Hi ALROT... ;D...!
On FS 2002, when I set up the custom cloud types/formations/weather conditions, etc, this will often change almost instantly during flight, from overcast, to clear blue skies.
The weather doesn't change slowly over a period of time.
I think it's just the way that the Flight Sim works.
...at least in FS 2002... ;)...!
The only way I find the changing weather to be more realistic, and gradual, is by using "Weather Maker"....>>>>
http://www.weather-maker.com/Cheers ALROT...!
Paul.
P.S. ..Weather Maker Personal Edition version 1.4 is a free download...a simple program, and highly recommended....

...!
Win 8.1 64-bit. DX11. Advent Tower. Intel i7-3770 3.9 GHz 8-core. 8 GB System RAM. AMD Radeon HD 7700 1GB RAM. DVD ROM. 2 Terra Byte SATA Hard Drive. Philips 17" LCD Monitor. Saitek Cyborg X Fly-5 Joystick. ...and a Briggs and Stratton Petrol Lawn Mower.