The D-Day landings on June 6, 1944, mark a turning point in the European theatre of World War 2 that would see the Allies go on to defeat Nazi Germany. Weather has played a significant role in many other historic events, but in this case, rather than being at the mercy of the weather, it was accurately predicted and taken advantage of. In a time before super computers and constant satellite monitoring the achievement is even more impressive.

Operation Overlord had been planned 2 years in advance and would have thousands of ships make a 17-hour journey across the English Channel and establish defensive positions on the French coast at Normandy.

Image: D-Day landings and ideal weather conditions. Source: UK Met Office.

Strategic weather parameters

With 2 years of planning for the largest amphibious assault in history, as well as the largest airborne landing at the time, its actual undertaking depended on the moon, tides and weather. The conditions needed were:

• A full moon
• Low tide at dawn, rising after dawn
• Little cloud cover
• Light winds
• Low seas

Once the lunar and tidal predictions were made, the dates of a June invasion were narrowed to the 5th, 6th and 7th. Then it all depended on the weather.

A powerful June storm

The 5th was initially chosen but the weather observation taken on the 3rd by a young postal worker on the west coast of neutral Ireland alerted weather forecasters of a storm coming from the North Atlantic.

This powerful storm indeed arrived late on the 4th with furious winds, large waves and thick cloud. It would have been a disaster if they didn’t have any knowledge about the coming storm.

Image: Hand drawn weather chart for 1am on Monday, June 5, 1944, showing a powerful low (977hPa central pressure) to the north of the UK and a cold front crossing the English Channel. Source: UK Met Office.

A similar low and cold front hit the UK during this week, and it gives us a chance to see such June storms from the modern perspective of satellites and computer models.

Image: Powerful low to the north of the UK and cold front crossing the English Channel at 7am on June 3, 2025. Source: EUMETSAT.

Image: The cold front produced strong southwesterly winds over the English Channel at 6am on June 3, 2025, according to the ECMWF model.

A break in the stormy weather

An observation from the 4th of June from a single ship 1000 km west of Ireland showed that pressure was rising over the North Atlantic. This allowed forecasters to deduce that a high pressure ridge was on its way and could provide marginal conditions for the operation to go ahead on the 6th. The sustained winds would still blow up to 40km/h and there would still be some cloud about, but conditions were much better than the previous day.

Image: Hand drawn weather chart for 1pm Tuesday, June 6, 1944. A ridge of high pressure over the Atlantic nudged towards the English Channel. The low near the UK had weakened (995hPa central pressure). Source: UK Met Office.

Conditions on the 7th would have deteriated again with winds and waves increasing. This is because the pressure gradient increased between the low to the east of the UK and the high pressure ridge to the west of the English Channel.

A historic forecast

It wasn’t only the case that the Allied weather forecasters had made an accurate forecast. Perhaps just as importantly the Germans hadn’t. The Allies had the weather observations for the European Continent because they could decipher German codes. But the Germans didn’t have the UK and North Atlantic weather observations. The German commander left Normandy for his wife’s birthday because he thought the weather would remain just as bad as on the 5th.

You can read more from the UK Met Office which was involved in this historic forecast:

D-Day - the most important weather forecast in history - Met Office