system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do they know what the possible high and low for the day’s temperature will be?

When a weather forecast says there is going to be a high of 18 and a low of 12 for example, how do they know the temperature range or possibility, especially over a weekly forecast?
gpt
It is done with mathematical models that use observational data from all over the world that including ground measurement and satellite observation.   The calculation is done by supercomputers to predict the future.

The models are not perfect and the data you input is limited so the farther you go into the future the lower is the accuracy of the prediction.

 Another reason for the error is that the calculation is done in grids that might be 2.5x2.5km in size. The weather is not identical in each grid and it might rain in part and the other side has sunshine but the weather models assume the same weather in all of the grid. The terrain is not the same in the whole grid and it can have an effect too.