Forecasting for D-Day 6 June 1944

Stagg and his teams relied on the synoptic charts for the information needed to produce the crucial forecast. These charts were used to identify both the bad weather which led to the postponement of the invasion on the 5th and the weather window which enabled it to go ahead. 

Early on the morning of 4th June it became clear that the weather would not be suitable for an invasion on the 5th. 

Eisenhower made the decision to defer it by a day. 

On this chart for the early hours of the 5th a front in the English Channel shows the forecast was correct.

The chart for the 5th June also shows the first indications that the weather would improve on the 6th. 

High pressure was building from the Atlantic and pressure observations were beginning to rise at the westernmost observation stations. 

The forecast was far from certain.

On the chart for the 6th June we can see that the cold front in the Channel has moved away, but the high pressure has not moved as far towards the UK as expected.

The isobars in the England Channel are quite closely packed together indicating breezy conditions and rather rough seas. Not the ideal conditions for the small Armada of flat bottomed landing craft carrying the allied troops.

Although conditions were not perfect, this actually helped the allies. 

German meteorologists lacked access to as much data from the Atlantic and advised that conditions were not good enough to invade. As a result 20,000 additional troops were not sent to the Atlantic Wall.

In addition to observations from the UK the chart also shows many observations from occupied Europe. 

These reveal that the allies had broken the German Enigma code. German weather signals were amongst those being read at Bletchley Park in England and even helped crack the code.

We have also learned that some of the observations on these charts were sent in by local resistance fighters who risked their lives operating covert radios to provide information for allied air operations.