The Origins Of Tomorrow Night's Snowfall

iWeather Online's synoptics expert Fergal Tierney reports: "A look at the 1800GMT Airmass RGB satellite picture (arctic airmass is visible in pinky purple hues) shows the cold front lying along the south coast of Ireland, and behind it a large area of snow showers feeding into the north and west. 

These showers are dissipating as they move inland, so much of the east and southeast remain relatively dry. Further north, an area of more enhanced cloudiness can be seen stretching from northern Scotland to northeast of the Faroes Islands. I have circled what seems to show the development of a polar low, and that will need to be watched as it moves southwards towards Scotland overnight. There are also further such features to the north of this (not shown).

Off the southern tip of Iceland, there is a visible kink in the isobars. This is due to lee effects as the strong northerly airstream flows over the terrain, causing a reduction in surface pressure (Lee Cyclogenesis). This is an area of interest to us as it is this process that will generate the low pressure system that will bring organised snowfall to Ireland tomorrow night. There is a strong northerly jetstreak over Iceland at the moment, and this will cause this lee effect to grow and form a deep trough to our northwest during tomorrow. 
The NAE model shows an associated potent little low pressure centre developing on it and moving into the northwest during the late evening, bringing organised bands of snow and strong winds. This could be a serious issue to those on the roads as it moves southeastwards during the night, so take care if you're one of those, and watch out for further updates on IWO."
2100GMT Friday, NAE