My mother is now retired, but she used to work in the National Wheatear Forecast Office in Argentina. She says that now the weather forecasting is very accurate.

When she started working there, they didn't have satellite images, only many local measurement stations in each big city and other places. In each one, someone measure all the things manually and then send a message to the central office.[1] They have to interpolate manually the data to cover all the country.

The idea is that if you have some rain clouds here, and the wind has this speed and direction, then tomorrow they will be approximately there. The clouds may run out of water. Sometimes clouds appear out of the blue if hot humid air collide with cold air.

If the clouds were moving over the sea and they now move over land, you must make some corrections. If there are some mountains, they may change the speed or direction of the clouds. In particular the Andes may stop the clouds for a few days and the change of height make them drop most of the rain in Chile, so it's difficult to predict manually. I'm not a meteorologist, so I don't know the details, but you can see that this involve a lot of general and local knowledge, approximations and guessing.

She was involved in getting the first satellite images here, and then interpreting them, because they only know how the clouds looked from below. [2] Much later they made a transition to digital forecasting. I still remember a few maps in ASCII code with the border of the country, and the predictions of the pressure, humidity and other stuff. She was not involved too much in the modeling part, but she was using the result to make the official forecast.

The first models were not very good, and sometimes they have to use the manual predictions. In particular the mountains in the Andes were not correctly modeled, and we have a big ocean in each side to make everything more complicated.

After a few/many years the model become better and better. Also, they have satellites that can measure visible light, infrared, IIRC humidity and a lot of other stuff. Computer models use more variables, smaller grids, more accurate mountains and have better formulas. So the models now are pretty good.

She now is retired, but she still makes unofficial forecast for the family. If I'm planning a pick nick for the weekend, she will give (unsolicited) advice. [3]

[1] Sometimes the data was inaccurate. Sometimes the data was missing. Sometimes the local person was lazy and lied instead of going outside to get the data. Sometimes the problem was in the transcription.

[2] There is some standard classification of clouds and you must understand how each one looks form above. For example dark clouds are not dark from above, so the darkness is not a good indication of how much water they have.

[3] A few years ago (after she retired), we made the birthday party of my daughter in Zoo a few miles away of Buenos Aires. Two days before the party she said something like:

> It will be rainy in the morning, but after 10am it will be fine, so don't cancel the party. Also, it will rain again after 6pm, so remember to return before that time.

It was pretty accurate, like half an hour of error in the time. She was seeing the forecast of the National Office and also the forecast of a few international pages. Some of them publish the easy forecast in words, and also the maps with the predictions of the models.