Walter Munk and the surf forecast that went ashore on D-Day.

Walter Munk was twenty-four years old and working out of the Scripps Institution of Oceanography in La Jolla when the United States Army came to him with a question no one had ever answered: whether the surf on the Atlantic coast of Morocco would allow landing craft ashore. It was 1942. The Allied planners were designing Operation Torch, the first major amphibious landing of the war, and the open beaches near Casablanca — exposed to the full fetch of the Atlantic — could be closed by surf for days at a stretch without warning. Munk and the institution's director, the Norwegian oceanographer Harald Sverdrup, set out to build the instrument the planners needed.

The problem had sabotaged amphibious operations before. At Gallipoli in 1915, landing craft had been swamped and driven sideways in surf that looked benign on a map. Planners in the Second World War understood the hazard but had no systematic way to forecast it. Surf height depended on swell — and swell could arrive at a beach from a storm a thousand miles away, days after the storm itself had passed and left no trace on the local weather. It was treated as something you observed and prayed about. What Munk and Sverdrup recognized was that ocean swell is not weather. It is physics. A storm generates waves of predictable period and height that travel outward at calculable speeds, organized by fetch and wind duration. Know the storm; know the surf — before it arrives.

The theoretical foundation came largely from Sverdrup, whose 1942 textbook The Oceans, co-authored with Martin Johnson and Richard Fleming, had just synthesized everything known about physical oceanography into a single volume. Munk worked the engineering end — translating the wave equations into operational forecasting procedures that a trained weather officer could use at a map table. By autumn 1942 they had their method in working form. On November 8, Torch landed Allied forces on three separate stretches of the North African coast. The beaches at Fedala, Safi, and Port Lyautey were all operable. Some landing craft were lost to surf in the roughest sectors, and the Atlantic was uncooperative in places, but the forces went ashore. The method had survived its first test.

What Munk and Sverdrup had built was a chain of calculation running from the weather map to the waterline. Given a storm's position, wind speed, and fetch, they could predict the height and period of the swell it generated. From swell characteristics they could calculate what would happen as the wave entered shallow water: shoaling, the process by which a wave slows, steepens, and breaks as the seafloor rises beneath it. The output was a surf height and period at the target beach, updated as new weather data arrived. By later standards it was a rough instrument — the computers that would refine it did not yet exist — but it was the first instrument, and it worked. The Navy Hydrographic Office distributed their classified reports, and a training course for Navy and Army weather officers was organized at La Jolla, run by Munk and Sverdrup on the same bluffs above the Pacific where the method had been conceived.

Those trained officers were part of the planning staff for Operation Overlord. When Allied planners assembled the weather requirements for the Normandy landings in the spring of 1944, surf at the five target beaches was a specific variable in the go/no-go calculation. The beaches faced north into the English Channel — less exposed than the Moroccan Atlantic coast, but still vulnerable to swell generated by storms pushing through the gap between Ireland and France. Munk and Sverdrup's method was in the hands of the officers who ran those numbers. Their assessment of what the surf would be at Omaha and the other beaches during the narrow meteorological window of early June: high, but manageable.

On June 4, 1944, the operation was postponed twenty-four hours. A storm system had moved through the Channel, and Group Captain James Stagg, chief meteorologist for the Allied Expeditionary Force, forecast a brief clearing on June 6 that would hold long enough for the first waves to get ashore. Eisenhower accepted the forecast. In the early morning of June 6, 156,000 Allied troops crossed the Channel. The surf at Omaha ran three to four feet — brutal for the flat-bottomed Higgins boats, which were shipping water over their ramps before they grounded. Men drowned. Landing craft were swamped and broached in the shallows. But the surf did not stop the landings. The forecast had held.

Munk never left La Jolla. He joined Scripps permanently after the war and spent more than seventy years on its bluffs above the Pacific — measuring deep-ocean temperature, tracking nuclear-test acoustic signals across entire ocean basins, and late in his career using sound to detect the signature of a warming sea. The Navy's oceanographic research presence on Point Loma, a few miles south, grew in part from the wartime bond between the institution and the fleet — a bond Munk and Sverdrup had forged in 1942 when the Army asked an oceanographer whether the surf would cooperate. Today, Naval Base San Diego is the largest surface fleet base on earth, and Scripps wave models run continuously inside Navy forecasting systems, updated by satellite altimeters and global weather data, producing hour-by-hour surf forecasts for every beach on the planet.

Walter Munk died in La Jolla on February 8, 2019, at 101. He is buried near the sea he spent a century decoding. The same Pacific swell that builds across a thousand miles of open ocean and runs through the mouth of San Diego Bay on any October morning — organized by fetch, governed by the equations he first worked out on a wartime map table — is being measured right now by the gauges on the Scripps Pier. The same physics. The same ocean. You could sail out into it tonight.