Although the standard lumber yard might be less common, often being replaced by the large indoor box stores and home improvement stores, they still represent a significant problem.
For this scenario we will assume a wind condition of 10 MPH sustained with some higher gusts.
1.) What protection systems might be available for outdoor rack storage? Have you checked and preplanned the ones in your area?
2.) What will you do for initial fire attack? Large handlines? Ground monitors?
3.) Did the main building start first or is it now an exposure to the volume of radiant heat?
4.) Do you commit apparatus into the secured yard area, or just attack lines?
5.) How high and how stable are the piles? What other materials can be found here besides just lumber?
Stay Safe and stay thinking!