- Public Safety
- Williamson Ready
- Stay Alert
- Weather Alerts and Terminology
Weather Alerts and Terminology
Ever wonder what weather alerts are issued in Middle Tennessee, and what the criteria are for issuing different types of weather alerts?
Local Weather Products
There are some National Weather Service alerts (also called weather “products”) that are available in other parts of the country but not used in Middle Tennessee. For example, an “extreme wind warning” is issued for land falling hurricanes, and therefore would never be issued in Williamson County. Here, we will only ever see wind advisories or high wind watches and warnings.
Weather product “criteria” also varies based on geographic location. For example, the amount of snow and ice required to issue a winter storm warning in Colorado is very different from the requirements for a winter storm warning in Middle Tennessee.
The terms and criteria explained below are everything that could be issued from the National Weather Service office in Middle TN. The full weather product guide is also available on the NWS Nashville website.
These are the general differences between a watch, advisory, warning, outlook, and statement:
Used when the risk of an event has increased significantly, but its occurrence, location or timing is still uncertain. It is intended to provide enough lead time so those who need to set their plans in motion can do so. A watch means that hazardous weather is possible.
Issued when an event is occurring, imminent or likely. Advisories are for less serious conditions than warnings, but conditions that could still cause significant inconvenience or lead to situations that may threaten life or property.
Issued when a hazardous weather or hydrologic event is occurring, imminent or likely. A warning means weather conditions pose a threat to life or property. People in the path of the storm need to take protective action.
These usually do not have any type of criteria, but are meant to be a first glance of hazardous weather potential.
These usually do not have any type of criteria, but are meant to highlight significant short-term weather that is not already covered in an advisory, watch, or warning product.
Outlooks and Statements
Hazardous Weather Outlook
The Hazardous Weather Outlook is issued at least once daily around 5 am and is updated as needed. It provides a “first glance” of any expected hazardous weather conditions over the next seven days. This includes, but is not limited to, severe weather, flooding, extreme heat or cold, winter weather, and dense fog. There is no specific criteria for an outlook, it is more of an informational product.
Special Weather Statement
The Special Weather Statement augments the current forecast and highlights significant short-term weather that is not already covered in an advisory, watch, or warning product. Examples include a line of strong, but not severe, thunderstorms or an area of moderately intense snow showers.
Since this is issued for a wide range of potentially hazardous weather, there is no specific criteria. These statements are used for any type of hazardous weather that does not necessarily meet “warning” or “advisory” criteria. Here in Middle TN, statements are often used for strong thunderstorms (usually a lot of lightning and/or winds 30-58 mph and/or hail of 0.25” to 0.88”), light snow accumulations (a dusting that isn’t expected to stick), patchy fog, etc.
Frost and Freeze
Frost and freeze products are tied to the growing season. For these, NWS typically coordinates with the Midwestern Regional Climate Center on when to “start” and “end” these products. The average first autumn frost/freeze date is October 23, with the last frost/freeze date being April 10 for Williamson County.
A freeze watch is issued when there is a potential for significant, widespread freezing temperatures (below 32 degrees) within the next 24-36 hours during the growing season.
A freeze warning is issued when temperatures are forecasted to be below 32 degrees within the next 12 hours during the growing season.
A frost advisory is issued when the minimum temperature is forecast to be between 33 and 36 degrees on clear, calm nights with light winds 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) and in the spring at the start of the growing season (when it is late enough to cause damage to new plants and crops).
Winter Storm Watch
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, only that it is possible. In Middle TN specifically, this means that at least 3 inches of snow is possible.
Winter Storm Warning
A Winter Storm Warning is issued when a significant combination of hazardous winter weather is occurring or imminent. In Middle TN specifically, this means that at least 3 inches of snow is imminent.
A Blizzard Warning means that the following conditions are occurring or expected within the next 12 to 18 hours: sustained winds or frequent gusts of 35 mph or higher with falling and/or blowing snow where visibility is reduced to less than 1/4 mile for at least 3 hours.
Ice Storm Warning
An Ice Storm Warning is issued when at least 0.25” of ice accumulation is occurring or imminent.
Winter Weather Advisory
A Winter Weather Advisory is issued when 1-3 inches of snow (alone or in combination with sleet and freezing rain) and/or less than 0.25” of ice accumulation is expected. In other words, enough winter weather to cause a significant inconvenience, but not serious enough to warrant a warning.
Fog and Wind
Wind Chill Advisory
A Wind Chill Advisory is issued when wind chills of 5 degrees F to -4 degrees F are occurring or imminent.
Wind Chill Warning
A Wind Chill Warning is issued when wind chills of -5 degrees F or below are occurring or imminent.
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 2 or more hours.
High Wind Watch / Warning
A high wind watch or high wind warning is issued when the following conditions are possible:
- Sustained winds of 40 mph or higher for one hour or more OR
- Wind gusts of 58 mph or higher for any duration.
A wind advisory is issued when the following conditions are expected or met:
- Sustained winds of 25 - 39 mph for any duration OR
- Wind gusts of 40 - 57 mph for any duration.
The Storm Prediction Center has a lot of information specifically about watches, available here:
Severe Thunderstorm and Tornado Watches are issued in collaboration with the Storm Prediction Center (SPC), NWS, and any neighboring offices that may be impacted.
Severe Thunderstorm Watch
A Severe Thunderstorm Watch is issued when severe thunderstorms are possible in and near the watch area, but the primary threat expected are severe thunderstorms. It does not mean that they will occur. It only means they are possible.
Severe Thunderstorm Warning
Severe Thunderstorm Warnings are issued for storms that are producing or expected to produce winds of 58 mph or more, hail 1 inch or greater diameter, or both. These are issued for counties or portions of counties, and are typically an hour or less in length.
Note: You may notice something called “tags” with some warnings:
- “Considerable” Damage Threat tag: maximum wind speeds between 70 to 80 mph and/or 1.75” to 2.5” hail expected
- “Destructive” Damage Threat tag: maximum wind speeds over 80 mph and/or 2.75” or larger hail expected
A Tornado Watch is issued when severe thunderstorms and tornadoes are possible in and near the watch area, but the primary threat expected are tornadoes. It does not mean that they will occur. It only means they are possible.
Tornado Warnings are issued for expected tornadic activity over a county, or a portion of a county. A warning may be issued based on Doppler radar indications of a tornado, reports of a funnel cloud, or observed tornado sightings. Tornado Warnings are typically valid for 30 to 60 minutes.
Note: You may notice something called “tags” with some warnings:
- “Considerable” tag: the forecaster is confident of a strong tornado occurring or imminent (EF-2 to EF-3)
- “Tornado Emergency” or “Catastrophic” tag: the forecaster is confident that a violent tornado occurring or imminent (EF-4+)
The Flood Watch is issued when heavy rainfall is forecast to cause significant or widespread flooding. This is usually issued at least 6 hours in advance of the expected event, and on occasion as much as a day or two in advance in high confidence situations. Updates to an ongoing flood watch are also issued under the FFA product. If the event is still a few days away, a Hydrologic Outlook may be issued instead, to give a heads-up to the potential of heavy rainfall.
Criteria: There is no “specific” criteria for when we will issue a Flood Watch. However, if we are expecting widespread rainfall amounts of 3+”, this typically exceeds our flash flood guidance. It is up to the forecaster's discretion and collaboration with surrounding NWS offices.
Flash Flood Warning
Flash Flood Warning is issued when life or property-threatening, short-term flooding is imminent. Flash Flood Warnings may be valid for up to 6 hours, and can be extended as necessary. This is for non-convective causes (i.e. dam or levee failures), not convective (thunderstorm) based, and the criteria is based on the forecaster’s discretion.
A Flood Warning is issued when flooding is expected to threaten life and property a few hours after the onset of heavy rain, ice jams, reservoir releases or snow melt. Flood warnings may be in effect for days or even weeks depending on weather and soil conditions, land topography, and the size of a river. Updated information will be found in Flood Statements. Flood Warnings will be re-issued if the river forecast changes significantly.
Criteria is based on stream and river levels. Typically the warning will mention if it is flash flooding (local creeks and streams, usually quicker in time) or areal flooding (larger rivers & streams, more prolonged).
A Flood Advisory is issued when a specific weather event that is forecast to occur may become a nuisance. A Flood Advisory is issued when flooding is not expected to be bad enough to issue a warning. However, it may cause significant inconvenience, and if caution is not exercised, it could lead to situations that may threaten life and/or property. The criteria is based on the forecaster's discretion.
Heat and Fire
Excessive Heat Watch
An Excessive Heat Watch is issued when heat index values greater than or equal to 110 degrees OR temperatures greater than or equal to 105 degrees are possible within the next 12 to 24 hours.
Excessive Heat Warning
An Excessive Heat Warning is issued when heat index values greater than or equal to 110 degrees OR temperatures greater than or equal to 105 degrees are occurring or imminent. An Excessive Heat Warning may be issued for lower criteria if it is early in the season or during a multi-day heat wave.
A Heat Advisory is issued when the heat index value is expected to reach 105 to 109 degrees OR temperatures between 103 to 105 are occurring or imminent within the next 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 watches are issued to alert land management agencies of the potential for a Red Flag event. A Red Flag event is a combination of a critical fire weather pattern and critically dry fuels. The Watch will be issued when the fire weather forecaster is reasonably confident that a Red Flag event will occur, 12-72 hours in advance of the expected onset. The watch remains in effect until it is upgraded to a Red Flag Warning or it is determined that the Red Flag event will not develop.
Red Flag Warning
Red Flag warnings are issued to alert land management agencies of developing weather conditions that, when coupled with critically dry fuels, could lead to large and dangerous wildfires. Weather conditions that would contribute to a Red Flag event would be strong winds, low relative humidity, or dry lightning. In Middle TN specifically, this means winds speeds greater than or equal to 20 mph AND relative humidity less than or equal to 25% along with dry fuels. Knowledge of fuel conditions is essential, therefore NWS coordinates their decision with the Tennessee Department of Agriculture for either a Red Flag Warning or a Fire Weather Watch.
Other Weather Products
The NWS Nashville Product Guide for Middle Tennessee explains all weather products used by the NWS Nashville office. The following weather products are available to provide various weather information to the public.
The Hydrologic Outlook is used to give an outlook of heavy rainfall and possible flooding more than 36 or 48 hours out in the forecast. In the late winter and early spring, this product can give information on the possibility of flooding due to snowmelt and expected springtime precipitation.
The Hydrologic Summary product is issued daily around 9:30 AM. It contains observed river and lake levels, along with precipitation amounts across Middle Tennessee.
Daily Climatological Summaries
Climatological summaries for data collected at Nashville, Clarksville, and Crossville are issued twice daily (1:30 AM CST/2:30 AM CDT, and 4:30 PM CST/CDT). These summaries contain information on temperatures, degree days, precipitation, wind, climatological normals, and records.
Monthly Climatological Summaries
Preliminary climatological summaries for the past month are issued on the 1st of each month, normally before 12:00 PM CST/CDT, for Nashville. The summaries include average temperature and maximum observed values, degree days, precipitation, and comparisons to normal.
The Record Report is issued when record conditions have been observed from Middle Tennessee and south central Indiana official observing sites.
Regional Temperature and Precipitation Table
The Regional Temperature and Precipitation Table is issued twice daily: between 9:00 and 10:00 AM CST (10:00 and 11:00 AM CDT), and again between 7:30 and 8:30 PM CST (8:30 and 9:30 PM CDT). It contains basic climate information from around Middle Tennessee from airport observation sites and cooperative observers. The data primarily consists of high and low temperatures, and measured precipitation, including snow depth if applicable.
Tennessee Temperature and Precipitation Roundup
The State Temperature and Precipitation Table is issued twice daily no later than 7:30 AM/PM CST (8:30 AM/PM CDT). It lists the maximum and minimum temperatures along with precipitation and snow depth (if applicable) for select cities across Tennessee.
Nashville Air Quality Index
The Metropolitan Nashville Air Pollution Control District monitors air quality at various locations around Nashville, Tennessee. At minimum four times each weekday, the Air Quality Index is issued by NWS Nashville which includes the current air quality, the elevated pollutant, the current pollution category, and a short-term forecast for the pollution index. In the case of dangerous pollutants, health advisories or alerts would be included at the end of the product.
An Earthquake Report is issued upon occurrence of an earthquake being felt or causing damage in Middle Tennessee. The information is relayed to the U.S. Geological Survey, who establishes the official Richter Scale reading for the quake.
Non-weather related messages
- Child Abduction Emergency (issued at request of TBI)
- Civil Emergency Message (issued by local or state emergency management)