A sign informing neighbors of a homeowner's condition is displayed… (Reuters )
As residents of Joplin, Mo., continued digging out from Sunday's deadly tornado, researchers prepared to visit the stricken city to assess the storm's intensity. Coming only three weeks after an unprecedented series of twisters wrought destruction across the Southeast, many were wondering whether the events were related and whether more severe storms were in store.
Here are answers to some questions about the science of tornadoes.
Was there a connection between Sunday's tornado and the ones in April?
No, according to Gregory Carbin, warning coordination meteorologist for the National Oceanic and Atmospheric Administration's Storm Prediction Center in Norman, Okla. They were separated by three weeks and were "completely different weather systems," he said. "If there is a relationship, that relationship is springtime in the Plains."
Photos: Tornado hits Joplin
What caused the tornadoes?
Warm, moist air from the Gulf of Mexico moved north until it encountered cold air brought south by the jet stream. When the warm and cold air from different altitudes come together, it creates wind shear and circular air motions that lead to a tornado.
The air from the south was unusually warm and moist for this time of year, according to Stu Ostro, a senior meteorologist at the Weather Channel. Some experts say this is because waters in the gulf are about 2 degrees warmer than normal for this time of year.
In April, the jet stream stalled in place for about two weeks, allowing a repeated series of local disturbances that translated into a series of tornadoes, which meteorologists call an outbreak. Then the jet stream moved back north and the weather pattern became more tranquil. If the rest of the month had matched the first three weeks, Carbin said, May would have been a record slow month for tornadoes.
How severe was the Joplin tornado?
Preliminary estimates are that the storm measured EF-5 on the Enhanced Fujita Scale of tornado intensity, meaning that it had wind speeds of more than 200 mph. It is not possible to measure the wind speeds of a tornado directly, so researchers go in after the event and assess the damage to get an indirect measurement.
How common are such storms?
Between 1950 and 1994, about 1% of all U.S. tornadoes were either EF-4 or EF-5. But during that period, 67% of tornado deaths were attributed to tornadoes in those two classes. Experts say the increasing damage and deaths observed in recent years is in part due to increased urban sprawl, which means that any tornado has a greater likelihood of striking a populated area.
Is this an unusual year for tornadoes?
It's shaping up to be. Over the last couple of decades, an average of about 1,200 tornadoes touched down each year in the continental United States. This year, about 1,076 had been counted even before this week's activity in Missouri and elsewhere.
As mentioned, the Gulf of Mexico is warmer than normal, sending more moist air over the South. The jet stream has also been moving somewhat farther south than usual, bringing it into contact with that excess moisture and triggering the large storms.
Are the stronger storms a result of global warming?
It is impossible to link specific storms and weather events to climate change. But one of the predictions of the climate change models is that we'll be in for more intense storms as average global temperatures climb. That is what appears to be happening, both in summer and in winter.
Some reports have called the Joplin tornado a "double cyclone tornado." Is that correct?
That's not a generally recognized meteorological term. A more correct term is a "multiple vortex tornado." Simply put, that means there is not only a central, large funnel, but also one or more smaller funnels surrounding it. Such tornadoes are not common, but they are also not rare.
Why weren't people warned about the approaching tornadoes?
They were, with warning periods ranging from 17 to 20 minutes. But many people didn't respond to the warnings for a variety of reasons. Some said they were unable to hear the warning sirens because they were drowned out by noisy rain and hail.
Psychologists also note that warning sirens are relatively common in the Midwest and that many people ignore them until they can actually see a tornado approaching or a friend or neighbor vouches that it is on the way. By that time, it is often too late to reach safety, particularly with a storm as massive as this one, said Jerry Brotzge, a senior research scientist at the University of Oklahoma.
"Once they have that secondary information, they're much more likely to take shelter," he said.
Photos: Tornado hits Joplin
Times staff writer Amina Khan contributed to this report.