National Weather Service Weather Warnings, Watches, Advisories and criteria.
Hazardous Weather Outlook
The Hazardous Weather Outlook will describe potential hazardous weather and hydrologic information of concern in Days 1 through 7.
The outlook contains two segments: One segment for the marine zones and adjacent land-based (i.e., coastal) zones and the other segment for the rest of the land-based zones. Each segment of the HWO will contain 3 sections: short term through Day 1, long term for Days 2-7, and spotter information.
Hazardous Weather (Winter)
Hazardous winter weather conditions are occurring, imminent or likely. Conditions will cause a significant inconvenience and if caution is not exercised, will result in a potential threat to life and/or property. The generic term, winter weather advisory, is used for a combination of two or more of the following events; snow, freezing rain or drizzle, sleet, blowing snow.
Winter Storm Watch
Conditions are favorable for hazardous winter weather conditions including heavy snow, blizzard conditions, or significant accumulations of freezing rain or sleet. These watches are usually issued 12 to 36 hours in advance of the event.
Specific criteria are subjective based on region, however generally speaking significant and hazardous winter weather is defined as a combination of:
1) 4 inches or more of snow/sleet within a 12-hour period or 7 inches or more of snow/sleet within a 24-hour period
AND/OR
2) Enough ice accumulation to cause damage to trees or powerlines.
AND/OR
3) A life threatening or damaging combination of snow and/or ice accumulation with wind.
Sterling Virginia Example
A Winter Storm Watch is issued when there is the potential for significant and hazardous winter weather within 48 hours. It does not mean that significant and hazardous winter weather will occur…it only means it is possible.Significant and hazardous winter weather is defined as a combination of:
1) 5 inches or more of snow/sleet within a 12-hour period or 7 inches or more of snow/sleet within a 24-hour periodAND/OR
2) Enough ice accumulation to cause damage to trees or powerlines.
AND/OR
3) a life threatening or damaging combination of snow and/or ice accumulation with wind.
The snow/sleet criteria for a Winter Storm Watch for the five westernmost counties (Allegany, Mineral, Grant, Pendleton, and Highland) is higher (6 inches or more within a 12-hour period; 8 inches or more within a 24-hour period).
Blizzard Warning
A Blizzard Warning means that the following conditions are occurring or expected within the next 12 to 18 hours.
1) Snow and/or blowing snow reducing visibility to 1/4 mile or less for 3 hours or longer
AND
2) Sustained winds of 35 mph or greater or frequent gusts to 35 mph or greater.
There is no temperature requirement that must be met to achieve blizzard conditions.
Winter Storm Warning
A Winter Storm Warning is issued when a significant combination of hazardous winter weather is occurring or imminent. Specific criteria are subjective based on region. The criteria listed below are consistent with the Chicago, IL (NWS) Office:
1) Heavy snow of 6 inches or greater.
AND/OR
2) Heavy sleet of half inch or more.
AND/OR
3) Conditions meet the criteria for two or more separate warnings. Example: If you have Heavy Snow Warning conditions along with Wind Chill Warning conditions, a Winter Storm Warning will be issued.
Sterling Virginia Example
A Winter Storm Warning is issued when a significant combination of hazardous winter weather is occurring or imminent.
Significant and hazardous winter weather is defined as a combination of:
1) 5 inches or more of snow/sleet within a 12-hour period or 7 inches or more of snow/sleet within a 24-hour periodAND/OR
2) Enough ice accumulation to cause damage to trees or powerlines.
AND/OR
3) a life threatening or damaging combination of snow and/or ice accumulation with wind.
The snow/sleet criteria for a Winter Storm Warning for the five westernmost counties (Allegany, Mineral, Grant, Pendleton, and Highland) is higher (6 inches or more within a 12-hour period; 8 inches or more within a 24-hour period).
Ice Storm Warning
An Ice Storm Warning is issued when freezing precipitation is expected with 1/4 inch or more of ice accumulation.
Ice Storm Warnings are generally issued 12 to 24 hours prior to the expected start of the event. There are no Ice Storm Watches.
Freezing Rain Advisory
A Freezing Rain Advisory is issued when freezing precipitation is expected, however ice accumulations are anticipated to be less than 1/4 inch.
These are generally issued 12 to 24 hours prior to the expected start of the event. There are no Freezing Rain Watches or Warnings.
Winter Weather Advisory
Hazardous winter weather conditions are occurring, imminent or likely. Conditions will cause a significant inconvenience and if caution is not exercised, will result in a potential threat to life and/or property. The generic term, winter weather advisory, is used for a combination of two or more of the following events; snow, freezing rain or drizzle, sleet, blowing snow.
Sterling Virginia Example
A Winter Weather Advisory will be issued when 2 to 4 inches of snow, alone or in combination with sleet and freezing rain, is expected to cause a significant inconvenience, but not serious enough to warrant a warning.
In some cities, if the event is expected to impact the metro areas during rush hours (4-9 am or 2-7 pm on weekdays) forecasted snow totals of one inch will necessitate the issuance of a winter weather advisory.
Freeze Watch
A Freeze Watch is issued during the growing season when there is a potential for significant, widespread freezing temperatures within the next 24-36 hours.
A Freeze Watch is issued in the autumn until the end of the growing season (marked by the occurrence of first widespread freeze). The normal end of the growing season is mid to late October west of the Blue Ridge and early November east of the Blue Ridge. However, during anomalously warm autumns, the growing season may be extended past the normal end of the growing season.
A Freeze Watch is issued in the spring at the start of the growing season (when it is late enough to cause damage to new plants and crops).
Freeze Warning
A Freeze Warning is issued when significant, widespread freezing temperatures are expected.
A Freeze Warning is issued in the autumn until the end of the growing season (marked by the occurrence of first widespread freeze).
The normal end of the growing season is mid October (10/12) in northwest Cook County, and late October (10/24) in the City of Chicago. However, during anomalously warm autumns, the growing season may be extended past the normal end of the growing season. Information subject to change without immediate notice.
A Freeze Warning is issued in the spring at the start of the growing season (when it is late enough to cause damage to new plants and crops).
The latest Spring freeze has been in late May in northwest Cook County (5/25), and in mid May in the City of Chicago (5/12). Information subject to change without immediate notice.
Frost Advisory
A Frost Advisory is issued when the minimum temperature is forecast to be 33 to 36 degrees on clear and calm nights during the growing season.
A Frost Advisory is issued in the autumn until the end of the growing season (marked by the occurrence of first widespread freeze).
The normal end of the growing season is mid October (10/12) in northwest Cook County, and late October (10/24) in the City of Chicago. However, during anomalously warm autumns, the growing season may be extended past the normal end of the growing season. Information subject to change without immediate notice.
A Frost Advisory is also issued in the spring at the start of the growing season (when it is late enough to cause damage to new plants and crops).
The latest Spring freeze has been in late May in northwest Cook County (5/25), and in mid May in the City of Chicago (5/12). Information subject to change without immediate notice.
Wind Chill Advisory
A Wind Chill Advisory is issued when the wind chill is less than or equal to -20°F AND the winds are greater than or equal to 10 mph.
Wind Chill Warning
A Wind Chill Warning is issued when the wind chill will be less than or equal to -30°F AND the winds are greater than or equal to 10 mph.
Dense Fog Advisory
A Dense Fog Advisory is issued when widespread fog is expected to reduce visibilities to 1/4 mile or less over a large area for an extended period of time (2 or more hours).
High Wind Watch
A High Wind Watch is issued when the following conditions are possible:
1) sustained winds of 40 mph or higher for one hour or more
OR
2) wind gusts of 58 mph or higher for one hour or more.
High Wind Warning
A High Wind Warning is issued when the following conditions are occuring or imminent:
1) sustained winds of 40 mph or higher for one hour or more
OR
2) wind gusts of 58 mph or higher for one hour or more.
Hurricane Watch
A Hurricane Watch is issued when a tropical cyclone containing winds of 64 kt (74 mph) or higher poses a possible threat, generally within 48 hours. These winds may be accompanied by storm surge, coastal flooding, and/or river flooding.
The watch does not mean that hurricane conditions will occur. It only means that these conditions are possible.
Hurricane Warning
A Hurricane Warning is issued when sustained winds of 64 kt (74 mph) or higher associated with a tropical cyclone are expected in 36 hours or less. These winds may be accompanied by storm surge, coastal flooding, and/or river flooding. A hurricane warning can remain in effect when dangerously high water or a combination of dangerously high water and exceptionally high waves continue, even though winds may be less than hurricane force.
Wind Advisory
A Wind Advisory is issued when the following conditions are expected for 3 hours or longer.
1) sustained winds of 31 to 39 mph
AND/OR
2) wind gusts of 46 to 57 mph.
Extreme Wind Warning
An Extreme Wind Warning is issued for surface winds of 100 knots (115 MPH) or greater associated with non-convective, downslope, derecho (NOT associated with a tornado), or sustained hurricane winds are expected to occur within one hour. There is no Extreme Wind Watch.
Small Craft Advisory
Small Craft Advisories are issued for the Tidal Potomac River and the Chesapeake Bay when one or both of the following conditions is expected to begin within 36 hours:
1) sustained winds of 18 knots to 33 knots
OR
2) frequent gusts (duration of two or more hours) between 18 knots and 33 knots.
OR
3) waves of 4 feet or higher
Gale Warning
Gale Warnings are issued for the Tidal Potomac River and the Chesapeake Bay when one or both of the following conditions is expected to begin within 36 hours and not directly associated with a tropical cyclone:
1) sustained winds of 34 knots to 47 knots
OR
2) frequent gusts (duration of two or more hours) between 34 knots and 47 knots.
Storm Warning
Storm Warnings are issued for the Tidal Potomac River and the Chesapeake Bay when one or both of the following conditions is expected to begin within 36 hours and not directly associated with a tropical cyclone:
1) sustained winds of 48 knots to 63 knots
OR
2) frequent gusts (duration of two or more hours) of 48 knots to 63 knots
Hurricane Force Wind Warning
Hurricane Force Wind Warnings are issued for the Tidal Potomac River and the Chesapeake Bay when one or both of the following conditions is expected to begin within 36 hours and not directly associated with a tropical cyclone:
1) sustained winds of 64 knots or greater
OR
2) frequent gusts (duration of two or more hours) of 64 knots or greater
Special Marine Warning
A warning of potentially hazardous weather conditions of short duration (up to 2 hours) affecting areas included in a CWF that are not adequately covered by existing marine warnings and producing one or more of the following:
1) Sustained marine convective winds (showers/thunderstorms) or associated gusts of 34 knots or greater
AND/OR
2) Hail three quarters of an inch or more in diameter
AND/OR
3) Waterspouts
Severe Thunderstorm Watch
A Severe Thunderstorm Watch is issued when severe thunderstorms are possible in and near the watch area. It does not mean that they will occur. It only means they are possible.
Severe thunderstorms are defined as follows:
1) Winds of 58 mph or higher
AND/OR
2) Hail 1 inch in diameter or larger.
Severe Thunderstorm Warning
A Severe Thunderstorm Warning is issued when severe thunderstorms are occurring or imminent in the warning area.
Severe thunderstorms are defined as follows:
1) Winds of 58 mph or higher
AND/OR
2) Hail 1 inch in diameter or larger.
Tornado Watch
A Tornado Watch is issued when severe thunderstorms and tornadoes are possible in and near the watch area. A Tornado Watch does not mean that they will occur. It only means they are possible.
A Tornado Watch is issued for a large area (roughly on the order of 100 square miles or more) by SPC when severe thunderstorms capable of producing tornadoes are possible in and near the watch area.
Severe thunderstorms are defined as follows:
1) Winds of 58 mph or higher
AND/OR
2) Hail 1 inch in diameter or larger.
Tornado Warning
A Tornado Warning is issued when a tornado is imminent. When a tornado warning is issued, seek safe shelter immediately.
A Tornado Warning is issued by the NWS when a tornado is occurring or imminent. The area warned is a localized area specific to the storm producing the tornado and the areas expected to be impacted. When a tornado warning is issued, seek safe shelter immediately. A time frame often accompanies the warning of the specific localized area.
Tornado Emergency
A Tornado Emergency is a more notorious version of Tornado Warning issued (rarely) at the discretion of the NWS during significant tornado occurrences in highly populated areas. A Tornado Emergency generally means that significant, widespread damage is expected to continue and a high likelihood of numerous fatalities is expected with a large, strong to violent tornado.
Tropical Storm Watch
A Tropical Storm Watch is issued when a tropical cyclone containing winds of 34 to 63 kt (39 to 73 mph) or higher poses a possible threat, generally within 48 hours. These winds may be accompanied by storm surge, coastal flooding, and/or river flooding.
The watch does not mean that tropical storm conditions will occur. It only means that these conditions are possible.
Tropical Storm Warning
A Tropical Storm Warning is issued when sustained winds of 34 to 63 kt (39 to 73 mph) or higher associated with a tropical cyclone are expected in 36 hours or less. These winds may be accompanied by storm surge, coastal flooding, and/or river flooding.
Coastal Flood Watch
A Coastal Flood Watch is issued when moderate to major coastal flooding is possible. Such flooding would potentially pose a serious risk to life and property.
Coastal Flood Watches are issued for coastal areas along the Tidal Potomac River and coastal areas along the Chesapeake Bay.
Coastal Flood Warning
A Coastal Flood Warning is issued when moderate to major coastal flooding is occurring or imminent. This flooding will pose a serious risk to life and property.
Coastal Flood Warnings are issued for coastal areas along the Tidal Potomac River and areas along the Chesapeake Bay.
Coastal Flood Advisory
A Coastal Flood Advisory is issued when minor or nuisance coastal flooding is occurring or imminent.
Coastal Flood Advisories are issued for coastal areas along the Tidal Potomac River and areas along the Chesapeake Bay.
Flash Flood Watch
A Flash Flood Watch is issued when conditions are favorable for flash flooding. It does not mean that flash flooding will occur, but it is possible.
Flash Flood Warning
A Flash Flood Warning is issued when flash flooding is imminent or occurring.
Flood Watch
A Flood Watch is issued when conditions are favorable for flooding. It does not mean flooding will occur, but it is possible.
Flood Warning
A Flood Warning is issued when flooding is imminent or occurring.
River Flood Watch
A River Flood Watch is issued when river flooding is possible at one or more forecast points along a river.
River Flood Warning
A River Flood Warning is issued when river flooding is occurring or imminent at one or more forecast points along a river.
Excessive Heat Watch
An Excessive Heat Watch is issued when there is a potential for the heat index value to reach or exceed 110 degrees within the next 24 to 48 hours.
Excessive Heat Warning
An Excessive Heat Warning is issued when the heat index value is expected to reach or exceed 110 degrees within the next 12 to 24 hours. An Excessive Heat Warning may be issued for lower criteria if it is early in the season or during a multi-day heat wave.
Heat Advisory
A Heat Advisory is issued when the heat index value is expected to reach 105 to 109 degrees within the next 12 to 24 hours. A Heat Advisory may be issued for lower criteria if it is early in the season or during a multi-day heat wave.
Fire Weather Watch
A Fire Weather Watch is issued to alert fire officials and firefighters of potentially dangerous fire weather conditions within the next 24 to 36 hours.
A Fire Weather Watch is issued by the National Weather Service (NWS) when potentially dangerous fire weather conditions are expected within the next 24 to 72 hours.
Specific fire weather criteria needing to be met for a Fire Weather Watch to be issued are subjective based on region. The following are the criteria needed for Northern Illinois and Northwest Indiana:
1) Sustained winds of 20 mph or higher.
2) Afternoon relative humidity less than 25%.
3) 10 hour fuel moisture at 8% or less for one day.
As an example of another specific region, a Fire Weather Watch is issued in Sterling Virginia to alert fire officials and firefighters of potentially dangerous fire weather conditions within the next 24 to 36 hours using the following three criteria:
1) Surface relative humidity (RH) less than 30 % for VA and MD; RH less than or equal to 25% for WV AND
2) Sustained surface wind of 20 MPH or greater AND
3) 10-hour fuel moisture less than 8% for VA; less than or equal to 8% for MD and WV
Red Flag Warning
A Red Flag Warning is issued to alert fire officials and firefighters of potentially dangerous fire weather conditions within the next 24 hours. Specific fire weather criteria needing to be met for a Red Flag Warning to be issued are subjective based on region. The following are the criteria needed for Northern Illinois and Northwest Indiana:
1) Sustained winds of 20 mph or higher.
2) Afternoon relative humidity less than 25%.
3) 10 hour fuel moisture at 8% or less for one day.
As an example of another specific region, a Red Flag Warning is issued to alert fire officials and firefighters in Sterling Virginia of potentially dangerous fire weather conditions within the next 12 to 24 hours when the following three criteria are met:
1) Surface relative humidity (RH) less than 30 % for VA and MD; RH less than or equal to 25% for WV AND
2) Sustained surface wind of 20 MPH or greater AND
3) 10-hour fuel moisture less than 8% for VA; less than or equal to 8% for MD and WV