Where the outlook comes from
The models
Atlas reads NOAA’s weather models. Across the contiguous United States it uses HRRR, in quarter-hour steps; everywhere else on earth it uses GFS, in hourly steps. Each block is one of the model’s own steps long, and says which model run it came from.
An estimate, not a warning
What Atlas shows is a model’s estimate of the weather, and models are sometimes wrong. It is never a warning. For anything that could hurt you, use your national weather service, and in danger call your emergency number.