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Distribution and synoptic origin of selected heavy precipitation storms over Ireland

John G. Haughton, Michael S. Ó Cinnéide


Six storms with one-day precipitation of 50 mm or more at lowland stations in Ireland were analysed for the period 1968–1973. Each of the storms caused widespread continuous precipitation in some part of the country for a period of one to five days. An analysis of surface and 500 mb weather maps and tephigrams at Valentia and Long Kesh for each period showed that most of the storms accompanied highly meridional upper-air flow, a quasi-stationary front with slow-moving depressions at the surface, and a saturated layer several km deep above the surface near the time and location of heaviest precipitation. These conditions, and their accompanying rainfall, indicate a degree of convergence and lifting not found in most synoptic situations. To verify this precept, the classification of British Isles weather types based on surface airflow was evaluated for each storm day, and it was found that except for the cyclonic type, the common surface airflow types are not generally those which cause heavy precipitation events. Cyclonic flow accompanied the two rainstorms which affected Northern Ireland, while those in central and southern Ireland were mostly caused by complex multi-directional or unclassified flow patterns. The Westerly type, which brings Ireland a large proportion of its total precipitation, did not occur by itself on any of the heavy storm days, a major factor being the rapid movement of most Westerly-type depressions which do not remain over Ireland long enough to cause precipitation of extended duration.

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