It's 6th June 1944, in Normandy, France. The Allied troops prepare the largest amphibious operation in history; against Nazi Germany. D-Day has come. While the infantry and armored divisions wait for the green light, the weather charts are constantly being updated.
A recently declassified dossier shows that swell, surf height and wind were crucial variables taken into consideration during Operation Neptune. It involved tides, winds, waves, visibility both from the air and the sea, and the combined engagement with land, air and sea force that could not (and still cannot) be fully foreseen.
On the 1st February, the British Admiralty's Naval Meteorological Service activated a Swell Forecast Section in order to obtain accurate wave predictions for D-Day, the Big Storm (19th-22nd June 1944), and other over-the-beach supply operations following the destruction of the artificial harbor at the Omaha beachhead.
Two years before Operations Overlord (Normandy landings) and Neptune, Franklin Roosevelt, the US President, sent a message to Winston Churchill, the British Prime Minister, showing his concern. "...bad surf on the Atlantic beaches is a calculated risk."
Nearly one million trained troops were known to be extremely sensitive to wave action. It would not be easy to deploy everyone and everything, quickly and efficiently, at the landing spot if the seas had been too rough prior to arrival. The report from Charles C. Bates, then Lieutenant Colonel of the U.S. Air Force, indicated that the operation "...needed a four-day period of low seas" in order to be successful.
Studying the conditions of the ocean was crucial. Back then, waves meant casualties, not pleasure. It was decided that the Royal Naval Meteorological Service would take on the sole responsibility for generating sea, swell and surf forecasts for strategic decision making.
Graphs for relating wave height and period to wind speed and associated fetch and duration, as well as an estimate of their decay rate were created. Knowledge was power.
Wave forecasters for the Allied force received top secret aerial photographs of what the surf zones were like at the designated beachheads, in order to complement their information and predictions. "Notes on the Sea, Swell, and Surf in the English Channel" was the first confidential report issued by the surf forecast team.
A 6-way conference call was established to get updates on the condition of the waves. The day of the attack was going to depend on the surf conditions. The staff of meteorologists involved included Donald Yates (Colonel, USAF), James Stagg (Group Captain, RAF) and John Fleming (Instructor Commander, RN).
Tension built on the 3rd June 1944. Eisenhower demanded morning and evening swell updates. D-Day was coming. On the 4th June, the forecast for the following 48 hours read 2-4 foot wind waves for the assault areas.
What was considered "The Most Important Weather Forecast in the History of the World" arrived on Monday, 5 June 1944, at 0400 DBST. "Okay! Let's go!" said Dwight Eisenhower. The observed wave conditions at Operation Neptune's beachheads on the 6th June were: 2-6 foot waves with "choppiness [that] makes personnel transfer difficult."
Although the Swell Forecast Section's wave forecasting models became quickly outmoded post-war, it is widely accepted that the early models made enormous contributions to the science of predicting swells and surf. The Allies of World War II won and the planet was free from Nazi domination.