The sentence that has the proper subject-verb alignment is he first option, “those severe weather conditions are a threat.”
This is because the subject, weather conditions, is plural and requires a plural verb. This makes the second option incorrect, as “is” is singular and doesn’t match in number with the plural subject in the sentence. Subjects and verbs in sentences must have the same case and number in order to be grammatically correct.
I hope this helps! :)