Mackerel Sky and Mare's Tails Make Lofty Ships Carry Low Sails
Here's a fish story that actually holds up -- as long as you keep a tight rein on the details.
A mackerel sky, known in Germany and France as sheep clouds (German: schaefchenwolken; French: nuages moutonneux), is a large, spreading assemblage of clouds that resembles a series of waves or fish scales, with blue sky peeking out between the puffs. It consists of cirrocumulus or altocumulus clouds, which indicate moisture high up (around 18,000-30,000 feet (6,000-10,000 meters) in a cold sky (the blue bits indicate that these clouds are breaking up due to instability in the air). Mare's tails, meanwhile, are long, threadlike cirrus clouds, often stretched by strong high-level winds. Both clues suggest an impending storm, typically 6-8 hours away [source: Weather Online].
That is, assuming your mackerel sky is caused by ice clouds. It's also possible, if the clouds mostly consist of bigger, darker altocumulus, that you're dealing with a lower-level, water-droplet-based version. This could mean better weather in the short run, but keep a weather eye on that sky: If they continue to develop, a cold front and thunderstorms might soon be on the way [source: Weather Online].