1
general::
Appear imminent in a threatening, magnified form. For example, The possibility of civil war loomed large on the horizon, or Martha wanted to take it easy for a week, but the bar exam loomed large. This term employs loom in the sense of "come into view," a usage dating from the late 1500s.
American Heritage Idioms
2
general::
Phrase(s): loom large (on the horizon)
Cliché to be of great importance, especially when referring to an upcoming problem, danger, or threat. • The exams were looming large on the horizon. • Eviction was looming large when the tenants could not pay their rent.
McGrawhill's American Idioms And Phrasal Verbs