Spotter Reference Page

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What causes thunderstorms to form?

Thunderstorms begin to develop when the atmosphere becomes sufficiently unstable. This instability can arise from daytime heating, an inflow of higher dew points in the lower atmosphere, or cooling in the upper atmosphere. There must also be sufficient moisture in the air to form the clouds and rain, and enough lifting in the atmosphere to lift the parcels of air to form the clouds.

When enough lifting is present, the parcels of air rise into the atmosphere and are cooled until the air eventually reaches saturation, causing cumulus clouds to form. If there is enough instability and lifting in the atmosphere, the cumulus cloud will continue to grow vertically into a cumulonimbus cloud or thunderstorm.

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What makes a thunderstorm SEVERE?

Over 100,000 thunderstorms occur across the United States every year. Of these, only about 10% would be classified as severe storms. That is not to say the storms which are not severe are not dangerous! The number one thunderstorm killer every year happens to be flash flooding, and lightning which occurs in ALL thunderstorms averages 93 deaths and 300 injuries each year. Storms are classified as SEVERE by the National Weather Service when any of the following 3 things occur:

  • A tornado occurs
  • Hail greater than or equal to 3/4"
  • Winds in excess of 58 MPH

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I've heard the term "downburst" before, but what is it exactly?

All thunderstorms have rising and sinking air, the rising air keeping the thunderstorm active, and the sinking air or downdraft usually associated with the area of rainfall within a thunderstorm. Sometimes in the stronger storms these downdrafts become very powerful in a small area beneath the thunderstorm.

These "downbursts" can cause damaging winds in excess of 100 mph. These winds usually approach from the same direction and are often referred to as "straight-line" winds. These straight-line winds can often times cause the same type of damage which a tornado produces.

So when the National Weather Service issues a Severe Thunderstorm Warning, you should take the appropriate safety measures to protect yourself from potential damaging winds.

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What products do the NWS issue when Severe weather is expected or occurring?

The Storm Prediction Center (SPC) issues several convective outlooks every day. These outlooks will describe the reasoning why and areas where severe weather and general thunderstorms are expected. These products can be viewed in Graphical format, or in text format. Note: Text format is written using several NWS contractions.

These outlooks are written for day1, day 2, day 3, and days 4-8.

At the National Weather Service office in Aberdeen, a Hazardous Weather Outlook is issued around 600 AM every day and is updated when conditions warrant. These outlooks are issued to the public and include information about the possibility of dangerous weather expected within our county warning area.

For areas where severe weather is expected to develop within the next several hours the Storm Prediction Center will issue a Severe Thunderstorm Watch or Tornado Watch

When severe weather is occurring or will occur shortly within our County Warning Area, the Aberdeen National Weather Service office will issue, Severe Thunderstorm Warnings, Tornado Warnings, or Flash Flood Warning as needed. You can view these warnings on our Watches/Warnings page and several other locations.

Check out our Severe Weather Products List for more information on the products we issue for severe weather

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How can I find out if a warning is in effect for my area?

There are several ways to get warning information for your location. A few are listed below:

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