Strong to Severe Storms in the Upper Midwest; Bering Sea Low Continues to Impact Southwest Alaska

Scattered severe thunderstorms are possible Thursday across central and eastern Minnesota, Iowa, and western Wisconsin. A few tornadoes, isolated very large hail, and damaging winds may occur. In southwest Alaska, a Bering Sea low continues to cause strong winds, significant rainfall, and high seas, with further impacts expected Friday into the weekend from a North Pacific storm. Read More >

Tornadoes

Tornadoes are violent rotary winds that descend from severe thunderstorms. They produce the most violent winds found in nature. In their strongest form, tornadoes are capable of producing wind speeds in excess of 200 mph! The highest recorded wind speed associated with a tornado was on May 3, 1999, near Moore, OK, when a portable Doppler radar measured a wind speed of 318 mph at a height of about 250 feet above the ground. Tornadoes of this magnitude produce devastating damage, causing total destruction to everything in their path. Fortunately, these types of violent tornadoes are extremely rare, representing only one to two percent of the hundreds of tornadoes that occur across the United States each year.

As is the case with hurricanes, tornadoes are assigned a rating. However, because tornadoes develop rapidly, and because of their size, measuring their wind speeds in real time is practically impossible. Tornadoes are therefore rated by National Weather Service meteorologists after the fact based upon the type of damage that they produce. The rating system used to classify tornadoes is called the Enhanced Fujita Scale (EF-scale). Within the EF-scale, tornadoes can be further classified into weak (EF0 and EF1), strong (EF2 and EF3), and violent (EF4 and EF5).

Table 2. The Enhanced Fujita scale.
EF Rating	Estimated Wind Speed
EF0	65 to 85
EF1	86 to 110
EF2	111 to 135
EF3	136 to 165
EF4	166 to 199
EF5	200+

In the western Carolinas and northeast Georgia, weak tornadoes make up just under 80% of the approximately 12 tornadoes that occur across the area in a typical year. This means that the vast majority of the tornadoes that occur across the area each year produce wind speeds of around 100 mph or less, not much stronger than typical downburst winds. Meanwhile, strong

tornadoes account for about 20% of the total, while only 2% of the tornadoes that have occurred across the area since 1950 were classified as violent. No EF5 tornado has ever been documented in the states of Georgia, North Carolina, and South Carolina.

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