Many people with MS experience a temporary worsening of their symptoms when the weather is very hot or humid or they run a fever, sunbathe, get overheated from exercise, or take very hot showers or baths. For example, some people notice that their vision becomes blurred when they get overheated—a phenomenon known as Uhthoff’s sign. These temporary changes can result from even a very slight elevation in core body temperature (one-quarter to one-half of a degree) because an elevated temperature further impairs the ability of a demyelinated nerve to conduct electrical impulses.

Cold Can Also Be a Problem
Some people with MS notice that symptoms, particularly spasticity, become worse in cold weather. It is generally recommended that people with MS who are sensitive to temperature try to avoid extremes of either hot or cold, and that people who are considering a move to a "better" climate try to visit first to see if the climate change is, indeed, beneficial.

I live in West Texas and I can tell you dry heat with little humdity can still causing overwhemming fatigue.