The Meteo Times Update RE: Snow Potential Sunday/Monday (20/21 Jan 2013)


We are now looking at both a battle of air masses and a battle of weather models over the Sunday-Monday time frame. In other words, it's a forecasting nightmare but here is our interpretation as of this evening (Wednesday, 16 Jan 2013).

Snowfall would likely be elevation-based across the south with sleet closer to sea level, potential for 15-30 cm snow on higher slopes of Wicklow and Dublin, and 5-10 cm other parts of the south. Large parts of the north would miss the main effects of this system but could see streamers of mixed wintry showers coming inland on strong NE winds by Monday.

The below air pressure chart from the ECMWF (European) forecasting model this evening showing a deep low pressure system tracking eastsoutheast along Ireland's south coast. As it tracks ESE it will draw colder air across Ireland from Britain. Such a system is often referred to as a 'Slider Low' (see second image below).
Image c/o
A 'Slider Low' is explained in this illustration from British-based forecaster Matt Hugo.
According to TMT Senior Forecaster Peter O'Donnell: "It is a 3-1 advantage for the colder solution among the four models that I consider most reliable (most reliable recently in this order, ECM, GEM, UK, GFS). Only the GFS fails to develop a strong low on a southeast heading track Sunday-Monday. A blend of the other three would bring snow potential to most of the southern half of the country Sunday night into Monday morning.

"That might turn out to be elevation based but certainly the potential is there for 20-30 cm falls on slopes above 150m in the southeast where orographic uplift will help boost totals. About 5-10 cm potential elsewhere but with mixing issues for places like Cork and Waterford. Some parts of eastern Northern Ireland could be added in due to local streamers outside the storm precip shield but most of the guidance keeps Connacht and west Ulster dry. There again they could see a bit of snow during the period as winds back from ESE to NE.

"If the odd man out GFS (American forecasting model) is right, nothing much will happen and Ireland will stay in 3-5 C or milder with light rain and just a slight potential for snow on hills.

"Also most of the model consensus shows a deepening of cold by Wed-Thurs of next week with good potential for east coast snow streamers developing. The key to the solution I feel is the intensity of cold at present over Britain, this is setting up to enter the circulation whenever the pressure gradient backs around enough to pull the colder air across the Irish Sea. The Friday early morning frontal system fails to change the balance but then it dies out over western Britain and possibly this would begin to edge back west as a back door cold front that would slowly wrap around the approaching Atlantic low.

Nothing too clarified yet but potential is there for snow on Sunday-Monday, heaviest falls would be around 150-300m elevations on east slopes of Wicklow and Dublin Mtns so southern higher suburbs of Dublin could see perhaps 10-15 cm while it could be a sleety mix at sea level there. In a circulation like this projected low you would only need about -5 C at 850 mbs and 528 dm thickness to get snow," Peter concluded.