* WHAT...Temperatures as low as 33 to 36 degrees will result in frost formation. * WHERE...Central and Southern Willamette Valley, Northern and Central Coast Range Valleys and Mountains of Oregon, Tualatin Valley, Clackamas County Cascade Foothills, Cowlitz County Lowlands, North Clark County Lowlands, and South Washington Cascade Foothills. * WHEN...From 1 AM to 9 AM PDT Thursday. * IMPACTS...Frost could harm sensitive outdoor vegetation. Sensitive outdoor plants may be killed if left uncovered.