The diphthong /aʊ/ doesn't really make sense to me as a transcription. I'm a Southern British English speaker, and when I say and hear 'how' it sounds to me like /hæɒw/.
/a/ doesn't seem to exist as a monophthong on it's own in English, but I imagine that this sound is pronounced like a short version of /ɑː/ as in 'father', because that's the first vowel sound in the diphthong /aɪ/. And /ʊ/ is the vowel sound in 'book'. If I put those two sounds together (short father and book) when I say "how", it sounds like a Belfast "haoo" - it doesn't sound anything like my British pronunciation.
Why does /aʊ/ transcribe the sound in 'how'?