To get a watch (severe thunderstorm watch, winter storm watch, flood watch, etc.), there has to be at least a 50% chance that the weather will meet warning criteria, usually within 1-3 days. The watch is upgraded to a warning when the chances of meeting the criteria reach 80%, usually within 1 day or less.
All criteria listed below come straight from the National Weather Service
SEVERE WEATHER
Severe Thunderstorm Warning:
-Wind gusts at least 58mph, and/or
-Hail greater than 1 inch in diameter, and/or
-Tornado
Tornado Warning:
-Radar supports the possibility of a tornado, or
-A storm spotter confirms a tornado is on the ground
Tornado Emergency:
-A strong, life-threatening tornado is confirmed on the ground causing catastrophic
damage
HEAT
Heat Advisory
-Heat index values of 95-99° for at least 2 hours over 2 consecutive days, or
-Heat index values of 100-104° for at least 2 hours over 1 day
Excessive Heat Warning
-Heat index values of 105+° for at least 2 hours
Heat Wave
-A heat wave is declared when a location reaches 90+° for at least 3 consecutive days
COLD
Wind Chill Advisory
-For southern New England: Wind chills values of -15 to -24° for at least 3 hours
-For northern New England: Wind chill values of -20 to -29° for at least 3 hours
Wind Chill Warning
-For southern New England: Wind chill values at or below -25° for at least 3 hours
-For northern New England: Wind chill values at or below -30° for at least 3 hours
Frost Advisory
-Overnight low temperatures to drop to 33-36° during growing season
Freeze Warning
-Overnight low temperatures to drop to 32° or lower during growing season
WINTER
Winter Weather Advisory
-Snow or sleet accumulation average of 3 inches for southern New England or 4 inches
for northern New England in 12 hours
-Sustained winds (or frequent gusts) of at least 25mph accompanied by falling or
blowing snow reducing visibility
-Any accumulation of freezing rain on roadways, and/or
-Widespread black ice
Winter Storm Warning
-Snow or sleet accumulations average of 6 inches or more in 12 hours, or
-Snow or sleet accumulations average of 8 inches in southern New England or 9 inches
in northern New England in 24 hours
Blizzard Warning
-Sustained winds of at least 35mph and considerable falling or blowing snow that
reduces visibility to less than a quarter of a mile for at least 3 consecutive hours
Ice Storm Warning
-Half an inch or more of ice accumulation
Snow Squall Warning
-Short-lived burst of very heavy snowfall over a small area that causes sudden whiteout
conditions
FLOOD
Flood Advisory
-Expected overflow or inundation by water which does not threaten life or property
Flood Warning
-Expected overflow or inundation by water which causes damage and/or a threat to life
River Flood Warning
-Water level at river forecast point is expected to reach flood stage
Flash Flood Warning
-Rapid flow of high water into a normally dry area, or
-Rapid water level rise on a stream or creek, or
-Dam or levee failure, or
-Release from an ice jam causing rapid rise in a stream, creek or river
Flash Flood Emergency
-Extremely heavy rain causing severe threat to life and catastrophic damage
Coastal Flood Advisory
-Minor coastal flooding/splashover expected
Coastal Flood Warning
-Widespread moderate to major coastal flooding expected that is expected to damage
property and threaten life
WIND
Wind Advisory
-Sustained winds of 31-39mph for at least 1 hour, or
-Gusts of 46-57mph
High Wind Warning
-Sustained winds of 40mph or more for at least 1 hour, or
-Gusts higher than 57mph
Extreme Wind Warning
-Sustained winds of 111mph or more, usually signifying the approach of an eyewall of a
major hurricane
TROPICAL
Tropical Storm Warning
-Sustained winds of 39-73mph associated with a tropical storm expected within 36
hours
Hurricane Warning
-Sustained winds of 74mph or more associated with a hurricane expected within 36
hours
Storm Surge Warning
-Danger of life-threatening inundation from rising coastal waters moving inland from
shoreline within 36 hours associated with tropical or post-tropical systems
FIRE
Red Flag Warning
-Dry fuels combined with 1 or more of the following weather conditions:
-Sustained winds or frequent gusts of 25mph or more
-Relative humidity at or below 30%
-Rainfall amounts for the previous 5 days less than a quarter of an inch
-Lightning
-Significant dry frontal passage
-Dry thunderstorms
FOG/SMOKE
Dense Fog Advisory
-Widespread visibility reduced to less than a quarter of a mile for at least 3 hours
Freezing Fog Advisory
-Very light ice accumulations from fog at or below freezing
Dense Smoke Advisory
-Smoke reducing visibility to less than a quarter of a mile
Air Quality Alert
-Air quality is expected to drop below healthy levels from pollution or smoke
Air Stagnation Advisory
-Atmospheric conditions stable enough to cause pollutants to accumulate in a given
area