TRENDS for the week of 22 to 28 Dec 2020
-- Temperatures will average about 2 or 3 deg below normal values. It will be turning much colder near the end of this interval after an earlier cooler period around starting today.-- Rainfall will average 25 to 50 per cent of normal, some precipitation will fall in wintry mixtures.
-- Sunshine will average near normal values.
FORECASTS
TODAY will be a cold and almost calm day with cloud spreading a bit further north to obscure sunshine after mid-day in the north. A northeasterly breeze will pick up in southern counties this afternoon. Some light outbreaks of sleety rain or mixed wintry precipitation will develop late in the day across parts of the south. Highs only 2 to 5 C north, 5 to 7 C south.
TONIGHT there may be wintry mixtures of sleet, snow and cold rain across the south, with the greater likelihood of snow on higher terrain and well inland from the coast. The northern counties will remain partly cloudy to clear and that will allow temperatures to fall back below freezing there, further south lows will be around zero C to +3 C near the south coast. By morning there could be slushy accumulations of snow in some parts of the inland south.
WEDNESDAY will see this mixture of sleet and light snow continuing in parts of Munster and south Leinster, mixing with sleety rain near the south coast, leaving a coating of 1-3 cm in some areas (more likely inland and above 100m asl). It will likely stay dry north of about Clare to north Wicklow, except for any wintry showers that may form over east Ulster and parts of Mayo due to weak bands of streamers. These may move further south later in the day and affect the Dublin region. Winds will increase from light northeasterly to moderate north-northeast 50-70 km/hr, adding chill to temperatures that will be steady in the range of 2 to 5 C. The wintry mix will end rather gradually across the south, by afternoon in most places, but not until evening in Wexford. Some significant snow accumulations are possible on hills in the inland south and southeast (up to 5-10 cms but rather wet and partly melting after falling).
THURSDAY (24th) will see partial clearing from early morning, with morning lows near -2 C. Sunny with cloudy intervals during the day, except where bands of wintry showers form near east coast and north Connacht, west Ulster; local snowfalls on hills may amount to 1-3 cm in places although many areas will remain dry. A sharp frost is likely after sunset on Christmas Eve, following daytime highs of 4 to 7 C.
FRIDAY (Christmas Day) will start out frosty in parts of Leinster and Munster, east Connacht and east Ulster, where lows could be -2 to -5 C. Any frost further west will likely be removed before morning by a slight increase in westerly winds and cloud cover. Hazy sunshine will follow the frosts in some eastern counties, more overcast further west, but generally dry with moderate westerly breezes setting in. Highs near 4 C east to 7 C west, 9 C coastal Munster.
SATURDAY (St Stephen's Day 26th) will become windy with temperatures peaking at a rather early hour (possibly during the night of 25th-26th) at around 7-9 C, then it will begin to turn colder in stages with winds northwesterly at about 70-100 km/hr and passing showers, becoming wintry on hills later in the day as temperatures fall to around 3 or 4 C. Some heavy rain turning to mixed wintry showers may develop in this strong northwest flow, the risk of wintry conditions will be minimal at first but significant by evening.
SUNDAY (27th) and MONDAY (28th) are looking windy and cold with scattered wintry showers, many of them snow but a few of sleet or hail too near coasts, in strong northerly winds 60 to 100 km/hr, which will add quite a chill factor to temperatures barely above freezing in the daytime (1-3 C). Nights will not be exceptionally cold because of this constant strong wind, but could drop to -1 or -2 C inland. At some point around Monday 28th a low may form in this strong northerly flow, reducing the wind speeds as the slacker gradient crosses northern counties, but increasing the risk of widespread sleet or snow.
The OUTLOOK calls for further wintry weather conditions towards New Years with a growing possibility of significant snowfalls, as low pressure areas begin to form in the broad northerly flow providing a focus for more organized bands of snow or sleet at times. Temperatures will be well below average around zero or +1 C in the daytimes and around -3 C at night. Some of the guidance suggests that the cold spell will dig in and persist for some time into January, and there are also some hints of battleground scenarios where Atlantic moisture over-runs the cold air across Ireland leading to more snowfall. Conditions could become quite harsh in isolated higher terrain with blizzard like conditions possibly affecting free ranging livestock. At the same time, I should underscore the fact that there is some uncertainty about details and it seems likely that conditions will be rather variable from place to place in this coming cold spell, not everyone is going to see the same amounts of snowfall for example. The latest guidance looks very wintry indeed at times in January, and there is no trend towards any fast return to milder Atlantic-dominated regimes, in fact what seems more likely is a second strong northerly setting in around 4th January.