The D-Day landing of June 1944: extrat- ropical cyclones and surface winds in June 1944 compared with a climatology based on the Twentieth Century Reanalysis Christoph Welker,1 Melanie conditions (e.g. light winds and non-stormy caught by surprise and the landings on the 2 2 seas) would permit the largest military inva- Normandy beaches marked the beginning Bieli, Nicolas Piaget and sion in history to go forward on 5 June 1944 of the end of World War II. Michael Sprenger2 or not. On top of that, the forecast of these 1Oeschger Centre for Climate change conditions had to be made at least 2 days in advance – at a time when forecasting Reconstructing the D-Day Research and Institute of Geography, weather for several days and beyond was weather – goals and questions University of Bern, Switzerland still unknown scientific territory, especially The historical background of the D-Day 2Institute for Atmospheric and Climate for the highly variable UK weather. landings sets the stage for our investiga- Weather – July 2014, Vol. 69, No. 7 69, No. Vol. – July 2014, Weather Science, ETH Zurich, Switzerland Given this background, Stagg and his team tion of the weather conditions in June 1944. hoped for a stable ‘blocking high’ associated Our analyses are based on the Twentieth Introduction with persistent calm weather. However, as Century Reanalysis (20CR; Compo et al., Historical overview from a May turned to June, the placid weather 2011), a novel global atmospheric reanalysis turned into a ‘forecaster’s nightmare’. It was meteorological perspective which extends back to 1871, as well as his- exceptionally stormy (e.g. Montgomery, torical weather charts provided by the Met Near the end of World War II, as the D-Day 1948; Ambrose, 1994; Fleming, 2004), Office. The weather evolution during the invasion of Normandy by thousands of ships with conditions more like those during D-Day landings is examined with emphasis was approaching, the whole enterprise was April. Under these conditions, climatology on wind and atmospheric moisture, which totally dependent on an accurate prediction and analogue forecasts could not provide we regard as the key variables determin- of weather. The lives of more than half a much of a guideline for the predictions. ing sea swell and visibility in the landing million soldiers were at stake, as well as the With D-Day approaching, the commanders area. Because historical reports stressed the fate of the war in Europe (for a very recent became more and more anxious, and the uncommonly unsettled and stormy weather account on D-Day, see Atkinson, 2013). three teams were struggling to arrive at a conditions over the British Isles and north- Under the pressure of these highly dramatic final consensus. Behind the scenes, during ern France during June 1944, the ‘stormi- circumstances, six meteorologists working long telephone conferences over scrambled ness ’ of this month is assessed in terms of in three different teams were responsible signals, critical differences among the weather cyclone frequency and near-surface wind for the D-Day forecasts. Their decision to forecasters, especially between the Norwegian speed in comparison to the climatology. go ahead would come down to the most Sverre Petterssen and the American Irvin important weather forecast ever made. Krick, were fought out in an atmosphere of Operation Overlord, the operation that increasing tension and occasional hostility The 20CR and objective launched the invasion of German-occupied (Cox, 2002). Krick’s team consistently found cyclone identification | downloaded: 26.9.2021 western Europe, would use allied air, naval historical analogues that called for accept- The 20CR data set and land forces. It was decided that this able weather on 5 June 1944. On the other joint multinational undertaking should be hand, Petterssen’s upper-air analyses and Retrospective analyses (or reanalyses) have accompanied by a combined effort on the Douglas’s intuition, both meteorologists become an important tool in studying weather forecasting side as well (see e.g. working together at Dunstable, just as con- weather and climate variability. By incorpo- Douglas, 1952; Stagg, 1971; Petterssen, sistently indicated deteriorating weather rating a variety of measurement data into 2001). Three separate teams, from the (Ackerman and Knox, 2006). numerical models, they produce a tem- British Meteorological Office (referred to as Finally, the bleak, windy forecast that porally and spatially consistent synthesis Dunstable group), the British Admiralty, and Stagg presented to General Eisenhower led of observations and analyses of variables the US Air Force, were assembled by the to the postponement of the D-Day landings. that are not easily observed. To reconstruct Allied Supreme Commander, General (later However, at the time this decision was taken, the weather in early June 1944, we made US President) Dwight Eisenhower. They first the weather in the Channel region was still use of the 20CR data set, which contains made separate forecasts and then sought fairly calm, because the weak anticyclonic global weather conditions and their uncer- consensus in telephone conferences, with flow associated with the high-pressure sys- tainty in six-hourly intervals for the period the British meteorologist James Stagg lead- tem over the Azores was still prevailing. The from 1871 to present. Surface-pressure ing the debate and presenting the findings decision to invade on 6 June was made in observations were combined with a short- to Eisenhower – an early example of what the morning of 5 June and was based on term forecast from an ensemble (with 56 the forecast of a short period of improved ensemble members) of integrations of a https://doi.org/10.7892/boris.58528 is referred to as ensemble forecasting today. While the Allied combat troops were weather that would open a small ‘window numerical weather-prediction model using waiting in a state of what their commander of opportunity’ for the assault. Indeed, the Ensemble Kalman Filter technique to called ‘suspended animation’ (Cox, 2002), the invasion weather on 6 June was mar- produce an estimate of the complete state 176source: this group of forecasters had to decide if ginally acceptable. The German army was of the atmosphere, and the uncertainty in that estimate. Monthly sea-surface temper- the months of June in the period 1871–2008 the landing by 1 day (Douglas, 1952). Due ature (SST) and sea-ice distributions from based on the 20CR ensemble data set. to the rapid movement of the cyclones, the the HadISST data set (Rayner et al., 2003) cyclones were expected to pass the Channel served as boundary conditions. For further region on the morning of 5 June. Early June 1944 in 20CR The D-Day landing of June 1944 details on the 20CR refer to Compo et al. For the period in early June 1944 analysed (2011). Among others, the 20CR data set and historical weather charts in this section, the ensemble spread among was intended for the assessment of decadal- In this section, we provide only a brief the 56 SLP fields of the 20CR for the NA scale climate variability. Furthermore, the overview of the weather dynamics during and European sectors is generally small and 20CR can be used as a kind of ‘time machine’ D-Day and the preceding days, because ear- none of the fields shows a fundamentally to study historical weather events. lier studies have investigated the meteoro- different pattern; in particular, the ensemble In this study, we analysed different logical situation (e.g. Douglas, 1952; ECMWF, members hardly differ in the spatial extent atmospheric variables from the 20CR data 2004). The purpose is rather to show that of the Azores high. It is therefore sufficient set to assess on the one hand the synoptic the 20CR is able to capture the main fea- to analyse the ensemble-mean SLP fields weather situation, in particular the location tures of the D-Day weather, compared with in Figure 2, without studying all ensemble and development of cyclones in the North historical weather charts provided by the members individually. 7 69, No. Vol. – July 2014, Weather Atlantic (NA) and European sectors during Met Office. On 4 June (0700 UTC) the centre of the east- June 1944; on the other hand, the 20CR was According to historical weather charts, ern cyclone, which had rapidly deepened, was used to describe surface winds and asso- two relatively weak surface cyclones were located northwest of Scotland (Figure 1(b)). ciated wind-generated waves that could located over the NA on 3 June 1944 At the same time, another cyclone was have seriously endangered the ships and (0700 UTC): one south of Iceland, head- already forming near Newfoundland. The paratroopers in the English Channel region. ing towards the British Isles, and another 20CR (Figure 2(b)) again agrees well with Furthermore, atmospheric moisture is used one east of Newfoundland (Figure 1(a)). At the historical chart. In the afternoon of 4 to estimate the probability of bad visibility, this time, the English Channel region was June, the cold front associated with the deep cloud coverage, or even precipitation. located between two frontal zones. The cyclone over Scotland was located over Great synoptic situation depicted in the histori- Britain (not shown). An increase in atmos- Surface cyclones and associated cal charts is captured by the 20CR, with a pheric moisture over the English Channel cyclone centre east of Newfoundland and region (Figure 3(d)) was associated with the high wind speeds in the 20CR the possibility of a second cyclone south of approaching cold front, and near-surface ensemble Iceland, although not visible as a closed SLP wind speed increased substantially over the To further examine the synoptic weather contour in Figure 2(a). Based on the weather course of the day (Figure 3(a)).