by Meteorologist Matthew Martin on 08/02/2021 . This article is taken from our Meteorologist’s Commentary page.
Monday-Wednesday: Risk of snow showers, frost and ice
A cold easterly air-flow has now become established over Ireland and over the next few days the risk of snow showers will increase with eastern counties at greatest risk. The risk of snow accumulations will increase tonight (Monday night), through tomorrow (Tuesday) and for a time on Wednesday before the main focus for snowfall risk turns to the attempted frontal breakdown from the Atlantic on Thursday and Friday.
Sea-effect snow showers
The risk of wintry showers over the next few days will be driven by what is termed the ‘sea-effect’. Similar to lake-effect snowfall that is common place in North America during winter, sea-effect snow is a term used to describe the mechanism by which showers develop over the sea during winter. Sea/Lake effect snow occurs when cold polar or arctic air flows over relatively warm water, generating convective bands of showers downwind from the body of water. These shower bands can at times become quite narrow and intense giving significant falls of snow in very localised areas whilst some adjacent areas receive very little snowfall. Convective snow showers or bands are enhanced by lift due to frictional convergence and orography over the leeward shore and other factors such as wind fetch play a critical role in determining the intensity and extent of the sea-effect snow showers or bands. In Ireland, the Irish Sea is often the origin for many of our wintry showers during an easterly or north-easterly air-flow, however, they can also form in northern sea areas and in the Celtic Sea.
Typically, the temperature difference between the water surface and the lower atmosphere (850hPa) must be at least 13°C to support convective sea-effect shower formation over the Irish Sea. At present sea-surface temperatures are around +8°C whilst forecast 850hPa temperatures are around -10°C, resulting in a 18°C differential which should be sufficient to create some snow showers over the coming days.
Areas at greatest risk of seeing snowfall are highlighted in the white/green shades in the ECMWF forecast charts below. However, this is a rough guide and showers are likely to be more widespread than highlighted in these graphics.
Thursday & Friday: Frontal snow risk
On Thursday, frontal rain-bands in the Atlantic are forecast to move towards Ireland and as they encounter the cold air in situ over Ireland further snowfall is predicted. However, there is some uncertainty as to how far these frontal bands track north-eastwards over Ireland as high pressure over Scandinavia is dominating and acting to weaken these fronts as they move into the cold air. However, current projections indicate a spell of sleet and snow spreading north-eastwards over Ireland on Thursday and into Friday bringing accumulations of snow to many areas. However, the snow may transition to rain in some southern and western parts as milder air attempts to move in from the Atlantic.
At time of writing, a weather advisory is currently in operation and weather warnings are likely to be issued in the coming days.
As always, you can stay up to date with Met Éireann’s weather and warnings by monitoring the following pages on our website and please sign up for weather warning notifications on your smartphone Met.ie app.
Wintry Cold Spell from Feb 7th 2021
By Meteorologist Liz Walsh, 4th Feb 2021
Our weather is expected to turn progressively colder from early on Sunday 7th February, with the cold spell now looking likely to last well into next week. The origin of the cold air over Scandinavia will bring a northeasterly or easterly component to the wind-flow over Ireland. That wind will be especially noticeable on Sunday and Monday with gusty east to northeast winds likely to produce a significant wind-chill factor.
Daytime temperatures are expected to struggle in the low single figures, generally 1 to 3 degrees Celsius, which means any frost or ice that forms at night will be slow to clear, if it clears at all, so the risk of icy roads and footpaths will increase.
Snowfall is expected in the form of showers from Sunday onwards, but more especially later on Monday and during Tuesday. Any showers are likely to be restricted to eastern parts of Leinster and Ulster initially, but a few may push further inland during Tuesday.
There is still a high degree of uncertainty in the forecast from Wednesday 10th February onwards, but it looks as though Atlantic frontal systems will attempt to push milder air in from the south to displace the cold air in situ over Ireland.
It is still too early to predict how energetic or vigorous these frontal systems will be, and that has a downstream effect on the impact that this clash of mild and cold air may have over Ireland. If the frontal feature is vigorous, for example, it would likely make quick progress northeastwards over the country and any associated snowfall would be a transient affair with rain following behind. However, if the frontal system has less energy as it comes towards us and starts to become slow-moving either on its approach or as it tracks over us, we could end up in a situation where some parts of Ireland could see more significant and lasting snowfall.
Whatever the case snow-ice and/or low temperature warnings are likely in the coming days for conditions from Sunday through Tuesday, and forecasts for the latter part of next week will be updated and fine-tuned as we move closer to the time.
As always, you can stay up to date with Met Éireann’s weather and warnings by monitoring the following pages on our website:
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