GLOSSARY OF TERMS REFERENCES RULES OF THUMB National Weather Service (NWS) If temperatures are below 18°F for a Blizzard period of 72-hours or more, salt becomes A storm which contains large amounts of snow Approximate snowfall to rain equivalency ratios: ineffective OR blowing snow, with winds in excess of 35 mph and visibilities of less than 1/4 mile for an If temperatures remain above 18° extended period of time (at least 3 hours). and icing is less than ½”, salt will Winter Storm Warning IDOT – Roadways/Traffic remain effective. Issued when hazardous winter weather in the Emergency Road Closures Every 72-hours of operation, 1/3 of the form of heavy snow, heavy freezing rain, or snow removal equipment that is in-service heavy sleet is imminent or occurring. Winter will be placed out-of-service for repair and Storm Warnings are usually issued 12 to 24 Roadway Weather Information System replacement. hours before the event is expected to begin. (RWIS) Winter Storm Watch Alerts the public to the possibility of a blizzard, heavy snow, heavy freezing rain, or heavy sleet. Winter Storm Watches are usually issued 12 to 48 hours before the beginning of a Winter Storm. Wind Chill Warning: Issued when wind chill temperatures are expected to be hazardous to life within several WIND CHILL CHART LINKS minutes of exposure. Ice Storm An ice storm is used to describe occasions when NWS – NOAA damaging accumulations of ice are expected during freezing rain situations. Significant ice National Weather accumulations are usually accumulations of ¼" or greater. River forecasts Ice Storm Warning Issued when freezing rain produces a significant and possibly damaging accumulation of ice. The Storm Prediction Center criteria for this warning varies from state to state, but typically will be issued any time more than 1/4" of ice is expected to accumulate in an area.
GLOSSARY OF TERMS REFERENCES RULES OF THUMB Blizzard A storm which contains large amounts of snow OR blowing snow, with winds in excess of 35 mph and visibilities of less than 1/4 mile for an extended period of time (at least 3 hours). Winter Storm Warning Issued when hazardous winter weather in the form of heavy snow, heavy freezing rain, or heavy sleet is imminent or occurring. Winter Storm Warnings are usually issued 12 to 24 hours before the event is expected to begin. Winter Storm Watch Alerts the public to the possibility of a blizzard, heavy snow, heavy freezing rain, or heavy sleet. Winter Storm Watches are usually issued 12 to 48 hours before the beginning of a Winter Storm. Wind Chill Warning: Issued when wind chill temperatures are expected to be hazardous to life within several minutes of exposure. Ice Storm An ice storm is used to describe occasions when damaging accumulations of ice are expected during freezing rain situations. Significant ice accumulations are usually accumulations of ¼" or greater. Ice Storm Warning Issued when freezing rain produces a significant and possibly damaging accumulation of ice. The criteria for this warning varies from state to state, but typically will be issued any time more than 1/4" of ice is expected to accumulate in an area. National Weather Service (NWS) Approximate snowfall to rain equivalency ratios: IDOT – Roadways/Traffic Emergency Road Closures Roadway Weather Information System (RWIS) If temperatures are below 18°F for a period of 72-hours or more, salt becomes ineffective If temperatures remain above 18° and icing is less than ½”, salt will remain effective. Every 72-hours of operation, 1/3 of the snow removal equipment that is in-service will be placed out-of-service for repair and replacement. WIND CHILL CHART LINKS NWS – NOAA National Weather River forecasts Storm Prediction Center
HAZIDCRD-WinterWeather.docx