Why weren't the plural forms used here like "blow gales"?
There aren't any plurals in this sentence because the phrase "blow a gale" is an idiom. It is a weather-related idiom that means to be very windy. Idioms don't necessarily follow grammatical rules. This idiom is a British English idiom, but isn't commonly used in the United States. Ex: It's blowing a gale outside. Ex: Today is the perfect day to fly a kite! It's blowing a gale out there.