Rain and Snow
When precipitation starts to fall, it is more advantageous to have your fantasy players playing on field turf as opposed to grass, but neither are the ideal condition for explosive plays and points to be scored. Be sure to always check weather reports and try to avoid drafting players in these situations at all costs!
Moisture is not conducive to throwing or catching a football so adjustments are made. But that does not mean the passing offense is completely voided by rain or snow. You just need to be aware of how it will be impacted. Teams will opt for higher percentage passes because some of the precision is lost when a quarterback is throwing a wet ball. This is a setback for sure, but it is not what you must be most concerned about when it is raining or snowing.
Rain and snow will make the playing surface slippery which is not good for fantasy statistics. Traction will be lost, it will be harder for ball carriers and receivers to make cuts, pass patterns will be harder to run, and players will slip. Receiver's yards per catch will be decreased and big plays will be reduced. Runners that run north to south will fare better in this type of condition as well as receivers who rely more on short to intermediate route running. However, precipitation alone is not the worst thing that can happen to fantasy owners. That is wind.
http://www.rotopicks.com/nfl/how-weather-affects-football-games.php