Great suggestions already, I'll add Mazzy Star, Rickie Lee Jones (the acoustic one if you need it to be totally mellow), Lucinda Williams, Lindi Ortega, Kelsea Ballerini, Phoebe Bridgers, Caitlin Canty, someone already mentioned Amber Rubarth and Natalie Merchant, but +1 on those.
Let us know what you thought of all these recommendations.