Severe storms

Thunderstorms — rain storms with lightning — can be dangerous by themselves and can cause destructive, deadly flooding. When they contain strong winds, hail and tornadoes they can turn violent. NOAA classifies a storm as “severe” when it produces wind gusts of at least 58 mph and/or hail one inch in diameter (about the size of a quarter) or larger and/or a tornado.

Lightning is caused by the attraction between positive and negative charges in the atmosphere, resulting in the buildup and discharge of electrical energy. This rapid heating and cooling of the air produces the shock wave that results in thunder.

Strong winds are often called “straight-line” winds to differentiate the damage they cause from tornado damage. Most thunderstorm winds that cause damage at the ground are a result of outflow generated by a thunderstorm downdraft.

Hail is a form of precipitation that occurs when updrafts in thunderstorms carry raindrops upward into extremely cold areas of the atmosphere where they can freeze into balls of ice. Hailstones can grow to several inches in diameter and may fall at speeds greater than 100 mph.

A tornado is a narrow, violently rotating column of air that extends from the base of a thunderstorm to the ground. About 1,200 tornadoes occur in the U.S. annually. The most destructive and deadly tornadoes come from rotating thunderstorms called supercells and can have winds estimated at more than 200 mph.

Flooding is an overflowing of water onto land that is normally dry. Floods can happen during heavy rains, when ocean waves come on shore, when snow melts too fast, or when dams or levees break. It is a threat all over the United States and occurs nearly every day.

National Weather Service forecast offices are the front line for severe weather forecasts and warnings. Meteorologists at 122 local offices issue daily forecasts, as well as severe thunderstorm, tornado, flood and flash flood warnings, ensuring the communities they serve receive the most accurate and timely information to stay out of harm’s way.

Days before storms form, the NOAA/NWS Storm Prediction Center in Norman, Oklahoma, provides timely and accurate forecasts and watches for severe thunderstorms and tornadoes over the contiguous United States. The SPC also issues forecasts for hazardous winter and fire weather.

How do NWS Storm Prediction Center meteorologists make a forecast? Check out this video.

Forecasting the weather is a complicated process. NWS meteorologists use their own knowledge and experience, as well as a variety of tools, including:

• numerical weather prediction models

• satellite image loops

• radar displays

• data from surface weather stations

• upper air data from balloons

Current weather watches and warnings for your area are available at:  http://www.weather.gov/

Advances in research and technology have helped forecasters increase average lead time for tornado warnings from just six minutes in 1994 to 13 minutes today. This means individuals and communities have more time to seek shelter and secure property.

Scientists are working in the laboratory and the field to enhance our understanding of severe thunderstorms and their hazards. How tornadoes form is still unclear, which is why NOAA researchers routinely conduct field campaigns, including the recent VORTEX2 project, to find the answers.

We do know, however, that the most destructive and deadly tornadoes occur from supercells, which are rotating thunderstorms with a well-defined radar circulation called a mesocyclone. Tornado formation is believed to be dictated mainly by things which happen on the storm scale, in and around the mesocyclone. Recent theories suggest that once a mesocyclone is underway, tornado development is related to the temperature differences across the edge of downdraft air wrapping around the mesocyclone. Another field research campaign, VORTEX SE, focuses on storms in the southeastern United States and aims to provide researchers with greater insight.

At NOAA laboratories, researchers are developing new products to help forecasters identify potential storms. Recently, NOAA’s Earth System Research Laboratory produced NOAA’s only hourly updating weather model that can predict individual thunderstorms over the U.S. This model gives forecasters and decision-makers fast, local guidance to help make more accurate severe weather forecasts and warnings across the country. It is currently being tested for operational use.

Also, at NOAA’s National Severe Storms Laboratory, researchers are working on new projects, such as Warn on Forecast and FACETS, that provide more information to the public to support a Weather Ready Nation. Warn on Forecast will increase lead times for severe weather warnings to reduce loss of life, injury, and damage to the economy. FACETs will help forecasters and decision-makers provide more focused information about ongoing severe weather threats to keep the public informed.

Severe weather can strike anywhere at any time. It is important to be aware of the forecast, so you can be safe. Don’t be caught off guard when storms are in your area.

Are you prepared for severe weather? Do you know what to do? The National Weather Service recommends that you have a plan before severe weather happens, and seek shelter when the weather threatens and a warning is issued.

What do the terms mean?

• Severe thunderstorm or tornado WATCH: Be prepared
• Severe thunderstorm or tornado WARNING: Take action
• Flood ADVISORY: Be aware
• Flood WATCH: Be prepared
• Flood or flash flood WARNING: Take action

It is important to have more than one source of weather information. This can include NOAA Weather Radio, cell phone apps, local TV and radio, and Wireless Emergency Alerts.

Watch these videos to learn the steps to take when tornadoes threaten your area:

• BEFORE the storm
• DURING the storm
• AFTER the storm

Whatever type of severe weather comes your way, having a plan and knowing what to do could save your life.