In Ireland and the UK, people can start preparing for storm Eowyn. Currently, the low-pressure system is located near Bermuda, but it is moving rapidly northeast while intensifying at an extraordinary rate. In this blog, we will explore how this storm can develop so quickly and what its impact will be, particularly for Ireland.
Storm development: Divergence
The jetstream is a powerful river of wind high in the atmosphere. This river meanders across the globe, driven by the clash between cold air from the polar regions and warm air from the tropics. At the boundary between these two air masses, a strong west-to-east wind develops high in the atmosphere.
The left exit of the jetstream plays a crucial role in the formation of storms like Eowyn. Figure 1 shows the jetstream for Thursday at the 300 hPa level according to ECMWF. The black circle indicates the left exit of the jet stream. The purple colours indicate windspeeds up to 180 kts at 300 hPa. Just as a river can split into different streams, the air at this location diverges. The wind accelerates, and the air begins to spread out in the upper layers of the atmosphere. This process is known as divergence.
Divergence creates a so-called "shortage" of air in the upper atmosphere. To compensate for this deficit, air at the Earth's surface starts to rise. As the air rises, atmospheric pressure decreases, leading to the formation of a low-pressure system. In this case this process happens very rapidly and explosively. When large amounts of air flow divergently at high speeds in the upper atmosphere, the rising air at the surface intensifies, causing the pressure to drop even faster. As long as divergence continues, the pressure will keep decreasing.
This is why storms often develop at the left exit of the jetstream. The divergence of air currents high in the atmosphere is the driving force behind this phenomenon.
What to expect?
Ultimately, the low-pressure system will deepen to a central pressure of approximately 940 hPa. It will reach this pressure early on Friday morning when the low is positioned just northwest of Ireland (see Figure 2). Around this time, the pressure gradient will be at its peak over Ireland. This means that the winds will also be at their most extreme. Virtually all weather models are forecasting a very severe storm, with wind gusts reaching around or even slightly above 100 knots just off the west coast and along the western coastline of Ireland. Similar wind speeds could also be observed on hills or mountains further inland (Figure 3).
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Figure 2: Storm Eowyn just NW of Ireland on Friday morning according to ECWMF.
There is a high likelihood this will become one of the strongest storms in Ireland's recorded history. Even in the far interior of Ireland, localized wind gusts of more than 80 knots are expected. The strongest winds will occur on the southwestern side of the storm’s core. This means the most intense winds are expected over the half western Ireland. As the low moves eastward, the wind will gradually subside. This is because the divergence in the upper atmosphere will diminish, causing the low-pressure system to slowly fill.
However, strong winds will persist over the Irish Sea and Great Britain. Wind gusts of up to 80 knots are expected over the Irish Sea Friday afternoon and evening accompanied by Hs of 7-8m. Over Scotland and the northern North Sea, gusts could reach a maximum of 70 knots. These areas will experience the strongest winds primarily on Friday evening and into the night leading into Saturday.
Figure 3: Windspeeds according to ICON on Friday morning when storm Eowyn makes landfall in Ireland.
Conclusion
Ireland, in particular, should brace itself for a historic storm. Especially along the western coastline, wind gusts of up to or even slightly above 100 knots are expected. Further inland, gusts of 80-85 knots are also anticipated. These winds are likely to cause (significant) damage. Over the Irish Sea, wind gusts of up to 80 knots are expected, along with significant wave heights of 7-8 meters. The storm will move away via the northern North Sea later on Friday and into Saturday. Gradually, calmer will return during the weekend.
