Do changes in the weather (e.g. a strong heat wave or thunderstorm) signal that the volcano is in a state of unrest and may erupt at any moment?
Volcanoes are caused by processes beneath the surface of the Earth and the factors which may trigger eruptions are totally unconnected to the atmospheric processes which cause changes in weather. Heat waves, overcast conditions at the summit or any other weather phenomena do not indicate anything about the state of the volcano. However, heavy rains during an eruption may trigger mudflows (lahars) and lightening can occur from eruption columns.