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When winter weather threatens, the National Weather Service issues statements updating conditions and expected changes.
School, business, governmental, church, and other organizational closings from local media sources. A winter weather advisory is issued for winter weather conditions that will cause a slight mess and an inconvenience, but which are not as dangerous or as crippling as winter storms. A winter storm, according to the local definition used at NWS Greenville-Spartanburg, is a winter weather event that produces 4 or more inches of snow in 12 hours, 6 or more inches of snow in 24 hours, 1/2 to 1 inch or more of sleet in a 12-hour period, or 1/4 inch or more of glaze ice accumulation from freezing rain. A winter storm watch is issued 12 to 36 hours ahead of an expected winter storm. If a winter storm is expected immediately or within the following 18 hours, a winter storm warning may be issued. Winter precipitation may fall as snow, sleet, freezing rain, or a combination of those, possibly mixed with rain. Snow occurs when the entire column of air from high in the clouds down to ground level is at or below freezing. Sleet occurs when snow falls through a warm layer of air in the middle levels of the atmosphere, and then re-freezes at the lower levels and forms ice pellets. Freezing, or supercooled, rain occurs when liquid rain falls into a layer of cold air near the ground, where it cools to near freezing but stays liquid while it is still falling. When the supercooled rain lands on a cold surface, such as a tree limb, power line, or in some cases a roadway, it freezes very quickly, forming a glaze of ice. Should freezing rain, rain, snow, and/or sleet occur together in at least 2 of those types at the same time, a wintry mix occurs. Thanks to NWS Greenville-Spartanburg for the local winter storm definition. |