards the Netherlands during Saturday evening. 

The low moves east/northeast on its journey across the North Atlantic to the UK over the next 36 hours, however, when it reaches the UK it will then turn southeast towards the near continent. The change in track of the low is due to major Rossby wave-breaking in the upper flow taking place over the weekend - with an upper trough over NW Europe and far NE Atlantic at the moment disrupting as a strong ridge builds north to the west over the North Atlantic - this forms a cut-off upper low over mainland Europe, which will at the same time draw Storm Darragh low southeast into Europe too, as the trough disruption happens.

Wave breaking and trough disruption this weekend will send Darragh southeast into mainland Europe after it's done with the UK:

Although the Storm Darragh low won’t have a particularly deep central pressure, there is forecast to be large pressure rises on Saturday on the back of the low as it clears away, with this tight pressure gradient a recipe for unusually strong northerly winds to develop across the Irish Sea, Wales and England behind the low clearing east.  

Northerly winds are forecast to gust to 70-80mph through the Irish Sea Saturday morning, some models show higher gusts, with similar gusts affecting exposed coastal areas of N. Ireland, Ireland, NW England and Wales too. Inland across N. Ireland, NW England, Wales, Devon and Cornwall gusts of 60-70mph are possible.

A Met Office amber wind warning has been issued for these areas. So there is a risk of widespread travel disruption, property and tree damage, power disruption and also large waves bringing coastal flooding.

A broader yellow wind warning covers most of England and Wales for wind gusts of 40-50mph possible inland, locally in excess of 60mph, perhaps 60-70mph around coasts. Again, this could bring fallen tree branches and  travel disruption to other areas not in the amber warnings.

Heavy rain from Darragh could also cause issues, particularly across northern and western areas - where rain will be heaviest and most persistent for a time. The Met Office have issued yellow rain warnings across southern Scotland and Wales for 20-30mm widely; with as much as 50-60mm over higher ground in the south and west of these areas. Falling on saturated ground - this could lead to localised flooding and transport disruption.

As well as the strong winds and heavy rain, colder air undercutting the northern and western flank of the low from the north will likely bring snow rather than rain across higher ground of Scotland and later northern England as colder air gets further south, snow mostly settling above 300m. 

As mentioned, Storm Darragh will clear off into the Netherlands on Saturday evening, but we are keeping a close eye on potential for gales or severe gales towards coasts of eastern England, with potential for gusts in excess of 70mph for a time here, as some models suggest.

A strong northeasterly wind will follow on Sunday across much of England and Wales, lighter across Scotland, so feeling raw, with outbreaks of rain across central and eastern areas of England, though easing later in the day.