NOAA Weather Watches and Warnings Definitions

Convective Definitions

Tornado Watch
The Storm Prediction Center (SPC) issues Public Tornado Watches to alert the public, media and emergency managers to organized thunderstorms forecast to produce three or more tornadoes or any tornado which could produce F2 or greater damage. The forecast event minimum thresholds should be at least 2 hours over an area of at least 8,000 square miles.
Severe Thunderstorm Watch
The Storm Prediction Center (SPC) issues Public Severe Thunderstorm Watches to alert the public, media and emergency managers to organized thunderstorms forecast to produce six and more hail events of 3/4 inch (penny) diameter or greater, or damaging winds of 50 knots (58 mph) or greater. The forecast event minimum thresholds should be at least 2 hours over an area of at least 8,000 square miles.
Tornado Warning
Tornado Warnings are issued when there is radar indication and/or reliable spotter reports of a tornado.
Severe Thunderstorm Warning
Severe Thunderstorm Warnings are issued when there is radar indication and/or reliable spotter reports of hail of 3/4 inch (penny) diameter or greater, and/or wind gusts of 50 knots (58 mph) or greater.

Winter Weather Definitions

Watches and warnings:

Blizzard
Visibility frequently below 1/4 mile in falling or blowing snow, and wind speeds (sustained or frequent gusts) at least 35 mph. These conditions are expected to continue for 3 hours or longer.
Winter Storm
A snow accumulation of at least 6 inches in 12 hours, or at least 8 inches in 24 hours, in combination with one of the following: rain, freezing rain, sleet, wind, blowing snow or cold (wind chill).

(Note: lesser amount of snow can be forecast for mixed precipitation, blowing, etc.)

Lake Effect Snow
A pure lake effect snow event that is forecast to produce a snow accumulation of at least 6 inches in 12 hours, or at least 8 inches in 24 hours.

(Note: lesser amount of snow can be forecast for mixed precipitation, blowing, etc.)

Ice Storm
A significant, and possibly damaging, accumulation of ice. Normally a coating of at least 1/4-inch, but may be up to 1/2-inch if winds are less than 15 mph.
Heavy Snow
A snow accumulation of at least 6 inches in 12 hours, or at least 8 inches in 24 hours. Heavy snow accumulations are the only hazard expected.
Sleet
An accumulation of sleet of at least 1/2-inch.

Warnings only:

Wind Chill
Wind chill values fall to -30°F or colder, with winds equal to or greater than 10 mph.

Advisories:

Snow
Synoptic scale storm producing snow (average forecast range) of greater than 3 inches and less than warning criteria (6 inches) in 12 hours.

(Note: lesser amount of snow can be forecast for first snowfall, long duration between snow, etc.)

Snow and Blowing/Drifting Snow
Situations that cause significant inconveniences, and do not meet warning criteria. Sustained wind or frequent gusts of 25 to 34 mph accompanied by falling and blowing snow, occasionally reducing visibility to less than 1 mile for three hours or more. If caution is not exercised, this could lead to life-threatening situations.
Freezing Rain or Freezing Drizzle
Situations that cause significant inconveniences, and do not meet warning criteria. If caution is not used, this could lead to life-threatening situations. Ice accumulation causes driving or walking problems, but no damage to trees or power lines.
Lake Effect Snow
Pure lake effect snow event that is forecast to produce snow (average of forecast range) of greater than 3 inches and less than warning criteria (6 inches) in 12 hours.
Winter Weather
A synoptic scale storm producing a combination of winter weather that presents a hazard, and does not meet warning criteria. Snowfall in this case does not have to reach snow advisory criteria.
Wind Chill
Wind chill values drop between -20 and -29°F inclusive, with winds equal to or greater than 10 mph.
Blowing Snow
Blowing snow reducing visibilities to 1 mile or less and winds less than 35 mph.
Sleet Advisory
Situations that cause significant inconveniences, and do not meet warning criteria. If caution is not used, this could lead to life-threatening situations. Ice accumulation causes driving or walking problems, but no damage to trees or power lines.

Hydrology/Flooding Definitions

Flood Watch
Flood watches inform the public of hydrometeorological conditions which may cause flooding when the flooding is neither certain nor imminent. Flood watches may cover states, counties, rivers, portions of states, portions of counties, or portions of rivers (e.g., one or more forecast points). Flood Watches provide advance notice and up-to-date information on the possibility of flooding within 36 hours.
Flash Flood Warning
Flash flood warnings are issued when flooding is imminent. This product will be reserved for those short-term events which require immediate action to protect lives and property, such as dangerous small stream flooding or urban flooding, and dam or levee failures. Flash flood warnings may cover counties, portions of counties, well-known geographical areas (e.g., deserts, valleys), or river basins.
Flood Warning
Flood warnings are issued for any high flow, overflow, or inundation not covered by flash flood warning products. There are two general types of flood warnings - areal flood warnings and river flood warnings issued for specific forecast points. Flood warnings for forecast points usually include information on the impacts of expected flooding at locations upstream and/or downstream from covered forecast point(s) on a river or stream. Areal flood warnings are issued for areas along rivers and streams not associated with a forecast point, or for counties or portions of counties (with the areas covered described in the same way as for flash flood warnings).
Urban and/or Small Stream Advisories
Flood statements issued to provide information on elevated river/stream flows or ponding of water in urban or other areas, when such events warrant notification of the public in a product less urgent than a warning.

Non-Precipitation Definitions

Watches and warnings:

High Wind
Sustained non-convective winds greater than or equal to 40 mph lasting for one hour or longer, and/or gusts greater than or equal to 58 mph for any duration.
Excessive Heat
A heat index of 110°F or higher is expected for a period of 3 hours or more. An excessive heat warning shall be continued through the overnight hours, following a day with excessive heat, if the heat index is not expected to fall below "around 80°F".

Warnings only:

Freeze
Freezing temperatures (shelter temperature 32°F or below) occurs during the growing season.

Advisories:

Dense Fog
Widespread visibilities reduced to less than or equal to 1/4-mile in fog.
Wind
Sustained non-convective winds greater than or equal to 30 mph lasting for one hour or longer, or winds greater than or equal to 45 mph for any duration.
Heat
A heat index of 100°F or higher is expected for a period of 3 hours or more. A heat advisory shall be continued through the overnight hours, following a day with excessive heat, if the heat index is not expect to fall below "around 80°F".
Frost
Conditions that would lead to frost formation during the growing season, but warmer than freezing conditions. Generally clear, calm night with shelter temperatures of 33 to 37°F.