Obviously there were very few weather stations in 1944 out in the middle of the Atlantic Ocean, so predicting the weather for the D-Day landings was fraught.

One would think that the steady line of and troop transports, especially liberty ships, constantly crossing that pond were sending real time observations for the benefit of the logistics folks on both sides. Still, without the satellite technology we enjoy today that allows surfers to precisely follow giant swells across the ocean, forecasting was what it was.

Anecdote. My uncle tried to enlist as soon as he got the US. He really wanted to become a machine gunner, go home and shoot Nazis. The Army wouldn't take him since he was an Austrian deserter from the German army. Having acquired his PhD in Vienna, he got hired by Villanova's math dept. His first job; teaching Navy guys math necessary for shipboard operations (navigation, radio, etc). He found it humorous that they tasked with that because, except for crossing the channel to the UK and then to the US in 1938/
39, he had zero exposure to the ocean.