American English Phonetics

6.8.2 Centralization

In some dialects speakers have a tendency to retract or centralize their vowels in all contexts, but especially after /w/. Centralization tends to affect the front lax vowels /ɪ, ɛ/. It is a common feature in the California variety of GA, particularly with younger speakers. So, for some speakers, west and twenty would be pronounced: [wɛ̈st] and [twɛ̈ni], where [ɛ̈] may begin to sound like /ʌ/.