Scan the slate-gray sky on a December morning, and you may be puzzled: Do those low clouds foretell a blizzard, or just an overcast day? The surprising answer: It depends where you are. According to Tom Moore, senior meteorologist at The Weather Channel, each region offers its own clues.
Be wary of wind blowing from the south, especially if it suddenly shifts to the northwest–and the mercury plunges. "Clouds can develop quickly over the mountains and release heavy snow," Moore says.
Below the Great Lakes–from Illinois to West Virginia and up to New York–frigid, moisture-rich air can produce lake-effect snow hours after a cold front (a low line of dark clouds) has passed.
Atlantic storms spawn the most precipitation, "but it has to be sufficiently cold [for them] to produce snow," he says. Nor'easters develop when Arctic high-pressure air collides with southern low pressure. Watch for winds blowing from the east.
Warm, moist air from the Gulf of Mexico rises as it flows north, producing snow if there's frigid air at ground level. Clouds rolling in from the southwest indicate approaching storms.
No matter where you are, high-altitude cirrus clouds indicate precipitation within the next 24 to 36 hours, as does a ring around the moon or a sudden drop in the cloud deck.