Ireland And Lake-Effect Snow

This week signals the true arrival of winter as the Atlantic storms of recent weeks make way for Arctic cold from the northeast, writes Fergal Tierney.  

Nights will turn bitterly cold, with severe frosts slow to clear as daytime temperatures struggle into single figures. Showers, which brought snow above 500 metres to the mountains of Wicklow yesterday, will also pop up in north and northwest coasts in the next few days as winds swing around to a north or northwesterly direction. The snowline will lower considerably, but exactly how low is still a subject of hot debate. But why are only coastal areas being affected by these showers, why can't we all share the snow?

The reason is because this is what is called "Lake-effect Precipitation", named after the Great Lakes in the USA, where this phenomenon regularly gives major snowfalls to areas surrounding the lakes. The "lakes" in our case will be the Irish Sea and Atlantic waters off the north and west coasts.

At this time of the year, sea-surface temperatures (SSTs) are still quite warm, ranging from 11-13°C around our coasts.

As this week's Arctic airmass moves southwestwards from Scandinavia, over the North Sea and continues on to the Irish Sea, we have a situation of increasingly cold air above warm water, which is a highly unstable situation. The surface layer of air becomes warmed by the underlying water and rises, producing cumulus clouds as it reaches the Lifted Condensation Level (LCL). 

Depending on several factors, these cumulus clouds may develop further into cumulonimbus as they continue their track across the sea. These cumulonimbus bring precipitation to coastal areas, and may intensify further as they are forced to rise over the higher terrain, dumping considerable falls of rain, hail and snow. This is what has been happening in the east already this week, and will happen in northern and northwestern areas too as the flow becomes more northerly.
Lake-effect cloudiness brings snow to Michigan, USA.
For lake-effect showers to form, there are some conditions that need to be fulfilled:
    * SSTs should be at least 13°C warmer than the temperature at the 850hPa level (~5,000ft)
    * The airmass should extend to at least the 700hPa level (~10,000ft)
    * The surface wind should be at least 10mph and not vary in direction by more than 30° from the 850hPa wind or 60° from the 700hPa wind
    * There should be at least a 30km fetch over open water

At the moment, 850hPa (upper air) temperatures are around -5°C over the Irish Sea, yielding an 18°C differential between this level and the SST (which is just under 13°C). These upper temperatures are still not cold enough to give snow to low levels, but the situation over towards the end of the week becomes increasingly more conducive, with anywhere from -8 to -10°C 850hPa temperatures expected, depending on which model you choose.
(c) Meteociel
850hPA temps. (c) Meteociel
With the windspeed also set to increase, this will reduce the amount of modification that will take place before reaching us, allowing lower Theta-W* (850hPa wet-bulb potential temperature) values a better chance of reaching us and lowering the snowline towards sea level.

All this leads to a high probability of heavy lake-effect showers forming in the Irish Sea and northern Ulster and Connaught towards the end of the week, bringing substantial accumulations of snow on higher ground, less so the lower down you go. It still remains to be seen exactly how much snow, if any, will fall at the lowest levels, and Peter's daily forecast is the place to keep an eye on for this and other detailed analysis nearer the time.

But whatever the snow situation, we're still in for quite a cold blast for at least the next week to ten days.

*Theta-W (850hPa wet-bulb potential temperature) = The temperature that a parcel of air at any level would have if, starting at the wet-bulb temperature, it were brought at the saturated adiabatic lapse rate to the standard pressure of 1000 hPa. It gives an idea of how evaporitive cooling will offset the latent heat of fusion released by snow as it melts, thus limiting the amount of melting that can take place. To be sure of snow at sea level, theta-w should be around -1°C.

Theta-W forecast