This is probably the way to make weather in STALKER as dynamic as it can be done without making abrupt and ugly weather switching.
How: a number of separate weather cycles are defined. These configs don't need to match at any time of day, i.e. totally independent cycles. A program loads all of these cycles and generates a config file with intermediate phases for each possible transition for every hour of the day.
Then some scripting: every hour a random number is generated with range from 1 to number of cycles. This is then used to select the next cycle. If the cycle is different from current cycle then a transition phase is initiated. The next hour the transition completes by changing to the new cycle.
The idea is very simple, really but the implementation is not completely simple: you don't want to generate transition sections between two very different looking weathers because the engine works by cross-fading the paramters and does not care if it cross-fades clear sky with sun into black thundercloud with rain. Some rules need to be set up and this significantly reduces the number of generated transitions.
Enough talk, the files for the proof of concept are fully usable (with non-weather mods or vanilla) and you can get them here:
http://zhat.dk/stalker/files/dynamic_weather_v0.1.zip