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[ Level one alert ] -- Heavy thunderstorm rainfalls in Connacht and south Ulster later, overnight

  • 08-09-2021 7:25am
    Registered Users Posts: 13,921 ✭✭✭✭M.T. Cranium

    With the new format I don't see how to create the usual tag for alert level so I improvised. I also feel this could be more of a localized level two situation with a wider level one impact.

    Frontal troughs undercutting the warm, humid air mass later today into tonight will create clusters of heavy thundery showers, already somewhat evident on radar to the west of Ireland now. This activity may spread into western counties during the day and join with other rather disorganized clusters that could form over the midlands during the warmer part of the day.

    The GFS model shows a local rainfall maximum quickly reaching 75 mm after heavy rain starts this evening, over north Connacht (specific location is near the mutual borders of Mayo, Sligo and Roscommon counties). There's an axis of heavier rainfall depicted from that peak value into south Ulster where 20-40 mm seems likely and the UKMO has already issued a heavy rainfall advisory for Thursday.

    Most of the heavier rain I am expecting would come overnight into Thursday morning although there could be some preliminary heavy downpours in some places during the day today.

    Just leaving this as a foundation for a discussion thread that mods may wish to alter (in part because of the question of the level designation shown in the thread title). A rainfall of 75 mm in 18-24 hours would certainly cause some major localized flooding; the problem is that the GFS model could be accurate in that amount but a little off in its placement, and almost any movement of the rainfall maximum in any direction would place it over more populous areas of Connacht than it is shown to be developing over at present time. If that much rain fell on any given town in the region it would certainly lead to severe flooding locally. So the hope would be either the GFS has over-cooked this or has the position correct, I had a look on google earth and can see one drainage system leading north towards Ballina that should be watched if this comes to pass. My hunch is that it will actually happen a few miles further south or west which could be problematic for places along the Mayo coast or in north Galway.


  • Registered Users Posts: 10,196 ✭✭✭✭igCorcaigh

    Yellow thunderstorm warning from Met.

  • Registered Users Posts: 13,921 ✭✭✭✭M.T. Cranium

    So far a few heavy and possibly thundery showers have dropped 5-10 mm amounts, with 2-4 mm an hour rates noted at a number of locations.

    The 12z GFS has shifted the rainfall max further south and west, placing it over the coast from Galway around to Newport and beyond. Amounts for the previous max location have been dropped to only about 5 mm, with 40-50 mm amounts now shown (in addition to what already fell, totals of 50-60) in the new maximum zone. South Ulster has also been downgraded. Central Kerry is also shown with potential for 30 mm.

  • Heavy, heavy rain unloading in Galway City now, like a cloudburst but just going on.