Long range weather forecast for Ireland (17 December 2020)

Long range weather forecast for Ireland by Peter O'Donnell.

TRENDS for the week of 17 to 23 Dec 2020
-- Temperatures will average near normal as a generally colder trend develops. The southeast could remain somewhat above average with parts of the north falling below average.
-- Rainfalls will average 50 to 100 per cent of normal, probably most places closer to the 100% top of the range, but there is some uncertainty about how much rain might fall later in the period.
-- Sunshine will average around normal amounts, possibly a bit above.

TODAY will have some brighter intervals at times and it won't be all that breezy, but eventually a moderate southerly wind will set in with rain spreading into the west during the afternoon. Highs 9 to 12 C.
TONIGHT will have variable amounts of cloud with bands of showers following behind the initial rainfall band, about 10 to 15 mm is expected in total. Lows around 7 C.
FRIDAY will continue breezy and rather mild, with further showers, winds becoming southwest 50 to 80 km/hr, and highs 9 to 11 C.
SATURDAY will be partly to mostly cloudy with passing showers, moderate southwest winds, and a bit cooler with lows near 4 C and highs near 7 C.
SUNDAY will be partly cloudy and cool with isolated showers, still rather breezy at times, lows near 2 C and highs near 7 C.
MONDAY presents some uncertainties with low pressure (the same system that is bringing a snowstorm to the northeastern U.S. currently) arrives, and may track either close to the southeast or through parts of central Ireland on a northeastward track. This track will determine whether the outcome is a mild, wet and windy scenario for the south and east, with cooler temperatures but also rain further north and west, or possibly more of a wintry mix with only the southeast getting mild enough to stay in all rain. Watching this closely as model guidance is somewhat split on the exact details. Would expect temperatures to be in the 5 to 10 C range for most regions, but it could be colder than that in some parts of the north leading to mixed wintry forms of precipitation.
TUESDAY sees the colder air taking more control and if a second wave of this storm attempts to track into Ireland, that has a greater chance than the Monday portion to become a wintry mixture, as temperatures could be only in the range of 1 to 4 C in many parts of the country, with winds turning north to northeast. Some snow could be in that mix although sleet seems more likely at this point.
WEDNESDAY will become brighter but staying quite cold, with moderate north to northeast winds, and some chance of wintry showers forming in northeast winds over the Irish Sea and parts of the northwestern counties as well. Lows around -2 C and highs 3 to 6 C.
THURSDAY (24th) will be bright and cold with isolated wintry showers likely, a sharp frost followed by highs only into the 2 to 6 C range.
FRIDAY (Christmas Day) at this point looks very cold with some sunny skies but a risk of localized snow flurries or mixed wintry showers coming in from the surrounding seas. There is some uncertainty at this time range on details, winds could be backing around from northerly to northwest and then west, with an Atlantic frontal system trying to push somewhat milder air back in, but that could lead to some late sleet or snow lasting into part of the 26th. Temperatures look like they might be as cold as -4 C in the morning, to highs near 3 C (but it could be closer to 7 C in a few parts of the south and west).
The model guidance for the past day or so has been "all over the place" on details for the weather scenario between Christmas and New Years. 
Some model runs have brought in persistent cold and a threat of snow, others relent quickly from the cold at Christmas to milder days later as the flow becomes more westerly again. A lot of this is pushed on from the earlier uncertainty about the Monday (21st)- Tuesday (22nd) evolution, the colder that turns out, the more robust the later cold is likely to be. So these questions are linked and the forecast will depend on how the U.S. storm develops once it gets clear of North America by Friday into Saturday. (the low is presently south of New York City).

In the USA, the east coast storm brought a nasty mixture of snow, sleet, freezing rain and rain to various parts of the heavily populated region between Washington DC and Boston, but it's all heavy snow further inland, with reports of 30 to 50 cms already on the ground, while it continues to snow hard in northeast PA, central NY and much of New England (late overnight there now). The snow may continue much of today in those regions so some very heavy falls are likely in some places. Closer to the coast in the larger cities (other than Boston which will see mostly snow), the precipitation has changed over to sleet or a cold rain in some places, and some suburban areas have icing problems. There are numerous highway closures in the region and dangerous travel conditions in general.