Heavy rainfall breaks October record

MET ÉIREANN:MONDAY'S RAINFALL, which caused so much disruption to Dublin, was an October record, according to Met Éireann.

A total of 82mm of rain (more than three inches) fell on Monday at Casement Aerodrome, one of Dublin’s main weather stations. The average monthly rainfall is about 65mm.

Not only is it an October record, but it is also an event likely to occur only once every 20 years.

Much of the torrential downpours were concentrated in a six-hour spell in the afternoon between 2pm and 8pm when 65.7mm of rain fell. That is a once in every 80 years event, according to Met Éireann.

The 23mm (almost an inch) of rain that fell between 4pm and 5pm was the greatest amount of rainfall ever recorded at the aerodrome in a single hour since records began in 1954.

The stations at Dublin airport and the Phoenix Park also had significant daily rainfall totals of 69.1mm and 71.3mm respectively. These too are rainfall events only likely to occur every 20 to 25 years.

Though the rainfall on Monday was huge, it was not a record for Dublin. More than 180mm was recorded on June 11th, 1963, during a violent thunderstorm in the Mount Merrion area of south Dublin. Exactly 30 years later on June 11th, 1993, more than 100mm of rain fell in a 24-hour period in the Dublin area.

Fortunately, there is no repeat forecast of the monsoon-like conditions. Today and tomorrow will see outbreaks of drizzle. Rain will push in from the west on Friday and it will feel cool with fresh southerly winds. The forecast on Saturday and Sunday is for some wind and rain, but nothing out of the ordinary for this time of year.

The heaviest rainfall on Monday was nearly all concentrated in the Dublin and Wicklow areas, though there was heavy rain elsewhere along the eastern seaboard. Johnstown Castle in Co Wexford recorded 22.4mm of rain and Dunsany in Co Meath 37.8mm.

Met Éireann spokesman David Rogers said a combination of factors caused more than a month’s worth of rain to fall in a short period in Dublin. Ireland was at the centre of a slow-moving frontal depression which stretched from western France to south of Iceland.

The Wicklow mountains caused a process called orographic uplift when moist air is forced to rise by a mountain barrier, condenses and then falls as rain. There was also a convergence of isobars, indicating atmospheric pressure, off the east coast which also caused moist air to rise and fall as rain. “The whole lot conspired to produce excessive rainfall,” Mr Rogers said.

Though most of the attention has been focused on the east, the west too experienced heavy rainfall over the week. Met Éireann’s station at Valentia in Co Kerry, which is traditionally the wettest in the country, experienced 92mm of rain on Saturday and Sunday, amounting to the equivalent of almost four inches.

Over the weekend Belmullet in Co Mayo experienced 42mm of rain, Cork airport 40mm and Shannon airport 30mm.

October has been one of the most dramatic weather months in recent years with near record temperatures at the start of the month, followed by a cold snap and then by the heavy rains.