-- Temperatures will average 4 to 5 deg below normal, with a slight moderation likely near the end of the week.
-- Precipitation estimates are rather uncertain as most of the liquid equivalent will be late in the period and details about the "breakdown" phase of the cold spell are quite uncertain; my estimate is that near average amounts of precipitation are likely overall.
-- Sunshine will probably not reach average mid-February amounts in very many locations, some western counties may approach those (2.5-3.0 hours a day). The average further east may be quite low due to frequent cloud cover.
FORECASTS
TODAY will begin to feel considerably colder as the first outer edges of the impending arctic air mass push across the Irish Sea on moderate northeast winds 50 to 70 km/hr. Skies will be mostly cloudy with a few breaks most likely towards the west coast. Some bands of mixed wintry showers will form and hail could be one of the more frequent components, along with cold rain, ice pellets and snow on some hills. Highs today only 3 to 6 C. It may be somewhat more moderate in parts of west Munster which will be feeling the full effects somewhat later.
TONIGHT will continue mostly cloudy with mixed wintry showers, cold with northeast winds 40 to 70 km/hr. Lows will be in the zero to 2 C range, feeling more like -3 C.
SUNDAY will be windy and cold with scattered wintry showers, as the colder air will not be fully assembled over Ireland until late in the day, the mixture of showers will be considerable and will take various forms, but some snow could begin to fall especially on higher terrain and in Ulster. At this point, storm "Darcy" as it has been named by various met services will be dropping potentially large amounts of snow, sleet and mixed wintry precipitation over southeast England, on higher ground there could be as much as 30 cms. The precipitation to hit Ireland during the cold spell is probably not directly related to "Darcy" but I'm sure the general public will think of the entire event as related if not integrated with Darcy, so having made that comment I am going to avoid talking about Darcy after today since I only consider the southeast England outcome really the direct product of this storm. Many other factors are going to be involved in what happens from late Sunday to the end of next week.
MONDAY will become very windy at times, especially near the south coast, as a disturbance begins to move into France from the Atlantic by afternoon or evening. The day may start out fairly dry although quite cloudy and very cold, with a gradual increase in the coverage of westward-moving snow streamers. This passing storm's strong wind gradient will mean that any snow or sleet streamers that hit the south coast (around Cork) will be wind-driven and potentially quite stormy in that region, as winds could reach 70-100 km/hr, especially towards the overnight hours into Tuesday morning. ... The east coast will be seeing winds of 50-80 km/hr and bands of wintry precipitation gradually increasing in their extent, probably more snow than any other forms, but sleet or hail are possible, along with ice pellets and thunder. These bands will be both sporadic and shifting around in their locations, the net effect for most places in Leinster and the midlands will be intervals of snow or sleet and some brighter spells, accumulations of 5-8 cms will be fairly widespread, and 10-15 cms possible (especially on higher ground). Parts of the north will likely have more continuous light to moderate snow in east winds of 40 to 70 km/hr. Temperatures in all regions will be in the range of -1 to +2 C and feeling like -5 C in the strong winds.
TUESDAY will probably see a repeat of these conditions, or a continuation of them. The tendency will be for conditions to deteriorate gradually on Monday especially towards afternoon and evening so that if there is any respite overnight it could quickly become stormy again on Tuesday morning, with temperatures steady around -1 C (possibly near +1 or +2 near some exposed coasts). Further scattered accumulations of 5 to 15 cms of snow are quite possible in many areas including the south coast, most of Leinster and Ulster, parts of south Connacht and more hit or miss in Munster away from the south coast snowfall area near Cork. There are chances that some areas will get heavy snowfalls and severe disruption although this is not a certain outcome (at least I don't consider it to be as certain as in storm Emma's rampage in the southeast in 2018). Hopefully later forecasts will give you more precise details and timing on disruptive snow events. At this admittedly early point in the preparations, I would say the best guess is localized maxima around 20-30 cms, more widespread 5-15 cm, and some lower accumulations even within the snowfall zones, with localized zero snowfall outcomes in the west.
WEDNESDAY could see some continuation of these harsh wintry conditions although it may begin to abate somewhat depending on the details of a gradual warming trend that could begin to show itself in the south by later Wednesday. There seems to be a fairly high risk of snow or sleet developing after mid-day Wednesday but before that starts, a more stable atmosphere and a shift to southeast winds could cut off a lot of the streamer activity for a time. Also temperatures could begin to edge back up into the mixed precipitation zone (2-5 C). Ulster is the most likely region to see significant warm frontal snows from late Wednesday into Thursday.
By THURSDAY this developing warming trend will probably begin to get considerably more organized and fronts will be advancing slowly but surely north and east, turning snow or sleet over to rain in southern then central areas, as temperatures rise slowly in the range of 3 to 7 C. Winds southeasterly 40 to 70 km/hr as a rough estimate (the details may turn out more intense however).
The warming trend seems fairly robust on most guidance although I noticed the Canadian model was changing its theme from warming to a stalling out of moderate cold southeasterly winds and the chance of reloading the severe cold into the following week. I think that option is probably fairly unlikely to verify unless we see other guidance starting to trend in that direction, for the time being, the most reliable models are showing quite a strong warming trend that pushes temperatures up above 10 C eventually around the weekend of 13th-14th.
So the summary would be this -- a gradual entry into the coldest air of this winter season, with full entry into the cold achieved by Sunday night into Monday morning from east to west, then increasingly harsh conditions likely reaching their most intense around Tuesday, followed by a gradual withdrawal from the worst of the chill and snowfall but perhaps with intervals of warm frontal snow to round out the wintry interval around late Wednesday into early Thursday (this part less certain at this more distant point in time). And any snow on the ground by that point will likely just melt away fairly gradually on Friday into Saturday due to milder temperatures and a little rain, hoping that the breakdown does not also feature heavy rainfall because that could produce a new flooding risk. (this does not appear all that likely given the sort of mild, dry pattern that is signalled for the weekend of 13th-14th).
As to the geographical extent of disruptive snow, I am pretty sure we're going to end up with a patchwork quilt sort of outcome of various snowfall amounts as streamers come and go but tend to favour certain locations over others, but it should be underscored that parts of Cork need to be on alert for their own somewhat separate event due to the occasionally good potential produced by these winds between east and southeast at various times, whether this amounts to widespread light snow or localized heavier snow, it will be rather unusual for that part of the country. Central Leinster is probably somewhat more used to snowfalls and certainly they are reasonably frequent in parts of Ulster. The northwest will be more like a hit or miss zone of either no snow or some snow, as there's a lot of terrain to cross and the Irish Sea is rather narrow at that latitude for production so that it might take streamers hugging the north coast as cold air pours out of Scotland to get snow into some parts of Donegal for example. Snow could fall almost anywhere in this spell but if you had to guess places that would escape it, look for local examples of subtropical vegetation, it tends to grow in places that avoid wintry weather a lot.
PETER O'DONNELL for IWO