From Halopedia, the Halo wiki
According to non-canonical blog posts by Halo Wars writer Graeme Devine, Tigard is in the northern regions of the planet, near a Forerunner relic. During the glassing of the planet by the Covenant it became an inhospitable wasteland. A UNSC base was established in 2526, Alpha Base, which managed to capture and observe a Forerunner relic in 2531 for a short amount of time before the Covenant retook it, leading the Spirit of Fire to Arcadia.
Given that the majority of Harvest's population were originally from the American mid-west, Tigard may have been named after the town of the same name.