* WHAT...Temperatures falling to around 30 to the mid 30s will result in areas of frost north of Interstate 80. To the south of Interstate 80, frost may be more patchy as temperatures only bottom out in the mid to upper 30s in some locations due to clouds being slower to diminish. * WHERE...Portions of north central and northwest Illinois, east central, northeast, and southeast Iowa, and northeast Missouri. * WHEN...Until 8 AM CDT this morning. * IMPACTS...Frost could harm sensitive outdoor vegetation. Sensitive outdoor plants may be killed if left uncovered.