I am a HUGE fan of using the metaphor, "ominous clouds on the horizon." First, it's something everyone can relate to, in terms of actually having seen it. Second, it's very accurate in terms of stressing the need to be able to literally look ahead of you and think about issues like, "How fast are those clouds moving towards me?" and "When will the Clouds arrive here?"
I particularly prefer "ominous clouds on the horizon" or "dark clouds on the horizon" to "sunny skies." The obvious collolary values of sunny skies are relaxation and general laziness, whereas dark/ominous clouds on the horizon connote a watchfulness and attentiveness- IMPORTANT TRAITS TO HAVE.