Severe Thunderstorms

What is a Severe Thunderstorm?
A severe thunderstorm is a thunderstorm that produces one or more of the following: hail that has a diameter of one inch or larger, winds greater than or equal to 58 mph, and tornadoes.  About 10% of all thunderstorms in the U.S. meet severe criteria.

Severe thunderstorms can occur at any time of year, although the most common time of occurrence is during the spring months of March, April, and May.

There is also a lesser known secondary season during the fall, in November and early December.



What is the Difference between a Watch and a Warning?
A severe thunderstorm or tornado watch means that conditions are favorable for severe thunderstorms or tornadoes to develop.  These are issued by the Storm Prediction Center in Norman, OK, typically a few hours before severe weather develops.

A severe thunderstorm or tornado warning means that a severe thunderstorm or tornado has either been detected on radar or witnessed by storm spotters firsthand.  Your local NWS Forecast Office issues these when severe weather is developing or occurring.


Safety Tips

• Have a plan.   Prepare ahead of time so you and your family know what actions to take when severe weather occurs.
• Get indoors! There is no safe place outdoors during a thunderstorm.
• Stay informed! When severe weather threatens, stay tuned to NOAA Weather Radio, local television and radio stations, or the National Weather Service webpage at www.weather.gov for up to date information on the weather situation.
• Know what county you are in. When a warning is issued, the threatened area will be identified by the counties that contain it.
• Have a NOAA Weather Radio.  This is the best way to receive the latest and most up to date weather information from the National Weather Service.

Information provided by the NWS Nashville Office Severe Weather Awareness Week brochure 2013.

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Severe Weather / Lightning

Severe Thunderstorms and Lightning

Did you know that lightning kills more people on average each year than hurricanes and tornadoes combined? Lightning can strike up to 10 miles away from a rain area.

The 30/30 Lightning Safety Rule:

During thunderstorms no place outside is safe but you can minimize your risk by assessing the lightning threat and taking the appropriate actions. Count the number of seconds from when you see the lightning flash until you hear the thunder. If you count 30 seconds or less you are in immediate danger. Even if you can't see the lightning, just hearing the thunder means lightning is likely within striking range.

If you are caught outdoors in a severe thunderstorm or when lightning threatens:

1. Immediately seek shelter in a substantial building.
   1. DO NOT seek shelter under trees during thunderstorms.
   2. DO NOT seek shelter in unprotected open structures such as picnic pavilions, rain shelters or bus stops.
   3. If a substantial building is not available, a metal-topped vehicle with the windows up is your next best choice.
2. Avoid contact with metal fences, metal bleachers, or metallic structures.
3. Avoid using the telephone or any electrical appliances. Use phones ONLY in an emergency.
4. Do not take a bath or shower during a thunderstorm.
5. Wait 30 minutes or more after hearing the last thunder before leaving the safe location

If Caught Outdoors and No Shelter Is Nearby:

1. Find a low spot away from trees, fences, and poles that is not subject to flooding.
2. If you feel your skin tingle or your hair stand on end, squat low to the ground on the balls of your feet. Place your hands over your ears and your head between your knees. Make yourself the smallest target possible and minimize your contact with the ground. DO NOT lie down. If you are swimming, get out of the water immediately.

REMEMBER: If you can hear thunder, you are close enough to be struck by lightning.

Once you have taken shelter indoors, you should monitor a weather radio, commercial radio/television station, internet or other weather service provider. Even when a specific storm cell has passed beyond the area, conditions may still be right for high winds, lightning, and other hazardous weather conditions.

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Severe Weather Awareness Week - Flood Safety

NWS Flood Products: What Do They Mean?Flash Flood Warning, Areal Flood Warning, River Flood Warning or Urban and Small Stream Flood Advisory

A variety of flooding and related phenomena place middle Tennessee at risk throughout the year. Flooding can result from a number of weather systems including slow-moving or stationary frontal systems, inland moving tropical cyclones and intense summertime thunderstorms. These systems can produce flash flooding in low lying flood prone areas and along small creeks and streams, as well as river flooding along mainstreams.

Sometimes, it can be difficult to differentiate between our flood products and what they mean, so let's take a look at each of them in hopes of clearing up any misconceptions or misunderstandings. 

Flood or Flash Flood Watches

Much like our Severe Thunderstorm and Tornado Watches and Warnings, we issue Flash Flood Watches and Warnings.  Flood Watches are issued to inform the public, cooperating agencies, and other interests that the current and/or developing weather pattern is such that there is a potential for rapid flooding (also known as flash flooding), more widespread areal flooding, or river flooding.  The occurrence of flooding is neither certain nor imminent.  Persons in the watch area are advised to check flood action plans, keep informed, and be ready to take necessary actions if a warning is issued or flooding is observed.  A Flash Flood Watch may also be issued for a potential dam break.

 Flood or Flash Flood Warnings

A Flood or Flash Flood Warning is issued to inform the public that flooding is imminent or in progress.  It focuses on specific counties, communities, streams, or areas. Flash Flood Warnings are issued for flooding usually occurring within 6 hours of heavy rain.  An Areal Flood Warning is issued for flooding that occurs more gradually, normally from prolonged and persistent moderate to heavy rainfall.  A River Flood Warning is issued when a river is forecast to go above its designated flood stage at the forecast point.

Areal Flooding vs Flash Flooding

These two types of flooding may be confusing, so we thought we would take a minute to try and clarify the difference between them.

A Flash Flood Warning is issued for flooding that normally occurs within six hours of heavy or intense rainfall.  This results in small creeks and streams quickly rising out of their  banks.   Dangerous flooding in areas near these creeks and streams, as well as low-lying flood prone areas, develops very quickly and is a significant threat to life and/or property.

An Areal Flood Warning is normally issued for flooding that develops more gradually, usually from prolonged and persistent moderate to heavy rainfall.  This results in a gradual ponding or buildup of water in low-lying, flood prone areas, as well as small creeks and streams. The flooding normally occurs more than six hours after the rainfall begins, and may cover a large area.  However, even though this type of flooding develops more slowly than flash flooding, it can still be a threat to life and property.

Pictured is a typical example of areal flooding.

Flood Advisories
A Flood Advisory is issued when flooding that is not considered a significant threat to life or property is expected or occurring.   This usually deals with nuisance flooding problems, such as flooding in low-lying areas and areas of poor drainage, as well as minor flooding of streets and roadways.  This type of flooding causes some inconvenience, but is not generally considered a significant threat to life or property.

Flood Advisories are normally issued as an Urban and Small Stream Flood Advisory. 

Other Important Information

Unlike our Severe Thunderstorms or Tornado Warnings, flood products are normally issued for extended periods of time.  These warnings are normally issued for 2 to 4 hours, or longer.  Even though rainfall may have subsided, flooding may persist for some time.  The longer warning time allows for rainwaters to recede while keeping the public aware that flooding is still occurring and there is still a threat to life and/or property in the warned area.  

Flood and Flash Flood Warnings will be transmitted on NOAA Weather Radio, and WILL alert your radio.  Advisories do NOT alert your radio.  You can also follow these products from the front page on our website, as well as from the Advanced Hydrologic Prediction Service (AHPS) web page.

It is important that the public is aware of this information, but equally as important is the reporting of any type of flooding.  If you see any of the above, contact the NWS Office!

       

Storm Reporting Hotline   1-800-267-8144
Twitter: @NWSNashville or #tnwx
Facebook

Information provided by the National Weather Service Nashville Office

http://www.srh.noaa.gov/bna/?n=floodinginfo

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What to do if you are in the path of a tornado

In a house with a basement: Avoid windows. Get in the basement and under some kind of sturdy protection (heavy table or work bench), or cover yourself with a mattress or sleeping bag. Know where very heavy objects rest on the floor above (pianos, refrigerators, waterbeds, etc.) and do not go under them. They may fall down through a weakened floor and crush you.
In a house with no basement, a dorm, or an apartment: Avoid windows. Go to the lowest floor, small center room (like a bathroom or closet), under a stairwell, or in an interior hallway with no windows. Crouch as low as possible to the floor, facing down; and cover your head with your hands. A bath tub may offer a shell of partial protection. Even in an interior room, you should cover yourself with some sort of thick padding (mattress, blankets, etc.), to protect against falling debris in case the roof and ceiling fail.
In an office building, hospital, nursing home or skyscraper:Go directly to an enclosed, windowless area in the center of the building -- away from glass and on the lowest floor possible. Then, crouch down and cover your head. Interior stairwells are usually good places to take shelter, and if not crowded, allow you to get to a lower level quickly. Stay off the elevators; you could be trapped in them if the power is lost.
In a mobile home:Get out! Even if your home is tied down, you are probably safer outside, even if the only alternative is to seek shelter out in the open. Most tornadoes can destroy even tied-down mobile homes; and it is best not to play the low odds that yours will make it. If your community has a tornado shelter, go there fast. If there is a sturdy permanent building within easy running distance, seek shelter there. Otherwise, lie flat on low ground away from your home, protecting your head. If possible, use open ground away from trees and cars, which can be blown onto you.
At school:Follow the drill! Go to the interior hall or room in an orderly way as you are told. Crouch low, head down, and protect the back of your head with your arms. Stay away from windows and large open rooms like gyms and auditoriums.
In a car or truck: Vehicles are extremely dangerous in a tornado. If the tornado is visible, far away, and the traffic is light, you may be able to drive out of its path by moving at right angles to the tornado. Otherwise, park the car as quickly and safely as possible -- out of the traffic lanes. [It is safer to get the car out of mud later if necessary than to cause a crash.] Get out and seek shelter in a sturdy building. If in the open country, run to low ground away from any cars (which may roll over on you). Lie flat and face-down, protecting the back of your head with your arms. Avoid seeking shelter under bridges, which can create deadly traffic hazards while offering little protection against flying debris.
In the open outdoors: If possible, seek shelter in a sturdy building. If not, lie flat and face-down on low ground, protecting the back of your head with your arms. Get as far away from trees and cars as you can; they may be blown onto you in a tornado.
In a shopping mall or large store: Do not panic. Watch for others. Move as quickly as possible to an interior bathroom, storage room or other small enclosed area, away from windows.
In a church or theater: Do not panic. If possible, move quickly but orderly to an interior bathroom or hallway, away from windows. Crouch face-down and protect your head with your arms. If there is no time to do that, get under the seats or pews, protecting your head with your arms or hands.

Info from the Storm Prediction Center

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NWS - Warnings, Watches, Advisories, and Statements defined

What the NWS Warnings, Watches, Advisories, and Statements mean to you.

1. WARNINGS - The hazard (tornado, flash flood, etc) is imminent. The probability of occurrence is extremely high. Warnings are issued based on eyewitness reports or clear signatures from remote sensing devices such as radar and satellite. Lead-time for thunderstorm type events is generally 30 minutes or less. Lead-time for hurricanes, river floods, and winter storms can be 6 to 18 hours.

2. WATCHES - Meteorologists have determined that conditions appear right for the development of the hazard. Probability of occurrence is greater than 60% in the watch area. Watches generally cover larger areas than warnings. In the case of thunderstorms, less than 30% of the watch area may experience the hazard. However, with larger storms such as hurricanes and winter storms, the entire watch area may be affected. Severe thunderstorm and tornado watches are usually issued 1 to 2 hours before the event begins. With flash floods, it can be 3 to 12 hours. For hurricane, river flood, and winter storm watches, lead-times are usually 12 to 36 hours.

3. ADVISORIES - An advisory is issued for weather that is expected to be a disruption to the normal routine and an inconvenience, but it is not expected to be life-threatening. Advisories are issued for 1 to 3 inches of snow, dense fog, minor street flooding, etc. The time frame is similar to that of a warning.

4. STATEMENTS - statements are issued to update current weather situations or highlight significant changes to come. Statements are also used to explain why watches, advisories, or warnings have been issued. Three special types of statements are ...

a) "Outlooks" or "Potential " Statements - During the warm season, the NWS Forecast Office issues "Thunderstorm Potential Statements" each morning discussing where and if storms will develop that afternoon and how intense they may be. When a winter storm may develop in the next 2 to 4 days, "Winter Storm Potential Statements" are issued. Outlooks may also be issued for possible heavy rain and flood events. The National Hurricane Center issues "Tropical Outlooks" for the potential for tropical storm and hurricane development. The National Severe Storms Forecast Center issues special statements when there is the potential for a severe thunderstorm or tornado outbreak.

b) Short-Term Forecasts - These statements discuss the short-range forecasts for the next 1 to 6 hours. During active weather, these statements may be issued hourly.

c) Public Information Statements - These statements provide information of special interest such as a summary of recent records set, snowfall, weather safety information, special activities (weather related) that may be occurring, etc.


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#tSpotter Reporting Information

We have partnered with the National Weather Service (Nashville) to immediately relay severe & winter weather reports you tweet to the NWS. We call this tSpotter.

If you see:

        TORNADO
        FUNNEL CLOUD
        FLOODING
        HAIL >= 1/2 INCH
        WINDS > 50 MPH (MEASURED)
        STRUCTURAL DAMAGE
        TREES OR POWER LINES DOWN
        and
        SNOW/FLURRIES

Let us know ASAP by tweeting your report with hashtag #tSpotter.  Tweets with photos and specific locations (geotagged) are best. Your reports assist the NWS in issuing severe weather warnings and advisories. The NWS only has radar, which can not see what is happening on the ground. Your report is a public service which improves the warning process, furthering the NWS mission of protecting life and property.

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Severe Weather Awareness Week - Weather Radio

NOAA Weather Radio All Hazards

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NOAA Weather Radio All Hazards (NWR) is a nationwide network of radio stations broadcasting continuous weather information directly from the nearest National Weather Service office. NWR broadcasts official Weather Service warnings, watches, forecasts and other hazard information 24 hours a day, 7 days a week.
Working with the Federal Communication Commission's (FCC) Emergency Alert System , NWR is an "All Hazards" radio network, making it your single source for comprehensive weather and emergency information. In conjunction with Federal, State, and Local Emergency Managers and other public officials, NWR also broadcasts warning and post-event information for all types of hazards – including natural (such as earthquakes or avalanches), environmental (such as chemical releases or oil spills), and public safety (such as AMBER alerts or 911 Telephone outages).
Known as the "Voice of NOAA's National Weather Service," NWR is provided as a public service by the National Oceanic and Atmospheric Administration (NOAA), part of the Department of Commerce. NWR includes 1000 transmitters, covering all 50 states, adjacent coastal waters, Puerto Rico, the U.S. Virgin Islands, and the U.S. Pacific Territories. NWR requires a special radio receiver or scanner capable of picking up the signal. Broadcasts are found in the VHF public service band at these seven frequencies (MHz):

162.400

162.425

162.450

162.475

162.500

162.525

162.550

Information provided by The National Weather Service

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