There is no specific time of day or weather condition that will directly improve your casting distance in Stardew Valley. However, some fish are only available during certain times of day or weather conditions, so it's important to keep that in mind when fishing. Additionally, certain fishing spots may be more productive during certain times of day or weather conditions, so it's worth exploring different spots to find where the fish are biting. Overall, practice and patience are key to improving your casting distance and catching more fish in Stardew Valley.