The weather in each section of the Wild Area is normally randomized based on your Switch system's date. This can be somewhat annoying since certain Pokemon will only appear during certain weather conditions. However, by setting your Switch system's date to a specific date (which differs depending on the weather condition you want), every section of the Wild Area will be experiencing the desired weather condition. The dates (listed in MM/DD/YYYY format) for each weather condition are as follows.

Clear: 5/1/2020
Sunny: 7/1/2020
Cloudy: 3/1/2020
Rain: 10/1/2020
Thunderstorm: 11/1/2020
Snow: 12/1/2020
Hail: 2/1/2020 (must have at least four badges)
Sandstorm: 4/1/2020 (must have at least four badges)
Fog: 6/1/2020 (must have defeated the Champion)