How it works
Picking a day to land
Eisenhower's planners could only land on a handful of days each month. They needed calm enough seas for landing craft, a bright moon for the paratroopers dropping in the early hours, a low but rising tide just after first light so beach obstacles were exposed, and winds light enough to fly and sail. On 5 June 1944 the wind was wrong, so the landing moved to the 6th. The next suitable tides were two weeks away, around 19 June.
This planner rates every day of the current month and the next two against those same conditions, at 10 coastal sites in Europe, the US and Latin America. The data updates every morning.
The rating
Each day gets a score from 0 to 100, a weighted blend of the factors below. A score of 75+ is GO, 50–74 is MARGINAL, and anything lower is NO-GO. Two conditions make a day NO-GO whatever its score:
- Offshore waves of 8 ft or more. Landing craft can't operate. Waves are measured 5 miles offshore, roughly where craft would be launched.
- Sustained wind of 25 mph or more (about Force 6, as on 5 June 1944).
| Factor | Weight | How it's scored |
|---|---|---|
| Waves (offshore) | 30% | Under 8 ft earns 80–100, calmer is better. At or over the limit, the day is NO-GO. |
| Moonlight at drop | 25% | How full the moon is, multiplied by how much of the 00:00–02:00 drop window it's above the horizon. A full moon that has set gives no light. |
| Low tide at dawn | 15% | Full marks when low tide falls between 90 minutes before and 30 minutes after sunrise. On 6 June 1944 at Omaha, low tide was about half an hour before sunrise. The score falls to zero 3 hours outside that window. Sites with small tides (under 2 ft a day) skip this factor, and its weight goes to the others. |
| Wind | 10% | Full marks up to 10 mph, zero at 25 mph. |
| Cloud cover | 10% | Less cloud is better for bombers, naval gunfire and navigation. |
| Rain | 7% | Chance of rain (forecast), or whether it rained (recorded and outlook days). |
| Sunshine | 3% | Share of daylight hours with sun. |
Where each day's data comes from
- Recorded (days already past this month): Open-Meteo's model record of the actual weather and waves.
- Forecast (today and about 15 days ahead): Open-Meteo's forecast. Waves are only forecast about a week out. After that, a forecast day uses the typical wave height for the time of year, labelled "typical".
- Climate outlook (everything later): no forecast reaches that far. Instead, the day's real moon and tide are paired with the weather and waves from every day within 3 days of that date in each year since late 2021, when Open-Meteo's wave archive starts. That's about 35 past days per date. The day's score is the average across them, and the panel shows how often those past days would have been a GO. Treat outlook days as odds, not predictions.
Moon and tides
Moon phase, moonrise, moonset and sunrise are calculated, so they're exact for any date. Tides are harder, since Open-Meteo only has modelled sea level for about three months back and ten days ahead. The planner fits the main tidal constituents to that record (a standard harmonic analysis) and extrapolates. Checked against two weeks of data held out from the fit, the errors are small next to each site's tidal range, but these are still not official tide tables. For anything real, check SHOM (France) or NOAA (US) predictions.
Notifications
Each morning the planner pushes a short summary for Omaha Beach to the same ntfy.sh topic as the weather station: the best remaining day this month and the next GO day.