thunderstorm vs violent storm

thunderstorm and violent storm both are nouns.

Word	Noun	Adjective	Verb	Adverb
thunderstorm	Yes	No	No	No
violent storm	Yes	No	No	No

As nouns, violent storm is a hypernym of thunderstorm; that is, violent storm is a word with a broader meaning than thunderstorm:

thunderstorm: a storm resulting from strong rising air currents; heavy rain or hail along with thunder and lightning
violent storm: a violent weather condition with winds 64-72 knots (11 on the Beaufort scale) and precipitation and thunder and lightning

Other hypernyms of thunderstorm include storm.

thunderstorm (noun)	violent storm (noun)
a storm resulting from strong rising air currents; heavy rain or hail along with thunder and lightning	a violent weather condition with winds 64-72 knots (11 on the Beaufort scale) and precipitation and thunder and lightning