Thursday, March 28, 2013

Facts About Thunderstorms





Thunderstorms are a common occurrence each spring and summer across the Midwest. Each year the Midwest sees hundreds of severe thunderstorms and thousands of non-severe thunderstorms. Thunderstorms can be extremely dangerous storms which may bring deadly tornadoes and lightning, damaging high winds and hail and can lead to flash flooding.

The National Weather Service issues severe thunderstorm warnings for thunderstorms that are producing or are capable of producing winds of at least 58 mph and/or hail at least 1 inch in diameter. Oftentimes severe thunderstorms may be much stronger than the minimum criteria.


What is the difference between a Severe Thunderstorm WATCH and a Severe Thunderstorm WARNING?
A Severe Thunderstorm WATCH is issued by the NOAA Storm Prediction Center meteorologists who are watching the weather 24/7 across the entire U.S. for weather conditions that are favorable for severe thunderstorms. A watch can cover parts of a state or several states. Watch and prepare for severe weather and stay tuned to NOAA Weather Radio to know when warnings are issued.

A Severe Thunderstorm WARNING is issued by your local NOAA National Weather Service Forecast Office meteorologists who watch a designated area 24/7 for severe weather that has been reported by spotters or indicated by radar. Warnings mean there is a serious threat to life and property to those in the path of the storm. ACT now to find safe shelter! A warning can cover parts of counties or several counties in the path of danger.

Thunderstorm Types

Often called “popcorn” convection, single-cell thunderstorms are small, brief, weak storms that grow and die within an hour or so. They are typically driven by heating on a summer afternoon. Single-cell storms may produce brief heavy rain and lightning.
A multi-cell storm is a common, garden-variety thunderstorm in which new updrafts form along the leading edge of rain-cooled air (the gust front). Individual cells usually last 30 to 60 minutes, while the system as a whole may last for many hours. Multicell storms may produce hail, strong winds, brief tornadoes, and/or flooding.

A squall line is a group of storms arranged in a line, often accompanied by “squalls” of high wind and heavy rain. Squall lines tend to pass quickly and are less prone to produce tornadoes than are supercells. They can be hundreds of miles long but are typically only 10 or 20 miles wide.

A supercell is a long-lived (greater than 1 hour) and highly organized storm feeding off an updraft (a rising current of air) that is tilted and rotating. This rotating updraft - as large as 10 miles in diameter and up to 50,000 feet tall - can be present as much as 20 to 60 minutes before a tornado forms. Scientists call this rotation a mesocyclone when it is detected by Doppler radar. The tornado is a very small extension of this larger rotation. Most large and violent tornadoes come from supercells.

A “bow echo” is a radar signature of a squall line that “bows out” as winds fall behind the line and circulations develop on either end. A strongly bowed echo may indicate high winds in the middle of the line, where the storms are moving forward most quickly. Brief tornadoes may occur on the leading edge of a bow echo. Often the north side of a bow echo becomes dominant over time, gradually evolving into a comma-shaped storm complex.

This is from the Dakota County Emergency Management Office.

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