Cold weather

• Winter weather advisory — A mixture of precipitation is expected, such as snow, sleet and freezing rain or drizzle.
• Snow — Alerts are issued in these categories:
  • Advisory — more than 2 in (5 cm), but not expected to accumulate more than 6 in (15 cm).
  • Heavy snow warning — 6 in (15 cm) or more in 12 hours or less; 8 in (20 cm) or more in 24 hours or less; light winds.
• Winter storm — Alerts are issued in these categories:
  • Watch — Conditions are favorable for the development of hazardous weather elements, such as heavy snow or blizzard conditions or significant accumulations of freezing rain or sleet. Conditions may occur singly or in combination. Usually issued 24 hours to 48 hours in advance of an event.
  • Warning — Heavy snow and some windiness are imminent or very likely, perhaps in combination with sleet or freezing rain/drizzle. Usually issued for up to a 12-hour duration, but can be extended to 24 hours.
  • Blizzard warning — Issued for winter storms with sustained winds or frequent gusts to 35 mph (56 kph) or greater, and considerable falling or blowing snow, reducing visibility to less than 1/4-mi (0.4 km). These conditions are expected to last at least three hours.
  • Ice storm warning — Damaging accumulations expected during freezing rain conditions, making walking and driving extremely dangerous. Significant ice accumulations are usually 1/4-in (0.64 cm) or greater.
• Wind chill — Alerts are issued in these categories:
  • Advisory — Wind chills will be -35ºF (-37ºC) or colder for a sustained period of time. Winds are expected to be at least 10 mph (16 kph).
  • Warning — Wind chills are expected to reach -50ºF (-46ºC) and wind speeds are 10 mph (16 kph) or greater. [Note: Some areas, due to the effects of humidity, give a warning when the wind chill is expected to reach -10ºF (-23ºC).]

• Excessive heat — Alerts are issued in these categories:
  • Advisory — Heat index above 105ºF (41ºC) is anticipated.
  • Watch — Conditions are favorable for the development of heat indices in excess of 105ºF (41ºC) during daytime hours in combination with nighttime low temperatures of 80ºF (27ºC) or higher, for 2 consecutive days.
  • Warning — Heat index equals or exceeds 120ºF (49ºC) for 3 hours or longer.

• Severe thunderstorm — Alerts are issued in these categories:
  • Watch — Conditions are favorable for the development of severe thunderstorms in and close to the designated area; usually in effect for several hours, but not more than 6 hours.
  • Warning — A thunderstorm has produced hail 3/4-in (2 cm) or larger in diameter or winds equal to or exceeding 58 mph (93 kph).

• Flood — Floods usually take 12 hours or more to develop. Alerts are issued in these categories:
  • Watch — Flooding is possible in or near the designated area. Individuals should be alert and ready to take action.
  • Warning — A flood is imminent or has been reported at a certain station or river basin.
• Flash flood — Flash floods take less than 6 hours to develop. Alerts are issued in these categories:
  • Watch — Flash flood conditions are possible in or near the designated area. Individuals should be alert and ready to take action.
  • Warning — Flash flooding is imminent. Those in the affected area should take immediate action.

Fog
A fog advisory means dense fog covers a widespread area and reduces visibility to 1/4-mile (0.4 km) or less.

• Watch — Conditions are favorable for the development of tornados in and close to the designated area; usually in effect for several hours, with 6 hours being the most common.
• Warning — A tornado is indicated by radar or sighted by spotters. Issued by the National Weather Service.

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