veiller au grain

Etymology
Expression arises from maritime use: is a sudden, violent gust of wind, accompanied with or without rain;  means to watch for, or be on guard for, unpredictable meteorological events that can endanger an ill-prepared crew or damage a frail, poorly sheltered boat.

Verb

 * 1)  to be on guard for a sudden, violent gust of wind
 * 2)  to keep an eye on things, to look out for squalls, to keep a weather eye open