Can a hot shower or weather aggrivate spinal stenosis and its symptoms?

Unlikely: Many symptoms may feel worse depending on mood, llife satisfaction, and innumerable other psychological influences, and the weather can certainly have effects on these things. However, weather almost certainly has no direct effect on spinal stenosis or its symptoms.

Not usually: It would be very unusual for this to cause an objective change in the stenosis. Subjectively, of course, you could feel something different from the heat, but that may more relate to decreases in muscular tone and/or spasm that may be associated with the stenosis.