The formal name is Ekaterina [Katherine]. Katya / Katia is the form used for childhood friends or very young girls, while Katrina is an informal form used among acquaintances in later years, say high school and after. Katya is a true diminutive, while Katrina would fall into the informal category.
It is to be hoped that Katia stays a much less mature storm than Katrina was.
]]>