TRENDS for the week of 29 Dec (2020) to 4 Jan (2021)
-- Temperatures will average 3 to 4 deg below normal, with a generally colder trend through the period.-- Precipitation will be fairly light for this time of year, but a portion of it will be wintry, so it may be significant more for disruption than moisture content.
-- Sunshine will average 25 to 50 per cent above normal.
FORECASTS
TODAY will continue quite windy although the stronger winds will begin to abate this afternoon and evening. Until then, winds northwest to north 50 to 80 km/hr with some higher gusts. Bands of showers will mix to sleet or wet snow on higher terrain, cold rain or hail at lower elevations. Highs 3 to 6 C, feeling more like -2 C in the winds.
TONIGHT will see partial clearing over the north and east, with a sharp frost likely, lows near -3 C. Overcast in the south and west, with some drizzle or sleet moving inland from the Atlantic late overnight, covering parts of south Connacht and west Munster. Lows around zero Celsius in this overcast area (+3 C outer headlands of Kerry, south Cork).
WEDNESDAY the band of precipitation may begin to produce snowfalls of 1-3 cm over parts of south/central Connacht, the midlands, and south Leinster, parts of inland east Munster. There is potential for this to increase to a higher amount and if I see justification for that I will update the forecast this evening. Even with this fairly small amount of snow, mixing with sleet and patchy freezing drizzle, roads may become icy especially untreated or lightly traveled routes. It will likely stay dry from around Dublin north into north Leinster and most of Ulster. Any change in the track or intensity of this system could bring snow further north however. Highs will be only around 2 C in many areas, light east winds to the north of the track of the low, and variable mostly westerly breezes in the milder sector, where temperatures (affecting most of Munster at some point if not all day) will reach 5-7 C with light rain.
THURSDAY will be back to partly cloudy and windy conditions with temperatures between -3 C for overnight lows and +4 C for daytime highs, scattered wintry showers are likely, along with fairly generous sunny breaks too.
(New Years eve around midnight it's likely to be -2 to +1 C, partly cloudy, a few wintry showers around)
FRIDAY (New Years Day) will also be partly cloudy and cold with scattered snow showers -- it seems like the freezing level will be very slowly descending to levels where snow could begin to fall at or near sea level, especially in any vigorous showers where the rate of fall doesn't allow time for the falling snow to melt. Lows near -4 C and highs of about 3 or 4 C.
The OUTLOOK continues to suggest a long cold spell with more and more anticyclonic and easterly influences, both of which would argue for slightly colder temperatures, and in Leinster, also more chance of significant snowfalls. I would say we're maybe one turn of the dial away from something memorable with this setup but on the other hand if the dial is turned the wrong way it could all end up seeming rather ordinary too. For winter weather lovers, at least we can say we're in with a chance this winter. And of course, probably 80% of memorable winter weather events tend to happen later than this, December 2010 was an anomaly in that regard. Late January to mid-March seems to be the part of the winter that produces more often than others, historically. It no doubt is related to colder ocean temperatures and the expansion of the arctic ice pack, but those are factors that have become weaker in recent decades, so we're fighting against the trends to get anything really memorable nowadays. (Late Feb 2018 showed that it can be done, however).
-- Peter O'Donnell is IWO's Senior Forecaster.