National Weather Service United States Department of Commerce

Hazardous Weather Outlook:
Definitions of Hazard Levels and Their Associated Triggers

 

Hazard Level 0 1 2 3 4 5
Threat to life or
property and/or
economic impact
Nil Minimal Small Medium Significant Very
Significant
 
Grassland Fire Danger Index     Very High Extreme Ongoing Fires  
Heat Index (°F)     100° 105° for 3 hours/
Advisory
110° for 3 hours/
Warning
110° for 3 hours/
Warning for 3 days
Wind Chill (°F)     -15° for 3 hours/
Advisory
-25° for 3 hours/
Warning
-25° for 3 hours/
Warning for 3 days
Blowing Dust or Smoke         Blowing Dust Advisory
(3/8sm - 1sm)
Blowing Dust Warning
Visibility < 3/8sm
Other Obstruction to Visibility (statute miles) Nil Obstruction to Visibility
in grids
Patchy 1/2sm Advisory Widespread
<1/8sm
 
Wind
(non-convective)
    Advisory High Wind Warning High Wind Warning for 3 hours or more Hurricane Force
Frost/Freeze Nil Frost
mentioned
Frost
Advisory
Freeze
Warning
   
Lightning Nil Thunder mentioned        
Hail Nil Below Significant
Weather Alert
Significant Weather Alert Severe Hail Likely >= 2 inches > 3 inches
Wind
(convective)
Nil Below Significant
Weather Alert
Significant Weather
Alert
Severe Wind Likely 75 mph Derecho
Tornado Nil   Cold Air
Funnels
Chance Likely Tornado
< EF4
EF4-EF5
Winter Weather Nil Mentioned Below Winter Weather
Advisory Criteria
Advisory
or Watch
Warning Paralyzing Winter Weather (ie., 1 inch or more ice or blizzard w/ heavy snow)
River Flood Nil   Action Stage
  Likely
Flood Warning
(minor-moderate)
Major
Flooding
Record Flooding
Flood/Flash flood Nil   Flood Warnings Possible
(FLW)
Likely Flood Warnings (FLW);
Possible Flash Flood Warnings (FFW), Flood/Flash Flood Watches (FFA)
Widespread Flooding,
or high probability of Flash Flood Warnings (FFW)
Dam Break/
Catastrophic Flooding