Example Tornado Watch for Central Iowa.
An example severe thunderstorm warning (yellow polygon) for central Iowa.
An example tornado warning (red polygon) for central Iowa.

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Severe Weather: Understanding the Forecast

Before it Strikes . . .  
Introduction
Planning Ahead
Understanding the Forecast
Receiving the Warning

Staying Safe . . .  
Tornadoes
Lightning
Hail and Straight Line Winds

 


How the National Weather Service Keeps You Informed

The National Weather Service issues severe weather outlooks, watches, and warnings to help keep you safe when hazardous weather threatens. It is critical for you to understand and pay close attention to these forecasts and alerts. This page describes each of these products in detail and what they mean to you.

  Click each product to find out more information
Product What It Means You Should...
Hazardous Weather
Outlook
Will there be severe thunderstorms or tornadoes in the next several days? If severe weather is expected, check back for later forecasts, information, and possible watches.
Severe Thunderstorm Watch Conditions are favorable for thunderstorms to produce large hail, damaging winds, and possibly a tornado in and around the watch area during the next few hours. If the watch includes your county, or one close to you, you should remain weather-aware and pay attention to what is going on. If there are storms nearby, check your weather source to see if there are warnings.
Tornado Watch Conditions are particularly favorable for the development of tornadoes, along with hail and damaging winds, in and around the watch area during the next few hours. If the watch includes your county, or one close to you, you should remain weather-aware and pay attention to what is going on. If there are storms nearby, check your weather source to see if there are warnings.
Severe Thunderstorm Warning A storm with large hail and/or damaging winds has been indicated on radar or observed by storm spotters. Listen closely to the warning - it will tell you exactly what to expect (hail size and wind speed). If you are outside, go indoors immediately.
Tornado Warning A tornado has either been seen, or there are signs on radar that a tornado could be forming. If you are in the warned area, now is the time to put your safety plan into action. Seek shelter immediately.

Outlooks

The hazardous weather outlook is a text product designed to give you information on any hazardous weather expected over the next seven days, including severe thunderstorms and tornadoes. The National Weather Service in Des Moines also generates a graphical Weather Story, which highlights the most significant weather expected in the next few days over central Iowa. Examples of these products are shown below.

Example Hazardous Weather Outlook Example Weather Story Graphic
THIS HAZARDOUS WEATHER OUTLOOK IS FOR PORTIONS OF CENTRAL IOWA.

.DAY ONE...THIS AFTERNOON AND TONIGHT
NUMEROUS THUNDERSTORMS ARE EXPECTED TO DEVELOP BY THIS EVENING ACROSS NORTHERN IOWA. A MODERATE RISK OF SEVERE WEATHER EXISTS IN NORTHERN AND CENTRAL IOWA DURING THIS TIME. THE POTENTIAL FOR VERY LARGE HAIL...DAMAGING WINDS AND TORNADOES WILL EXIST FOR THE AREA...ESPECIALLY THIS EVENING.

.DAYS 2 THROUGH 7...SUNDAY THROUGH FRIDAY
LARGE HAIL...DAMAGING WINDS AND A FEW TORNADOES WILL BE POSSIBLE ON SUNDAY AFTERNOON...ESPECIALLY ALONG AND EAST OF INTERSTATE 35.
Graphical Weather Story
Current Hazardous Weather Outlook Current Weather Story

The Storm Prediction Center (SPC) in Norman, Oklahoma issues severe weather forecasts for the entire United States. The severe weather threat is broken down into three risk categories: Slight, Moderate, and High. These categories are an indicator of the expected coverage/number of storms and/or the expected intensity of the storms. Slight risks are fairly common, but should not be ignored. It is important to remember that a slight risk does not mean the storms will only be slightly severe. Significant storms can occur in a slight risk area, or in an area that was not even in an outlook earlier in the day. A slight risk outlook for your area should be monitored closely for later changes.


Watches

Severe thunderstorm and tornado watches are usually issued hours before the development or arrival of severe weather. When a watch is issued, you should start paying close attention to the weather and be ready to take action should the weather worsen. The sky may be sunny when you first hear of the watch; therefore, do not be lulled into a false sense of security. Remember, a watch means that conditions are favorable for severe thunderstorms and/or tornadoes.

Tornado Watch
Example Tornado Watch

Do not ignore severe thunderstorm watches since tornadoes sometimes occur in these watches. It only takes one storm or one tornado to make the event significant to you. Severe storms can also occur before a watch is issued.

If conditions are ideal for the development of strong severe thunderstorms and/or tornadoes, a "Particularly Dangerous Situation," or PDS, watch will be issued. If you are located in a PDS watch, pay extra close attention to the weather. It is a good idea to review your emergency preparedness plan and ensure that everything is in order in case you need to use it.

You should pay attention to where the watch is and how it is oriented. For example, a tornado watch is issued for all of western Iowa, west of Interstate 35, from 3 pm until 9 pm. If you live in Des Moines, you are on the far eastern edge of this watch area. The sky may be blue outside when you first hear of the watch and storms may not reach your area until late in the watch period.

Watches may be extended in both space and time. This is why it is important for those living in and near the watch area to pay attention to the weather throughout the valid period of the watch. A watch that previously did not include you may be expanded to encompass your area or the watch may be extended past its original expiration time.

Just because you are on the edge of the watch area does not mean you will not see severe storms. Storms do not recognize political boundaries such as city limits or county lines, and they can develop or move into areas that are not under a watch or warning.


Severe Thunderstorm Warnings

A severe thunderstorm warning is issued whenever radar or storm spotters indicate that a thunderstorm is producing hail with a diameter of one inch or larger and/or winds in excess of 58 mph. The National Weather Service currently issues storm-based warnings that highlight the area expected to be impacted by the storm. The text for each warning is structured identically and is broken down into different segments highlighting the location of the storm, its speed and direction, places in the path of the storm, and what threats the storm contains. An experimental change to the warning midsection layout in 2013 now includes a bulleted listing of the expected thunderstorm hazard(s), how the hazard was detected, and the expected damage/impacts from the storm (more info). An example warning is shown below:

Severe Thunderstorm Warning Text

Severe Thunderstorm Warning

Graphically, the warnings are polygon shaped, with the warned area located inside of the polygon. An example polygon warning, along with a background radar image, is provided in the image on the right.

Do not ignore severe thunderstorm warnings! Severe thunderstorm warnings often precede tornado warnings, providing you with extra time to prepare for a dangerous storm. If there is a severe thunderstorm headed your way, you should monitor it closely, especially if a tornado watch is also in effect.

Television stations will often display the counties affected by different types of warnings with color coded maps on the screen. If your county is shown as being in a severe thunderstorm warning, you should try to get more detailed information as to what the storm is capable of doing. The National Weather Service website is an excellent source for information.

Keep in mind that even without producing a tornado, severe thunderstorms can be violent, dangerous and even deadly events. They can produce destructive hail larger than softballs, winds in excess of 100 mph, flooding rains, and deadly lightning. Severe thunderstorms occasionally do produce tornadoes without a tornado warning in effect.

These are examples of why you do not want to ignore Severe Thunderstorm Warnings!

Hail Damage Wind Damage
Hail Damage to a Vehicle Windshield A House Destroyed by Straight-Line Winds

Tornado Warnings

Tornado warnings cannot be issued for every single tornado that occurs. This is why you must take responsibility for your personal safety any time storms threaten. Do not wait until you get an official warning, either through television, weather radio, or an outdoor warning siren. If you feel threatened, you should follow your tornado safety plan. It is better to be safe than sorry.

Small, and usually weak, tornadoes can develop very quickly from any thunderstorm, so always be cautious and alert any time storms are in the area.

Tornado warnings are structured almost identially to severe thunderstorm warnings. Like severe thunderstorm warnings, the layout of the warning midsection underwent an experimential change in 2013 to include a bulleted listing of the expected thunderstorm hazards (in addition to tornadoes), how the tornado was detected, and the expected damage/impacts from the tornado (more info). If a tornado is confirmed by spotters, estimated arrival times may be appended to the general list of cities listed in the path of the tornado. A sample tornado warning is provided below:

Tornado Warning Text

Tornado Warning

If a large and potentially catastrophic tornado has been confirmed by multiple sources and is moving into a highly populated or vulnerable area, a
"Tornado Emergency" headline may be added to the top of the warning or statement. This headline will only be used in exceptionally rare situations where a large loss of life is possible.

IF YOU ARE IN A TORNADO WARNING AND SEE THE TORNADO EMERGENCY WORDING, YOU ARE IN IMMINENT DANGER! IF YOU DO NOT SEEK SHELTER NOW, YOU RISK SERIOUS INJURY OR DEATH.

While your National Weather Service strives to provide the most detailed and accurate information possible in tornado warnings, there may be occasions when your small town or community is in the path of a dangerous storm, but is not listed in the warning text. This also holds true for television path forecasts. You should be cautious when using detailed forecasts of time and location. Because of the way radar works and how storms behave, these times and locations could be off by several minutes and several miles. Allow yourself plenty of time to get to your tornado shelter.

National Weather Service tornado warning lead times (the amount of time between when the warning is issued and when the tornado strikes) have shown a steady increase over recent years, and there have been instances where a tornado warning has been issued tens of minutes before a tornado struck. However, given the state of tornado forecasting science and knowledge, you will not receive a long advance notice of every tornado. Your safety plans should account for this, and may require that you have an alternate plan for situations when you cannot reach your primary shelter area.


Severe Weather Statements

Severe weather statements are issued to update warnings and include new information about an ongoing severe storm. Here is a possible scenario:

5:05 PM       A severe thunderstorm warning is issued. The storm is producing golfball size hail.
5:15 PM       A severe weather statement is issued to update the warning. The storm is producing baseball size hail
                      and is also showing signs of rotation. A tornado warning might become necessary.
5:20 PM       A tornado warning is issued.

In this scenario, if you have been paying attention to the entire suite of severe weather products from the National Weather Service, you received an extra 15 minutes notice that a dangerous storm was nearby. If you had only payed attention to the tornado warning, this means you received less information and less advance notice.

 

National Weather Service forecasters work hard during an event to keep you informed with a continuous flow of information, from days before storms develop to reporting the aftermath of storms.


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