The anniversary of D-Day is coming this week, and a movie has just been released about the D-Day weather forecast (called Pressure).
The movie follows the plot of the play of the same name, describing how a weather forecaster (James Stagg) pushed to delay the landing due to a storm, was opposed by an egotistical American meteorologist, and that the German forecasters were wrong, leaving the German defenders unprepared.
New information from Allied and German archives and modern reconstructions of the weather during the landing period reveal that some of the "Pressure" story is not correct.
June 6, 1944
The landing in Normandy had several key environmental requirements.
A full moon was needed to provide sufficient illumination for gliders and parachute troops the prior evening. No low cloud base that would prevent close air support and accurate naval bombardment along the coast. Low tide conditions, so dangerous beach obstacles could be avoided. Furthermore, winds had to be light so that waves and swell would be minimal.
June 4-7th had the requisite lunar and tide conditions, with the next opportunity not until later in the month. And it was a matter of time before the Germans figured out where the real invasion would take place. There was strong pressure to move sooner rather than later.
Three forecast groups were represented in the overall meteorological team: two British (UK Meteorological Office and British Navy) and one American (headed by Dr. Irving Crick). The meteorological leader was the Royal Air Force's James Stagg.
The British meteorologists made use of traditional forecasting approaches that relied on the Norwegian Cyclone Model, with a typical evolution of cyclones and fronts (see below), while the Americans used a controversial "analog" approach, where one searched for similar weather situations in the past.
Weather prediction was a primitive, subjective affair at that time. There were no numerical weather prediction models, little understanding of jet streams and upper-level weather features, and an acute lack of observations, particularly over the ocean.
The situation on June 4, 1944, was problematic (see the Allies' sea level pressure analysis below). A fairly strong low center was approaching northern Scotland. The lines are isobars, lines of constant pressure, and where they are close together, the winds would be too strong. Plus, a cold front was approaching (the line with the triangles on it).
Not good for invasion: winds would be too strong on the beaches, with lots of wave action and high water level (could not see the obstacles).
The weather map on June 5th indicated that the low had strengthened but had moved to the northeast (see below). An area of high pressure centered west of Spain was starting to extend northward into the region. Winds would still be significant, but the situation might be improving.
The forecast team under Stagg predicted the low would continue northward, producing conditions just good enough for D-Day on June 6th. The actual weather map for June 6th (below) suggests that Stagg and associates made the right forecast for the wrong reason.
As noted in an excellent recent paper by Anders Persson, the storm did NOT move to the northeast as predicted but weakened rapidly and slid southeastward. Problematically, pressure differences over the channel were large enough to create significant wave action, which caused extensive sea sickness, high water levels on the beaches, and lost equipment and deaths.
The disliked American forecaster (Irving Krick) has been right about a pattern change that would push the low south rather than to the north, as predicted by Stagg and friends.
The situation was marginal, but just good enough for the landings in Normandy.
It has been thought that the Germans had poor weather maps over the eastern Atlantic because they had few weather assets over the Atlantic (and none over North America).
But as noted in the article by Anders Persson, it appears the Germans had decoded Allied weather communications and possessed relatively good weather maps. They assumed that the marginal conditions on June 6th were not good enough for an invasion, resulting in a lack of readiness and the absence of General Erwin Rommel to visit his wife on her birthday.
But the story does not end there.
During the past decade, the meteorology of the period has been simulated using modern weather models and data assimilation tools, giving us a far more accurate description of what occurred during the D-Day period.
Below is an animation of the hourly precipitation (color shading) and sea-level pressure on June 6, 1944, made by the European Center (the ERA5 analysis). You can see the low northeast of England weakening and moving southward. Not what Stagg expected.
Although marginal, conditions were just good enough for a successful landing on Normandy, although the high winds were very problematic.
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