but rather than simply pull data from some weather API he is actually using a sensor suite connected to the Arduino to measure things like barometric pressure in order to make a 12-hour forecast.

For example, the wind vane uses eight reed switches and an embedded magnet to communicate the current wind direction to the Arduino ... an off-the-shelf barometric pressure sensor, but we ...

Finally, your Arduino toolkit will be incomplete if you don't purchase some sensor and circuit modules. After all, you can’t build a precise weather station without a BME280, DHT22, DS18B20 ...