Weather Advisory - Heavy Rain/Hill Snow Risk For Ireland, Tuesday Night


Tuesday night's band of precipitation is throwing up some unusual forecasting difficulties regarding whether it will fall as rain or something more wintry. The usual atmospheric indicators are dismissing it as a rain event, but these can be misleading in certain setups, and tomorrow night qualifies.

As most of the population lives below 200 m above sea level (find out what altitude you live at here) it will be just another band of rain, with maybe the odd sleety bit thrown in. But above that we could see some extensive snow for a time tomorrow night through the east midlands and the Wicklow mountains. This will increasingly turn to rain toward dawn on Wednesday morning.

Meanwhile, rainfall totals of 15-20mm on Tuesday evening through to early Wednesday in West Cork, Kerry, West Limerick, Clare and west Connacht may also lead to some spot flooding overnight Tuesday in light of the already saturated ground here.

TMT Senior Forecaster Peter O'Donnell commented: "Snowfall potential Tuesday night will be about 5-15 cm on higher slopes in the inland southeast, mainly above 250m but some could see snow mixing in with rain on Wednesday morning, and meanwhile a greater potential for more widespread snowfall exists on Friday if the approaching Atlantic frontal system meets with enough resistance from cold air likely to be well-established over Britain and the North Sea. This is realistically about a 50-50 proposition for snow at lower elevations on Friday and we'll have a better idea by mid-week."

Meanwhile, TMT synoptics analyst Fergal Tierney added: "Tomorrow night is another occasion where looking at 500-1000 dm and the 850 level conditions could lead you astray and knock you off your guard. In the east during tomorrow night's precipitation, 500-1000 thickness will be as much as 540 dm. Going on that alone you would say absolutely no chance of snow. However, looking at the GFS forecast sounding for near Dublin (below image) shows that this is a very misleading parameter as there is a huge moist inversion from around 850 - 600 hPa. This large layer of warm and moist air is reducing the density of the layer and adding a disproportionate contribution to the thickness value, but if we look at the boundary layer - the lowest layer of the atmosphere, where snow lives and dies - we see that it is much colder than those upper temperatures would suggest, going on normal atmospheric lapse rates, and is in fact almost isothermal for the lower 500 m. It is also relatively dry for the most part, which will aid in evaporative cooling of the air as the snow falls."

Fergal continued: "Also looking at the 850 hPa level we can see how disproportionately warm that level is relative to the boundary layer. Its temperature, theta-e and theta-w values of -3, +15 and +3 °C would scream snow only above around 400 m, but again this totally disregards the cooler boundary layer and what is going in within it. So I reckon that tomorrow night's precipitation will fall as snow above around 150 - 200 m in the east to begin with, with this snowline rising as the warmer air moves a little further eastwards. The front will be dissipating as it stalls somewhere over the east coast so precipitation will die away during the day Wednesday."
GFS Forecast Sounding for Dublin area, 2100 Tuesday 15th January.
The red curve is temperature, the blue line is wet-bulb temperature, and the cyan curve is dewpoint. Image c/o
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