Translation:It is windy outside.
Characters may have 2 or more pronunciations. A particular pronunciation goes with a certain meaning or usage of the character and usually is consistent with the latter.
e.g. 外面刮着风，小心着涼。 The 1st 着 is a preposition indicating a continuous action, and is pronounced as zhe;
The 2nd 着 is a verb which means catch as in catching cold. It is pronounced as zhao.
刮风 is very common and natural, it is usually used when referring to strong or wide range wind. For example, we would say:
明天可能刮台风。(we don't use 吹 for typhoon) 今天上午刮风了。 我的衣服都被风刮跑了。
吹 is used when the wind is gentle or in a small range. For example: 微风吹动树叶。 风轻轻吹起了窗帘。 我用吹风机吹干了头发。
吹 does mean blow and it can substitute 刮. Only it is less usual to use 吹 without describing the strength of the wind. So more usually 吹著微風，吹著強風， 吹著很大的風，etc.
刮 already include the implication that the wind is strong, so using with or without magnitude are both natural, but it does not apply to weak wind.