Yes, you unfortunately can be terminated for refusing to go out in a thunderstorm. While an employer cannot ask you to do something too risky--e.g. scrap asbestos without training or safety gear--realistically, the chance of being hit by lightning is something like the chance of winning a lottery; while the upper atmosphere pyrotechnics are impressive, the actual  risk is so miniscule as to be almost unmeasurable. Therefore, it is not reasonable for you to refuse to go out in a thunderstorm, especially when there is a clear business need (no baskets for customers). The employer would therefore be free to terminate you without any risk of an improper termination lawsuit or other potential liability.