Thursday, February 21, 2013

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.




Tuesday, February 19, 2013


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.


Monday, February 18, 2013

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. 
street flooding

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.
flash flooding

 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.
flash flooding

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.
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. 
urban flooding

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.
Turn Around, Don't Drown

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
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Information provided by the National Weather Service Nashville Office