At times like these, you just have to be able to shrug off these small defeats and take it on faith that you will be able to pull it off. Decisiveness In critical situations, you just don't have a lot of time to mull over decisions. You often have to make critical calls on the fly, with a million other things going on. Planning helps, but races are fluid events, often creating situations that call for quick thinking and decisive action. But you can plan for all types of weather, which any smart RD will do.
Bad weather happens.
You just have to make the best of it. Know the effects of cold or wet weather, or excessive humidity on your runners. It was not excessively cold, but it was windy, which made it seem colder. Plus, it was a very dry day, one that set up marathoners for dehydration, since they drank less than they thought they might need. Know what kind of effect various types of weather has on runners and do your best to be ready for it.