WEATHER ALERT Thursday 16 Dec To Monday 20 Dec

Picture by Declan Kearney. 
UPDATE FROM iWeather Online forecaster Peter O'Donnell: After reviewing the most recent GFS weather model output, the only real change to previous forecasts is to upgrade the severity of the outbreak on Friday with a fast-moving reinforcement of the Thursday initial blast. This may give a mechanism for snow showers to reach the east and south at times, rather than keeping most of the initial two days snow potential confined to the west and north.

Beyond that, there is enhanced potential for snowfalls of 5-20 cms through the weekend and Monday.

Our readers in all parts of Ulster, Connacht and parts of West Munster, as well as now north and west Leinster, should be on alert for rapid onset of wintry conditions mid-day Thursday with several waves of heavy snow possible in some areas on strong NW winds. 

Don't be caught napping by the direction here, this air is super-cold and can withstand the Atlantic passage. There are sure to be reports of rain and hail at sea level where this sweeps in, but 90% of residents of the affected region are not at sea level, and even those who are may have to drive places.

I envisage a snowfall map by late Friday with 5-10 cms over a large part of inland Connacht, most of Ulster, and pockets of north Leinster, as well as some inland parts of Clare, Kerry and west Cork. Further east in Munster, it may be more hit and miss but these will be powerful streamers with potential to bring some snow almost across the country, and there will also be troughs embedded to promote some convection. Amounts of 2-5 cms could fall almost anywhere. 

For Dublin, the most likely amount by late Friday would be 1-3 cms coming from a few passing snow showers. Some places will get no covering but maybe fewer than we were speculating earlier.

Friday night and early Saturday could be bitterly cold with clearing skies following this fresh snow. Then outbreaks of snow appear likely through the weekend and these could even include normally temperate south coast regions because a lot of the emphasis will be on bands coming in from the southeast in association with low pressure circling around the west and later south coasts. But the whole country including the NI portions of Ulster are likely to be significantly affected with considerable snow potential extending across Britain also.

I would say the risk of disruptive amounts of snow is increasing with these model runs trending towards a snowy pattern. Will update my perspective after the 12z runs have come out.

Further updates will be issued throughout the day. Watch for updates on Twitter and Facebook and visit the IWO chatboard. For the weather forecast anywhere in Ireland and the UK visit our local weather page.