ForeFlight’s Prog Charts image set is drawn from the Weather Prediction Center’s (WPC’s) Short Range Forecasts and is updated at the same frequency. The WPC’s (and ForeFlight’s) images are updated twice per day according to the following schedule:
- 6 HR and 12 HR: approximately 1400 and 0200Z
- 18 HR and 24 HR: approximately 1630 and 0430Z
- 30 HR, 36 HR, and 48 HR: approximately 1930 and 0730Z
- 60 HR: approximately 2000 and 0800Z
Due to the staggered update schedule, there are brief 1—3 hour periods each day when some of the weather forecast images are updated before others in the same image set. This can make it seem that some of the images are duplicated or out of order when they are not.
For example, between about 1400Z and 1630Z, the 6 HR image will be refreshed with new data, but the 18 HR image will still show data that is around 12 hours old. Pilots viewing the images within that timeframe will see what looks like duplicate weather.
A close examination usually reveals differences in fronts, circulation, and pressure systems. This is a byproduct of how the WPC receives and publishes updated weather, and can be seen on the WPC website as well.
At 1530Z, ForeFlight’s 6 and 12 HR weather appears to be repeated on the 18 and 24 HR images.
At 1530Z, this screenshot from the WPC website shows the same apparent duplication between the 6 and 18 HR images, and between the 12 and 24 HR.
It's important to note that the precipitation shown on the Prog Charts comes from the local weather forecast offices of the National Weather Service and is updated every three hours. As a result, the issuance times indicated on the images may be updated more frequently than the times listed on the WPC page.
To avoid confusion, ForeFlight has added labels at the bottom of each image indicating the expected update time.