Severe weather awareness: What a 'tornado emergency' means

DETROIT – You may be aware of the terms "tornado watch" and "tornado warning," but you may not be aware of the term "tornado emergency."

That's because the National Weather Service (NWS) has never had to issue a tornado emergency in Michigan. 

• Watch the video above for the full explanation of when a tornado emergency is supposed to be issued.

NWS directices specify that tornado emergencies are reserved for rare situations when:

Understanding Michigan's severe weather alert terms

Thunderstorms

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:

• Winds of 58 mph or higher - AND/OR
• Hail 1 inch in diameter or larger.

This is issued by the National Weather Service when conditions are favorable for the development of severe thunderstorms in and close to the watch area. A severe thunderstorm by definition is a thunderstorm that produces one inch hail or larger in diameter and/or winds equal or exceed 58 miles an hour. The size of the watch can vary depending on the weather situation. They are usually issued for a duration of 4 to 8 hours. They are normally issued well in advance of the actual occurrence of severe weather. During the watch, people should review severe thunderstorm safety rules and be prepared to move a place of safety if threatening weather approaches.

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:

• Winds of 58 mph or higher - AND/OR
• Hail 1 inch in diameter or larger.

This is issued when either a severe thunderstorm is indicated by the WSR-88D radar or a spotter reports a thunderstorm producing hail one inch or larger in diameter and/or winds equal or exceed 58 miles an hour; therefore, people in the affected area should seek safe shelter immediately. Severe thunderstorms can produce tornadoes with little or no advance warning. Lightning frequency is not a criteria for issuing a severe thunderstorm warning. They are usually issued for a duration of one hour. They can be issued without a Severe Thunderstorm Watch being already in effect.

Severe Weather Risk Levels

What do the Marginal, Slight, Enhanced, Moderate, and High risk categories in the Convective Outlook mean? 

Marginal, Slight, Enhanced, Moderate, and High risks represent progressively larger threats for organized severe storm episodes. Here's a breakdown:

Thunderstorms (light green) - General or non-severe thunderstorms - Delineates, to the right of a line, where a 10% or greater probability of thunderstorms is forecast during the valid period.
1-Marginal (dark green) - Marginal risk - An area of severe storms of either limited organization and longevity, or very low coverage and marginal intensity.
2-Slight (yellow) - Slight risk - An area of organized severe storms, which is not widespread in coverage with varying levels of intensity.
3-Enhanced (orange) - Enhanced risk - An area of greater (relative to Slight risk) severe storm coverage with varying levels of intensity.
4-Moderate (red) - Moderate risk - An area where widespread severe weather with several tornadoes and/or numerous severe thunderstorms is likely, some of which should be intense. This risk is usually reserved for days with several supercells producing intense tornadoes and/or very large hail, or an intense squall line with widespread damaging winds.
5-High (magenta) - High risk - An area where a severe weather outbreak is expected from either numerous intense and long-tracked tornadoes or a long-lived derecho-producing thunderstorm complex that produces hurricane-force wind gusts and widespread damage. This risk is reserved for when high confidence exists in widespread coverage of severe weather with embedded instances of extreme severe (i.e., violent tornadoes or very damaging convective wind events).

Wind

Wind Advisory: A Wind Advisory is issued when the following conditions are expected: 

• Sustained winds of 31 to 39 mph for an hour or more - AND/OR
• Wind gusts of 46 to 57 mph for any duration.

High Wind Watch: A High Wind Watch 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.

High Wind Warning: A High Wind Warning is issued when the following conditions are expected:

• Sustained winds of 40 mph or higher for one hour or more - OR
• Wind gusts of 58 mph or higher for any duration.

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.

Fog

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).

Tornadoes

Tornado Watch: This is issued by the National Weather Service when conditions are favorable for the development of tornadoes in and close to the watch area. Their size can vary depending on the weather situation. They are usually issued for a duration of 4 to 8 hours. They normally are issued well in advance of the actual occurrence of severe weather. During the watch, people should review tornado safety rules and be prepared to move a place of safety if threatening weather approaches.

Tornado Warning: This is issued when a tornado is indicated by the WSR-88D radar or sighted by spotters; therefore, people in the affected area should seek safe shelter immediately. They can be issued without a Tornado Watch being already in effect. They are usually issued for a duration of around 30 minutes.

Floods

Flash Flood: A flood which is caused by heavy or excessive rainfall in a short period of time, generally less than 6 hours. Also, at times a dam failure can cause a flash flood, depending on the type of dam and time period during which the break occurs.

Flash Flood Warning: Issued to inform the public, emergency management, and other cooperating agencies that flash flooding is in progress, imminent, or highly likely.

Flash Flood Watch: Issued to indicate current or developing hydrologic conditions that are favorable for flash flooding in and close to the watch area, but the occurrence is neither certain or imminent.

Flash Flood Statement: A statement by the NWS which provides follow-up information on flash flood watches and warnings.

Urban and Small Stream Flooding: Flooding of small streams, streets, and low-lying areas, such as railroad underpasses and urban storm drains. This type of flooding is mainly an inconvenience and is generally not life threatening nor is it significantly damaging to property.

Urban and Small Stream Flood Advisory: This advisory alerts the public to flooding which is generally only an inconvenience (not life-threatening) to those living in the affected area. Issued when heavy rain will cause flooding of streets and low-lying places in urban areas. Also used if small rural or urban streams are expected to reach or exceed bankfull. Some damage to homes or roads could occur.

Lakeshore flooding intensities range from minor water overflow, with little or no damage, to significant water overflow, with extensive inundation and severe beach erosion. Lakeshore flooding is highly dependent on local coastline topography and nearshore bathymetry.

Lakeshore Flood Watch: Informs the public lakeshore flooding is possible. A watch should be issued 12 to 36 hours in advance.

Lakeshore Flood Warning: Informs the public lakeshore flooding, posing a serious threat to life and property, is occurring, imminent, or highly likely in the next 12 hours.

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.

Heat

Head Advisory: Heat index values 100 F; or heat index values 95 F for four consecutive days.

Excessive Heat Warning: A heat index of 105°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 75°F". An excessive heat warning can be issued for a heat index less than 105°F when the cumulative effect of successive days of near warning heat leads to life threatening conditions.

Fire

Fire Weather Watch: Fire Weather Watches are issued anytime the area has been dry for substantial amount of time (or for a shorter period during spring green-up or after fall color), the National Fire Danger Rating System (NFDRS) is high to extreme, and critical weather conditions are expected within the next 48 hours. 

Red Flag Warning: Red Flag Warnings are issued anytime there is an ongoing wildfire, or critical weather conditions will occur within the next 24 hours. These conditions are:

• Sustained winds averaging 15 mph or greater
• Relative humidity 25 percent or less
• Temperature 75°F or greater

Hail

Guide for determining hail sizes:

• less than 0.50" Pea
• 0.50" - Marble/Mothball
• 0.75" - Dime/Penny
• 0.88" - Nickel
• 1.00" - Quarter
• 1.25" - Half Dollar
• 1.50" - Walnut/Ping Pong
• 1.75" - Golf Ball
• 2.00" - Hen Egg
• 2.50" - Tennis Ball
• 2.75" - Baseball
• 3.00" - Tea Cup
• 4.00" - Grapefruit
• 4.50" - Softball