Weather is static at the start of a scenario based on what you neter or not... you also have the option of....
3-14.3 Variable Weather
If variable weather is used, the computer will automatically adjust the actual scenario weather. To accomplish this, users enter a series of specific conditions that will be “in effect” at certain times based on a pattern of regular “change intervals”. For example, if the change interval is 6 hours, and the scenario starts at 12:00 hours, the user would enter the desired specific conditions for 12:00, 18:00, 24:00, 0600, and so on through the time period covered by the scenario. If the scenario lasts longer than the weather periods entered, the conditions last entered will remain in force for the remainder of the battle.
So there you go...