Thermal signature on armor is not dependent of the weather as far as I know. Weather was introduced on 4.0, it is so new effect that it is unlikely to affect at all. However foliage, trees and bushed will have great effect to thermals now when field of view on thermals have to be adjusted.
However that will only give very minor advantage, as it really only makes identification of enemy difficult... not actual spotting. Where ever something is hot. It will be visible, no matter if thermal imager is adjusted or not.
Your best change for winning any war agaisnt modern equipment is to have good communications with other friendly units. That you know exatly what everyone, even enemy is doing.
And bring it close and personal. A knife fight. This is possible at thick woods, urban areas, or terrain that has many valleys and hills. Use flanking manauvers when ever possible, and get more than one unit, to attack same target, preferably on multiple directions.
With thermal and superior optics, enemy has advantage on long distance. Deny that advantage.
enemy tanks are more tall.. your are small and can hide. Use this advantage. - Communicate with others friendly forces to know when its best time to expose yourself to take killing shot to enemy. Remember... on short distance, its matter of who fires first.
On ambush, keep your vehicle silent. Turn off your engine, and wait.. patience is cold. When you need to... move fast and be very aggressive!