The spray comes from paired glands beside the anus, which release a mixture of sulphur compounds called thiols through small nipple-like ducts the animal can aim. The supply is finite, taking a week or more to replenish fully, and few full discharges are available at any time. A defence that expensive is not used casually.
The warning sequence is consistent enough to be described in field guides: foot-stamping, hissing, tail raised, and in spotted skunks a handstand display. Only if these fail does the animal turn, present its rear and spray, with an effective range commonly given as several metres. Most encounters between skunks and larger animals end at the display stage.
Two corrections are worth making. Tomato juice does not neutralise the thiols; it masks them, and the standard chemical remedy uses dilute hydrogen peroxide with baking soda and detergent, which oxidises the compounds. And skunks are not weasels: molecular work moved them out of Mustelidae into their own family, Mephitidae. Great horned owls are among the few predators taking skunks routinely, usually attributed to a poor sense of smell, though that has not been directly tested.