Stop Ahead Sign
MUTCD W3-1
A stop sign is ahead; prepare to stop.
What it looks like
A yellow diamond warning sign showing a black outline of a red-and-white stop sign to warn that a stop sign is ahead.
When you'll see it
It is posted before an actual stop sign, especially where the stop is hidden by a curve, hill, or trees and could be missed.
What to do
Begin slowing down so you can make a complete stop at the stop sign ahead.
Related signs
Source: Manual on Uniform Traffic Control Devices (MUTCD), 11th Edition — FHWA.