Extratropical Storm Ophelia - Technical Analysis Only - MOD NOTE post #1

Met Eireann

Weather News


Update on Storm Ophelia

14 October 2017

As of midday Saturday, Hurricane Ophelia situated at approx. 2,500km southwest of Ireland and 500km south-southwest of the Azores in the Atlantic Ocean is currently tracking east-northeast. The latest information from our colleagues in the National Hurricane Centre in Miami indicates that Ophelia will become a powerful post-tropical cyclone (from thereon in ex-Hurricane Ophelia) by Monday, and there is now generally high confidence that the centre of this system will track close to and possibly even over some parts of the west coast of Ireland.

Ex-Hurricane Ophelia is expected to be near the southwest tip of Ireland by Monday morning. From there it looks set to track northwards close to the Atlantic Seaboard at a fast pace to be roughly situated 100-200km northwest of County Donegal by midnight on Monday.

This will be a significant weather event for Ireland with potentially high impacts – structural damage and flooding (particularly coastal) - and people are advised to take extreme care Keep up to date with the warnings.

Met Éireann forecasters have been monitoring this situation closely all week. We have been liaising with our international colleagues, especially at the UK Met Office, and as we are now in the 48 hour window, with access to our high resolution models, we have decided to issue specific warnings. The warnings will be updated as required.

For further information on the evolution of this system, please continue reading.

What will happen with Hurricane Ophelia?

Hurricane Ophelia is expected to undergo an extra-tropical transition over the next 24 to 36 hours. That means its structure and appearance is going to undergo drastic changes as it approaches Ireland. It will lose its hurricane status but will become a powerful extra-tropical storm. Current indications are suggesting that Ex-Hurricane Ophelia will likely engage with an upper trough of low pressure in the mid-Atlantic at some point tomorrow afternoon. This engagement will cause the storm system to accelerate somewhat north-northeast towards Ireland and the UK.

Why will this happen?

As Hurricane Ophelia tracks north from its current position, it will encounter cooler seas and it will eventually come under the influence of the westerlies. The westerlies of the mid-latitudes increase in strength with height, a phenomenon known as vertical wind shear. This shear almost literally chops off the upper part of the hurricane and sweeps it away. Along with the lower sea temperatures of the mid-latitudes this destroys the positive feedback processes within the hurricane. What remains is the former hurricane’s low-level circulation which, if conditions are right, becomes the focus of further development.

What weather is expected?

Rain can be expected over the country on Sunday night and into Monday, though the heaviest and most significant rain will remain out to sea in the Atlantic on the western side of the surface low pressure. But there will also be some heavy and possibly thundery bursts rotating around the low centre itself, so it can be reasonably assumed that counties closest to the centre of the low will see the heaviest rainfall.

At the same time, damaging winds will accompany the low centre with wind gusts of 130km/h or more. The most severe winds will be in coastal counties, with lower impacts likely for central areas. In addition, we can expect very high waves. Sea conditions will be dangerous and large waves may lead to coastal flooding.

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