Jun 12, 2008 12:10 pm US/Mountain
Zeppelin: We're In Record Tornado Territory
By Jennifer Zeppelin, CBS4 Meteorologist
DENVER (CBS4) ―
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Damage at the Little Sioux Scout Ranch in Iowa Wednesday after a tornado struck.
CBS
You likely already know it's been an extremely active spring storm season across the United States. This month is no exception.
The country's Storm Prediction Center is reporting that a total of 277 tornadoes have been reported in the month of June.
Since it's still early in the month, June could end up being just as active as May, which was especially bad. More than 500 tornadoes were reported last month.
The most recent tornado outbreak has pushed our yearly total of tornadoes into record territory. The 10-year annual trend is 1,270 tornadoes per year; so far this year there has been a total of 1,577 tornadoes reported. The record yearly total occurred in 2004, with a total of 1,717 tornadoes.
It's not unusual to see this kind of activity; Colorado, Iowa, Kansas, Nebraska, and the Dakotas experience the most significant severe weather during this time of year. The plains of Eastern Colorado are particularly active during June and early July.
If you are wondering if the Windsor tornado on May 22 was unusual, it's not when you look at the time of year it occurred and the location.
Weld County has the highest number of reported tornadoes than any other county in Colorado.
The National Weather Service Denver office says that since 1950 there have been a total of 20 tornadoes of F3 and higher ratings within Colorado.
The Windsor tornado was the second F3 tornado reported in Weld County since 1950.
Meteorologists Can't Explain Exactly Why So Many Are Forming
"There are active years and we don't particularly understand why," said research meteorologist Harold Brooks at the National Severe Storms Lab in Norman, Okla.
"Right now we're on track to break all previous counts through the end of the year," said warning meteorologist Greg Carbin at the Storm Prediction Center earlier this month.
Global warming cannot really explain what is happening, Carbin said. While higher temperatures could increase the number of thunderstorms, which are needed to trigger tornadoes, they also would tend to push the storm systems too far north to form some twisters, he said.
La Nina, the cooling of parts of the Central Pacific that is the flip side El Nino, was a factor in the increased activity earlier this year -- especially in February, a record month for tornado activity -- but it can't explain what is happening now, according to Carbin.
A short-term answer is that the nation's heartland is stuck in a tornado rut with usually temporary weather conditions that can lead to tornadoes parked over the Plains, said Adam Houston, a professor of meteorology at the University of Nebraska-Lincoln. Cooler air at high altitudes and warmer moist air coming from the Gulf of Mexico are combining and settling over the region.
"You get day after day of severe weather and day after day of tornadoes until the pattern changes," Houston said.
But why that happens, Houston doesn't know. While scientists can forecast hurricane seasons, predicting their land-bound cousins is much harder, Brooks said. While tornadoes, like hurricanes, rely on large-scale weather phenomena, the crucial triggers are extremely local weather conditions.
On top of that, tornadoes have a "Goldilocks" issue. To make a tornado, the conditions have to be just right. Too much or too little of one ingredient and there is no tornado. For example, wind shear -- when upper and lower winds are at different speeds or coming from different directions -- is crucial to create a funnel cloud. Too little and there is no spin. Too much and the tornado falls apart.
And tornadoes form most often in late afternoon, between 5 and 9 p.m., so if a thunderstorm starts up early in the morning, it's far less likely to throw off a tornado, Brooks said.
In 1925, tornadoes killed 794 people. From 1916 to 1936, tornadoes killed an average of nearly 280 Americans a year. That's because tornado warnings were not as good, people couldn't hear them and housing was not as sturdy, Brooks said.
The difference between a tornado watch and a tornado warning
Tornado Watch: Is issued when conditions are just right for tornadoes to develop. It does not mean tornadoes are imminent, just that you should be aware that the possibility is there. When you hear or see that a watch is in effect, you should keep an eye and ear tuned to television or radio so you know what is going on with the weather.
Tornado Warning: Is issued when a tornado has been spotted, or indicated on Doppler radar. When a tornado warning is issued for your town or county you should take immediate safety precautions.
What is a Tornado?
A tornado is a rapidly rotating column of air dropping from a strong thunderstorm to the ground. That thunderstorm cloud is known as a cumulonimbus cloud.
What causes a Tornado?
Inside a cumulonimbus cloud or thunderstorm, you have updrafts and downdrafts. Warm rises up into the thunderstorm and cold air sinks down in the thunderstorm.
Now all air masses have a certain amount of spin, but, the updrafts and downdrafts in a thunderstorm increase that spin inside the storm. This motion creates a horizontal rotating column of air. The rising and dropping air tilt that column vertically and if that column of air extends to touch the ground, Bingo, you have a tornado!
The technical term for this process is called "Conservation of Angular Momentum." Have you ever seen an ice skater doing spins on the ice? When the skater pulls their arms in close to their body, they can spin faster and faster.
With a thunderstorm, the tighter the spinning air goes, the stronger the rotating column of air gets.
How long can a tornado last?
Some twisters may last only for a couple of minutes and are on the ground for a few feet to a few miles. While some can last several minutes to several hours and travel from a few miles to more than a hundred miles. A few of these huge tornadoes may have paths of destruction that are a mile wide.
When do tornadoes occur?
In Colorado, primetime for tornadoes is from mid May through mid August. Colorado twisters have been reported in 9 out of 12 months of the year.
The big month for tornadoes is June. Through the years most of the recorded tornadoes in our state have popped up in the month of June.
Most of our tornadoes happen between 1pm and 9 pm. In fact, 88 percent of them occur during this time period. And over half of all twisters threaten between 3pm and 6 pm.
If you break it down by county, Weld County in Northeast Colorado is the big winner or loser (depending on how you look at it) in number of tornadoes. In fact, Weld County on average has one of the biggest numbers of tornadoes across the country! The reason for that is primarily due to Weld counties size. It is 2 to 3 times larger than most counties in the United States.
Where are Tornadoes most likely to form?
Tornadoes can occur anywhere in the state of Colorado, but, most of them develop in Eastern Colorado. Most of those occur east of Interstate 25.
Twisters have been reported in every state in the United States. But, if we look at the nation as a whole, the bulk of tornado producing thunderstorms hit an area that meteorologist call "Tornado Alley." This is an area of land in the nation's midsection that is clobbered by more tornadoes than anyplace else in the world!
This area runs from North Texas through Oklahoma, Kansas, Missouri, and Ohio. That's not to say all tornadoes occur in this area. But, a large number of them do. Other areas that pick up a lot of tornadoes include many southern states like Mississippi, Alabama, Georgia and Florida.
How is the Strength of a Tornado measured?
Meteorologists use a scale called the "Enhanced F-scale" to measure the power of a tornado. It is a scale that is based upon how much damage any given tornado produces.
*** Important note about F-scale winds: These precise wind speed numbers are actually guesses and have never been scientifically verified. Different wind speeds may cause similar looking damage from place to place. Even from building to building. A new "Enhanced F-scale" will be implemented in February 2007.
The Enhanced F-scale replaced the old Fujita scale, (F-scale), which is named after Professor T. Theodore Fujita, a pioneer in the study fo tornadoes. The new Enhanced F-scale gives more detailed information about damage and winds.
Click here
to see the Fujita Scale.
Seth Borenstein, AP Science Writer, contributed the information in this report under the heading "Meteorologists Can't Explain Exactly Why So Many Are Forming." CBS4 meteorologist Dave Aguilera contributed the general information about tornadoes below that.
(© 2009 CBS Broadcasting Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. The Associated Press contributed to this report.)
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