|
|
https://weather.com/news/news/2019-06-05-d-day-weather-forecast-changed-history
D-Day: An Expert View of the Weather Forecast That Changed History
Seventy-five years ago, Allied forces landed in northern France as part of a military action that paved the way for the defeat of Nazi Germany, molding the course of world history in the process.
By Gavin Conway and Leon Brown, Chief Meteorologist, The Weather Channel U.K.•June 6, 2019
Seventy-five years ago, in early June 1944, Allied forces landed in northern France as part of a military action that paved the way for the defeat of Nazi Germany, molding the course of world history.
While careful planning, demonstrations of collective and individual bravery and, in many cases, the ultimate human sacrifice contributed to the operation, much of its success came down to the weather.
A team of six meteorologists – two each from the U.K. Met Office, Royal Navy and United States military – worked for months honing forecasting techniques, many of which have stood the test of time, before advising Allied commanders, led by General Dwight D. Eisenhower, as to the optimal time for attack.
The Allies ended up sailing and landing in relatively calm waters, but documents released in the intervening years showed just how close bad weather came to making the operation a complete failure.
“I don't think people realize how close-run it was," Dr. Lawrence Hogben, one of the Navy meteorologists, told The Telegraph. "Not much would have to have changed for D-Day to have been a failure, and a failure caused by the weather.”
Based on their own forecasts, two U.S. meteorologists, Irving Crick and Ben Holtzman, believed the attack should take place on June 5, an assertion Hogben and Geoffrey Wolfe of the Royal Navy and C. K. M. Douglas and Sverre Pettersen from the U.K. Met Office disagreed with.
Crick and Holtzman were so adamant their appraisal was correct that they phoned Douglas and Pettersen in an attempt to make them change their conflicting forecast.
But the U.K. Met Office pair refused to relent, forcing Eisenhower to postpone the landings for 24 hours. It was a decision that would prove crucial as, on the morning of June 5, there were rough seas and 25-30 mph winds in the English Channel.
“The weather was terrible that morning,” recalled Hogben. “The outcome of D-Day, perhaps the whole future of the western world rested on those forecasts, so I think you could say there was some pressure.”
Despite correctly shelving the June 5 plan, the mood was far from celebratory that day. Unless the weather conditions changed to allow the attack, the operation would have to be postponed until June 19 when the tides would be right for the landings.
The six-strong meteorology team then produced what would prove to be one of the most significant forecasts in modern history, declaring June 6 suitable for the Allied attack.
History reflects that despite a terrible human toll, the D-Day landings spelled the beginning of the end for Hitler’s reign of terror in Europe.
But, as Hogben explained, things could have been considerably, and tragically, different, or, “utter catastrophe, complete failure,” as he put it. “If we had been a little less certain and said no again, it would have had to shift to the 19th," he added.
“As it happened, on the 17th, all six of us produced a forecast for the 19th for almost perfect conditions, so they would definitely have gone ahead," said Hogben. "On 19 June the biggest storm of the 20th century came up. If they had landed that day, I doubt many landing craft would have even made it to the beaches. It does not bear thinking about.”
Back in the 1940s and during the Second World War, weather forecasting was still in its infancy. There were no satellites, weather radars or "supercomputers," and simply managing to get weather observations in a timely fashion was a challenge. The Weather Channel U.K.'s chief meteorologist, Leon Brown, takes a closer look at the weather conditions that prevailed that fateful day.
An Expert View of the D-Day Forecast
The map below shows an approximation of the weather pattern in the run up to D-Day. The jet stream must have been farther south and stronger than normal, steering deep depressions past northwestern Europe in early June.
(The Weather Company, an IBM Business)
Back in 1944, meteorologists didn't know much about the upper atmosphere or that jet streams existed, and there were very few observations from the Atlantic.
At the time the D-Day landings were being planned, Captain James Stagg was the Chief Meteorological Adviser to General Eisenhower and, along with a team of six specially selected meteorologists, had the unenviable task of trying to forecast the weather several days ahead. In fact, Eisenhower insisted on a five-day forecast, which can sometimes be a challenge even today.
Back then, forecasts were essentially nothing more than a guess beyond 24 hours ahead, which makes the story of the D-Day forecast all the more remarkable.
The Americans wanted the invasion to start early, and no later than May, while the British were more inclined to wait until August. Ironically, May 1944 was a month of fine, settled conditions, and then the weather suddenly changed dramatically in early June.
The initial plan was to invade the Normandy coast on June 5. However, on June 4, an unusually deep area of low pressure tracked over Ireland and then across northern Scotland with gales and heavy rain. Gusts over 60 mph were recorded around coasts in the west and north, and the English Channel would have been very rough.
So, it quickly became clear that the invasion had to be delayed, and the American and British meteorologists did their best to collect surface observations and plot them to create surface-pressure charts, then estimate the positions of any fronts.
In the fast-moving weather pattern at the time, this would have only given them a rough idea of the weather to come.
Captain Stagg advised that there should be a weather window for June 6. But there were a wide range of forecasts, with the British expecting more wind and rain and the Americans predicting three days of more-settled weather. The weather window was short during the event and the invasion was only just possible, but it was a great call from Captain Stagg's team.
Delaying much beyond June 6 would have meant waiting a few more weeks for the tides to be right, and by then, the Germans may have realized an invasion was imminent. In the event, the end of June was again quite stormy, so delaying may have been no better.
In fact, June 1944 was one of the windiest of the century in southeastern England, with only 1917 windier, going back to 1895. |
|
发表于 
