Many of us have heard about the extreme rainfall of August 1986 that was the result of ex-hurricane Charley and also the lesser known though still reported every now and then, 5th August 1986 deluge. However, you probably have not heard about the washout of 24th/25th August 1905.
On its own merits, August 1905 was a very poor summer month with below average temperature, well above average rainfall and below average sunshine. It followed on from a relatively warm July (CET of 17.2c) with a CET of only 14.7c. Phoenix Park had a mean temperature of only 13.6c compared to a typical August nowadays (1981-2010) which has a mean of 15.2c. The number of days with rain was high with up to 26 such days at Valentia Observatory and even Phoenix Park observed 24 days with rain. Roches Point observed the least with 19 such days.
The month was disturbed with Ireland being threatened by deep depressions on numerous occasions. The first of these depressions pushed up from the southwest on 4th August 1905. Valentia Observatory observed a minimum pressure of 982mb from this depression which is quite significant for the time of year although Belmullet would see a record low MSLP of 968mb in August 1959. This low would weaken and fill day by day along with its associated troughs but continue to meander around Ireland bringing further bouts of showery rain.
A ridge finally pushed up from the southwest by the 12th. This would bring a fine few days with reasonably sunny days though cloudy in the north at first. This would result in the only "warm" weather of the entire month. Birr Castle peaked at 23.5c on the 16th.
Westerlies came back with a bite from the 17th onwards with the high pressure collapsing. There was frequent showers and cool days with maximum temperatures stuck in the mid-teens at best as the westerly airflow brought air masses to Ireland from the North Atlantic.
On the 23rd, low pressure made its way eastwards across the UK and Ireland with high pressure from the Azores ridging to the southwest of Iceland. This forced another low pressure in the Atlantic to undercut to the southwest of Ireland. Through the next few days, this low would slowly progress eastwards across the south of Ireland as it got blocked somewhat by ridging over Europe. There was plentiful precipitation associated with this low.
This would become one of Ireland's worst "rainstorms" in recorded history. It resulted in a destructive flood at Bray according to UK Met Office report which is no surprise with such an exceptional rainstorm. Find a 28th August 1905 Irish Times extract below on Arklow streets being flooded along with a little report on the Shannon overflowing from the rain too:
Some of the highest rainfall totals recorded at Irish stations during the period:
Bray, Fassaroe, Co. Wicklow with 114.3mm on the 25th
Newcastle, Co. Wicklow with 104.4mm on the 25th
Dublin City with 87.4mm on the 25th
Phoenix Park, Co. Dublin with 85.1mm on the 25th
Trinity College, Co. Dublin with 83.8mm on the 25th
Clongowes Wood College, Co. Kildare with 74.4mm on the 25th
Roches Point, Co. Cork with 50.8mm on the 24th
Birr Castle, Co. Offaly with 49.3mm on the 25th
These totals are comparable to the exceptionally wet days of 25 August 1986, 11 June 1993, 9 August 2008, 24 October 2011 and 2 August 2014 in the east.
When the low that brought the intense rainfalls cleared, another one pushed in from the Atlantic on the 27th into the 28th bringing more rain to places that didn't need it.
This low would progress eastwards on the 29th and allow a northerly flow bringing cool air but drier weather too.
Data from the UK Met Office.