Atlantic storm brings high risk of coastal flooding

A powerful Atlantic storm peaked in intensity over the west-central Atlantic yesterday but will only be slightly less intense when it slams into western Ireland later today, writes IWO's Peter O'Donnell. 

Currently located at about 51N 28W, this low will be absorbed by another east of Iceland towards the end of the day, but its strong gradient winds will already be hitting Ireland during that process. Frontal troughs have formed out ahead of the storm and the first of these is already bringing some heavy showers to the east coast. Another one will move into the west soon and make steady progress east, then several more bands of showers and possibly thunderstorms will follow. Winds are fairly sedate now after peaking with a preliminary disturbance last evening. They will ramp up gradually during the mid-day hours and begin to approach warning criteria by late afternoon, staying strong all evening and only moderating slightly after midnight.

Temperatures will rise fairly steadily as this storm has tapped into subtropical air from north of the Azores and that is streaming in on southwest winds due to reach 70-110 km/hr later on. Highs of 13 to 15 C are likely, quite a bit milder than recent days. Some potential for gusts to 130 km/hr in a few exposed locations this evening, but perhaps a more potent hazard will be storm surge into Galway Bay and battering waves in other west-facing coastal areas, as ships were reporting waves out in the mid-ocean of ten metres. The timing of any coastal flooding around Galway would most likely be midnight to 0300h as there's a natural high tide at 0258h that will probably be enhanced a bit earlier than scheduled by the peak storm winds around midnight.

Rainfall amounts are not expected to be excessive but there could be some brief heavy downpours and amounts for most places should be in the 10-20 mm range.

Overnight the winds will veer more to due west which is why I am concerned about Galway Bay having shoreline flooding issues. They will slowly moderate slightly at least back down to 50-80 km/hr by morning. It will turn a bit colder after midnight with a morning temperature around 4 to 6 C in strong westerly winds. Passing squally showers will continue and some might become a bit sleety on higher ground with the freezing level coming down in an airstream that originated in the east Greenland Sea. However, another weak impulse travelling along in the strong westerlies will then arrive later Thursday and an interval of steady rain might be the result with a brief resurgence of stronger wind gusts. Friday sees another similar cycle with temperatures steady around 8 C and further rainfalls both days of about 5-8 mm. New moon is on Saturday so tides will be running fairly high all week with this sustained westerly flow, however the subsequent ones will probably stay below the levels achieved tonight.

By Saturday yet another weak disturbance is due with another round of showers and blustery westerly winds, temperatures near 7 or 8 C. Sunday sees the start of a milder trend that will be somewhat unsettled at first, until high pressure from the central Atlantic can build in close enough to dry out the flow by about Tuesday. So Sunday and Monday may both have a fair amount of cloud, showers or drizzle, and highs near 12 C. From Tuesday to about Friday it looks dry and perhaps mostly sunny with high pressure drifting in almost overhead by late in the week. Highs could edge up towards 15 C in bright sunshine but nights will continue to be rather chilly under the clear skies, and local fog patches are possible.

The further evolution of this high seems likely to take it a bit further north to begin a weak easterly flow at the surface levels over most of Britain and Ireland, but it may not turn that much cooler except for onshore sea breeze cooling in Leinster, so would suggest by the following weekend highs might have fallen back to 8-11 C there and remain 12-14 C in parts of the west. This wouldn't have to change too much to become like a notable warm spell under high pressure in March of 2012. But that may be a bridge too far for this pattern, eventually it looks like a rather variable regime will return with weak frontal systems, and transient highs peaking for a day or two, to provide a fairly average sort of outcome for the last third of March.