Recent Decembers in Ireland were mild, unsettled and Stormy. Anticyclonic conditions dominated 2016. Most stations reported below Long-Term Averages (LTA) for rainfall and above for temperatures. 2017 was unsettled and mild. Storm Dylan was the main feature with monthly rainfall and temperature totals above average and sunshine totals below average. 2018 was mild and unsettled with Storm Deirdre affecting Ireland. Many stations were near normal for rainfall totals but above in the South. Air temperatures were above LTA and sunshine totals below. 2019 was mild, bright and breezy. Storm Atiyah on the 8th and 9th and Storm Elsa on the 18th brought windy, wet conditions and coastal flooding. Monthly rainfall totals varied, temperature and sunshine totals were above LTA. 2020 was cool, wet and windy. Storm Bella brought windy and wet conditions on the 26th and 27th.. Rainfall and sunshine totals were above their LTA and temperature was below in most places.
Mild, changeable and windy at times
December was mild with low-pressure dominating the first and final thirds of the month, bringing above average rainfall. The middle third of the month was relatively dry with high pressure in control. The first week saw a mild, mostly westerly airflow bring several Atlantic weather fronts across the country with widespread rain or showers interspersed with clear spells. The week finished with a powerful mid-latitude storm, named storm Barra by Met Éireann, slowly crossing the country from west to east on Tuesday 7th. This brought widespread heavy rain, gales and storm force winds, which continued into Wednesday 8th. Low-pressure to the north and high pressure to the south maintained a mostly westerly airflow up to the 15th, with the high pressure to the south building north and gradually gaining more control. This brought mostly dry conditions between the 13th and 21st as the area of high pressure built over Ireland and slowly moved to the north. There were a few sunny breaks but also a lot of cloud with mist, fog and some frost at night. The weather broke on the 22nd, with the rest of the month dominated by low-pressure to the west of Ireland pushing active weath-er fronts over the country from the south. The rain was especially widespread and heavy on Wednesday 22nd, Saturday 25th (Christmas day) and Thursday 30th, with the South, East and Midlands receiving the bulk of the rain. It remained mild throughout this period.
Rainfall: Above average in most places, wettest in the South and East
The majority of monthly rainfall totals were above their Long-Term Average (LTA). Percentage of monthly rainfall values ranged from 79% (monthly rainfall totals of 108.1 mm, 98.8 mm and 115.6 mm) at Belmullet, Co Mayo, Claremorris, Co Mayo and Mace Head, Co Galway respectively to 166% (monthly rainfall total of 181.9 mm) at John-stown Castle, Co Wexford. Monthly rainfall totals ranged from 76.4 mm (101% of its LTA) at Casement Aerodrome, Co Dublin to 188.1 mm (114% of its LTA) at Valentia Observatory, Co Kerry. The highest daily rainfall total was 58.9 mm at Johnstown Castle, Co Wexford on Christmas day, Saturday 25th (its highest daily fall for December on record (length 80 years)). The number of rain days ranged from 15 days at Casement Aerodrome, Co Dublin to 26 days at Cork Airport. The number of wet days1 ranged from 10 days at Oak Park, Co Carlow to 21 days at a few stations. The number of very wet days3 ranged from 2 days at a few stations to 6 days at Johnstown Castle, Co Wexford.
Temperature: Above average everywhere
All mean air temperatures across the country were above their Long-Term Average (LTA) for the month. Deviations from mean air temperature ranged from 0.8 °C (7.2 °C mean temperature) at Malin Head, Co Donegal to 2.0 °C (7.0 °C , 7.5 °C mean temperature) at Claremorris, Co Mayo and Finner, Co Donegal respectively. Mean tempera-tures for the month ranged from 6.1 °C (1.8 °C above its LTA) at Knock Airport, Co Mayo to 9.6 °C (1.5 °C above its LTA) at Sherkin Island, Co Cork. The month’s high-est temperature was reported at both Phoenix Park, Co Dublin and Casement Aerodrome, Co Dublin both on Friday 31st with a temperature of 14.6 °C. The month’s lowest air and grass minimum was recorded at Mullingar, Co Westmeath with the lowest air temperature reported on Wednesday 13th with -2.1 °C, while the lowest grass minimum was -5.8 °C reported on Tuesday 14th. More than half of stations reported ground frost. The number of days with ground frost ranged from zero days at Roche’s Point, Co Cork to 19 days at Phoenix Park, Co Dublin. Less than half of stations reported air frost. The number of days with air frost ranged from zero days at a few sta-tions to 4 days at Casement Aerodrome, Co Dublin. Seven stations had their highest minimum temperature for December on record (record length ranging from between 11 and 60 years).
Sunshine: Sunny in the South and East, dull in the North and Northwest
Percentage of Long-Term Average (LTA) sunshine values were variable across the country. Percentage of monthly sunshine values ranged from 119% (monthly sunshine total of 53.5 hours) at Shannon Airport, Co Clare to 126% (the month’s highest monthly sunshine total of 60.4 hours) at Casement Aerodrome, Co Dublin. Monthly sun-shine totals were lowest at Malin Head, Co Donegal with 26.6 hours (No LTA comparison*). The highest number of daily sunshine hours recorded this month was 6.8 hours at Shannon Airport, Co Clare on Tuesday 14th. The number of dull days ranged from 10 days2 at Valentia Observatory, Co Kerry to 20 days at Johnstown Castle, Co Wexford.
Wind: Violent storm force 11 winds reported during Storm Barra
Monthly mean wind speeds ranged from 6.4 knots (11.9 km/h) at Ballyhaise, Co Cavan to 16.7 knots (30.9 km/h) at Mace Head, Co Galway. Gales were reported on 13 days during the month with strong gales reported on the 4th, 7th, 8th, 12th, 24th, 25th and 29th. Mean wind speeds reached storm force on Tuesday 7th during storm Barra at Valentia Observatory, Co Kerry, Roches Point, Co Cork, Sherkin Island, Co Cork, Malin Head, Co Donegal and Mace Head, Co Galway. Mean wind speeds also reached storm force on Wednesday 8th at Mace Head, Co Galway. Mean wind speeds reached violent storm force 11 on Tuesday 7th during storm Barra at Mace Head, Co Galway. The number of days with gales ranged from zero days at a few stations to 10 days at Malin Head, Co Donegal. The number of days with up to strong gales ranged from zero at a few stations to 6 days at Malin Head, Co Donegal. The number of days with storms force winds was up to 2 days at Mace Head, Co Galway. Both the month’s highest gust and 10-minute mean wind speed were reported on the 7th during storm Barra. The highest gust was 73 knots (135 km/h) reported at Sherkin Island, Co Cork (its highest maximum gust for December on record (length 17 years)), while the month’s highest 10-minute mean wind speed was 56 knots (103 km/h) at Mace Head, Co Galway.
Pressure: Storm Barra
The minimum mean sea level pressure observed during Storm Barra was at Mace Head, Co Galway with 957.2 hPa on Tuesday 7th. A pressure drop of approximately 50 hPa occurred at Mace Head, Co Galway over a 16-hour period between 20:15 on Monday 6th and 12:15 on Tuesday 7th, when the low-pressure centre made landfall in Galway.
The full report is available here