Q. Is global warming to blame?

A. You can't blame a single weather event on global warming. In any case, scientists just don't know whether there will be more or fewer twisters as global warming increases. Tornadoes arise from very local conditions, and so they're not as influenced by climate change as much as larger weather systems like hurricanes and nor'easters. They're not easy to incorporate in the large computer simulations scientists use to gauge the impact of global warming.

And when scientists ponder the key weather ingredients that lead to twisters, there's still no clear answer about whether to expect more or fewer twisters. Some scientists theorize that the jet stream is changing because sea ice in the Arctic is shrinking. And the jet stream pattern drives weather in the Northern Hemisphere.

Q. How does this tornado season stack up against previous ones?

A. The season got off to a quiet start this year. Typically, there are more during spring, and the numbers dwindle in the worst heat of the summer. An unusually cool spring kept the funnel clouds at bay until mid-May this year. The last two seasons illustrate the extremes in tornado activity. In 2011, the United States saw its second-deadliest tornado season. Last year, it was busy in April but there were few twisters after that.

Q. What happened in Oklahoma?

A. The tornado destroyed two elementary schools and flattened neighborhoods with winds estimated between 200 and 210 mph. The National Weather Service on Tuesday gave the twister a top-of-the-scale ranking of EF5. The tornado at some points was 1.3 miles wide, and its path went on for 17 miles and 40 minutes.

Q. How did it form?

A. Like the most destructive and deadly tornadoes, this one came from a rotating thunderstorm. The thunderstorm developed in an area where warm moist air rose into cooler air. Winds in the area caused the storm to rotate, and that rotation promoted the development of a tornado.

The FDA said on Monday it was taking a "fresh look" at the issue in response to the launch of a caffeinated gum, "and if necessary, will take appropriate action."

While the FDA did not name the gum in its statement, Wrigley launched the product this month. One piece of the gum contains 40 milligrams of caffeine, about as much as a half-cup of coffee, according to Wrigley, which is owned by privately held Mars Inc.

The company said it markets the gum as an energy product for adults aged 25 and older, and that it exceeds current regulatory requirements on labeling and disclosure. The gum has a more bitter taste that does not appeal to children, a higher price, and packaging that clearly separates it from other gums, she said.

"As the FDA refines its approach to caffeine, we welcome the opportunity to work with them on this important topic," a Wrigley spokeswoman said in an email.

The FDA seeks to protect consumers from unsafe foods. In addition to research and inspections, the agency has the power to recall and seize unsafe products and stop companies from producing them.

"The only time that FDA explicitly approved the added use of caffeine in a food was for cola and that was in the 1950s," Michael Taylor, FDA deputy commissioner for foods and veterinary medicine, said in a statement on its website on Monday.

"Today, the environment has changed. Children and adolescents may be exposed to caffeine beyond those foods in which caffeine is naturally found and beyond anything the FDA envisioned when it made the determination regarding caffeine in cola," he said.

Wrigley is not the first to market gum with energizing properties. Mondelez International Inc sells a line of Trident Vitality gums, with ingredients like ginseng, green tea and Vitamin C, and Stride Spark gum in "Kinetic Mint" and "Kinetic Fruit" flavors that have Vitamins B6 and B12 added.

A Mondelez spokesman said none of the company's gums contain caffeine.

(Reporting by Jessica Wohl in Chicago, Toni Clarke in Washington D.C. and Martinne Geller in New York; Editing by John Wallace and Jeffrey Benkoe)

Friday's draw for the clay-court Grand Slam tournament placed Nadal and Djokovic on the same half of the field, while Roger Federer could face David Ferrer in the other semifinal.

Federer, the owner of a record 17 major titles including the 2009 French Open, will face a qualifier in the first round and if he wins that, he'll play a qualifier in the second round, too.

Djokovic faces a far more intriguing start: The reigning Australian Open champion's first-round opponent is David Goffin, a 22-year-old Belgian who took a set off Federer in the fourth round in Paris last year after making it that far as a lucky loser.

No man has won the title at Roland Garros as many times as Nadal, who broke a tie with six-time champion Bjorn Borg by defeating Djokovic in last year's final and is 52-1 for his French Open career. Nadal also has reached the finals of all eight tournaments he's played in 2013.

But because the Spaniard missed about seven months with a left knee injury, his ranking slipped to No. 4, and the French Open decided not to bump him to a higher seeding.

If the tournament, which starts Sunday, had placed Nadal at No. 2, he and No. 1 Djokovic could have met only in the final; instead, a Nadal-Djokovic rematch for the championship can't happen in 2013.

"If you're not Nos. 1 and 2 of the world," Nadal said, "that can happen."

Jean Gachassin, president of the French tennis federation, said Friday his group has not yet discussed whether to switch to a seeding system similar to Wimbledon's, which takes into account past results on a specific surface rather than only the ATP rankings.

"That will be a debate that we will have after this French Open," Gachassin said.

Nadal has lost eight of his last 11 matches against Djokovic, including on clay at Monte Carlo last month.

"It definitely gave me the confidence boost," Djokovic said about that most recent encounter against the Spaniard. "Winning against Nadal on clay is not something that happens every day, you know. It's a big challenge."

Nadal is seeded No. 3 because second-ranked Andy Murray, the reigning U.S. Open champion, withdrew from the French Open with a back injury.

The possible men's quarterfinals are: Djokovic against No. 8 Janko Tipsarevic, his Davis Cup teammate for Serbia; Nadal against No. 7 Richard Gasquet of France; No. 2 Federer against Jo-Wilfried Tsonga of France; and No. 4 Ferrer against No. 5 Tomas Berdych of the Czech Republic.

Berdych was drawn to face Gael Monfils of France in the first round, drawing groans from some members of the audience.

The last man from France to win the French Open was Yannick Noah in 1983, a 30-year gap mentioned more than once at Friday's ceremony.

Serena Williams wants to end her own, shorter drought in Paris her lone French Open title came in 2002 and her bid for a second championship will begin against 83rd-ranked Anna Tatishvili in the first round.

A year ago in Paris, Williams lost her opening match to 111th-ranked Virginie Razzano, the American's only first-round loss in 50 career Grand Slam tournaments.

Williams is seeded No. 1 this year and is on a 24-match winning streak, the longest of her career.

Tatishvili, from the former Soviet republic of Georgia, is 2-10 this season. She's also 0-2 at Roland Garros and 6-8 overall at Grand Slam tournaments. Williams, in contrast, has won 15 major titles.

The possible women's quarterfinals are: Williams against No. 8 Angelique Kerber, defending champion Maria Sharapova against No. 7 Petra Kvitova, No. 3 Victoria Azarenka against 2011 champion Li Na, and No. 4 Agnieszka Radwanska against 2012 runner-up Sara Errani.

"It's very meaningful to come back as a defending champion. It means you have done something pretty good, and you're coming back into that position and you're trying to defend it," said Sharapova, who completed a career Grand Slam by winning last year's French Open. "I think it's one of the best honors you can have as a tennis player."

One noteworthy first-round matchup is No. 10 Caroline Wozniacki, who used to be No. 1, against 35th-ranked Laura Robson, a British teenager who reached the fourth round at last year's U.S. Open.

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Associated Press Writer Trung Latieule contributed to this report.

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Follow Howard Fendrich on Twitter at http://twitter.com/HowardFendrich

Bolden checked on the progress Thursday a month after the Obama administration unveiled its 2014 budget that proposes $105 million to jumpstart the mission, which may eventually cost more than $2.6 billion.

Engineers at the Jet Propulsion Laboratory in Southern California and Glenn Research Center in Ohio are developing a thruster that relies on ion propulsion instead of conventional chemical fuel.

Once relegated to science fiction, ion propulsion which fires beams of electrically charged atoms to propel a spacecraft is preferred for deep space cruising because it's more fuel-efficient. Engine testing is expected to ramp up next year.

During his visit to the JPL campus, nestled in the foothills of the San Gabriel Mountains northeast of Los Angeles, Bolden viewed an engineering model of the engine and peered through a porthole of a vacuum chamber housing the prototype.

NASA is under White House orders to fly humans to an asteroid as a stepping stone to Mars. Instead of sending astronauts to the asteroid belt between Mars and Jupiter, as originally planned, the space agency came up with a quicker, cheaper idea: Haul the asteroid close to the moon and visit it there.

Bolden said the original concept was impractical given the flat budget and praised the alternative as "ingenious."

"If you can't get to the asteroid, bring the asteroid to you," Bolden said.

The space agency would launch an ion-powered unmanned spacecraft to snare a yet-to-be-selected small asteroid in 2019 and park it in the moon's neighborhood. Then a spacewalking team would hop on an Orion space capsule that's currently under development and explore the rock in 2021.

Besides preparing astronauts for an eventual trip to Mars, NASA said the asteroid-capture mission is designed to test technologies to deflect threatening space boulders on a collision course with Earth.

"Anytime that you can get up close to an asteroid and understand its composition and its characteristics... that's getting to know the enemy," said Don Yeomans, who heads NASA's Near Earth Object Program at JPL.

Scientists have a dozen potential small asteroids in mind for the mission, but Yeomans said more observations are needed before settling on a target. Whatever asteroid NASA chooses to redirect, it won't pose a threat to Earth because it would burn up if it inadvertently plunged through the atmosphere, scientists said.

Bolden's JPL stop is part of his annual spring tour of NASA centers around the country. His California journey began Wednesday at the Dryden Flight Research Center in the Mojave Desert where Sierra Nevada Corp. is preparing its Dream Chaser spaceship for test flights later this year before it can make supply runs to the International Space Station. On Friday, Bolden was set to visit the Ames Research Center in the Silicon Valley where engineers are working on various space technologies.

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Follow Alicia Chang at http://twitter.com/SciWriAlicia

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That's because HTC hasn't had a hit since 2011. After , the once-popular Taiwanese smartphone competitor needed something like the One to lift it out of a bad year. The phone should have been good enough to do that, given all the laudatory praise. But it didn't: Sales of the device, which debuted in mid-April, started off slow, sources tell Ziegler — and that was a worse outlook than , which had the One "selling well," but not as well as its biggest competition, Samsung's Galaxy S IV. One source attributes the lag to "supply issues," which had to do with internal disorganization. But even if they get on track, HTC might be too late to fix this mess. The Samsung Galaxy S IV has already captured a lot of that high-end, non-iPhone smartphone market, gettingand selling 10 million devices in less than a month. (The Galaxy S III took 50 days to hit the same mark.) 

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Unfortunately, the One's decent sales are the better part of the news for HTC. The Facebook phone's  has only made things worse. One of Ziegler's sources called it an internal "disaster," which pretty much mimics its performance in the outside world. After a little over a month, the company discontinued the HTC First, despite the hype of a Facebook partnership, straight from Mark Zuckerberg himself. Things only look to get worse from here: HTC CEO Peter Chou told other senior executives he would step down if the One failed, . At this point, it remains unclear if that bigger, better phone is a failure, per se, but it's definitely not the type of huge and huger success that HTC really needs. 

HONG KONG (Reuters) - Three men were charged on Friday in a Hong Kong court with possessing false documents including a check for $460 million in connection with an investigation into the city's shuttered Hong Kong Mercantile Exchange.

The exchange, which operated a trading platform for gold and silver futures, surrendered its license to the Securities and Futures Commission at the weekend, saying it did not have enough trading revenues to support its operating expenses.

The SFC, the city's financial watchdog, on Tuesday said it had found serious suspected irregularities in the exchange's operations and referred the case to the police for a criminal investigation.

The three mainland Chinese men - Dai Linyi, 55, Li Shanrong, 49, and Lian Chunren, 50 - faced charges of "possessing a false instrument with intent", according to a charge sheet seen by Reuters.

The court gave no details of precisely how the men might be linked to the Hong Kong Mercantile Exchange.

No pleas were taken and the three were remanded in jail. The case was adjourned until July 19.

Barry Cheung, chairman and controlling shareholder of the exchange and a close political ally of Hong Kong leader Leung Chun-ying, has taken leave of absence from several high-profile public positions as the police investigation continues.

Cheung has not been accused of any wrongdoing.

The three men were accused of having in their custody or under their control instruments which were, and which they knew or believed to be false, according to the charge sheet.

It stated that the three intended to use the documents in question, including a letter of guarantee and telegraphic transfer, to induce somebody to accept them as genuine.

The abrupt closure of the mercantile exchange has raised questions over regulatory oversight in Hong Kong.

The case has also drawn unwanted attention to Hong Kong leader Leung nearly a year into his first anniversary in office due to his close ties with Cheung, who acted as his campaign chairman in the leadership election last year.

(Additional reporting by Grace Li; Editing by Anne Marie Roantree and Michael Urquhart)

SAN FRANCISCO/SEATTLE (Reuters) - Microsoft Corp is set to make a splash this week with the eagerly awaited unveiling of its new Xbox game console, eight years after the last version, as it seeks a larger share of the $65 billion a year global computer gaming industry.

But the small device faces some big competition from the PlayStation 4 by Sony Corp and the Wii U by Nintendo Co Ltd in a shifting market.

Gamers are gravitating to online play - suggesting the hey-day of console games are over - while Microsoft wants its sleek new toy to finally cross the bridge to the mainstream and become the family's entertainment center.

"Core gamers are very hungry for a new machine but the difference between 2005 and now is that the stakes are so much higher," said Ryan McCaffrey, executive editor at entertainment website IGN.com, harking back to Microsoft's last Xbox release. "The entire Xbox experiment from Microsoft was for it to be the center piece of your living room."

To that end, industry-watchers are expecting a raft of improvements from the new Xbox, when Microsoft unveils it at its Redmond, Washington, headquarters on Tuesday, from closer integration with the TV and link-ups with mobile devices to access to new and even exclusive content.

Console gaming still takes the lion's share of a growing gaming market - about 42 percent of the $65 billion world market, according to Microsoft. But playing games on smartphones and tablets, or as an offshoot to online social networks, is gaining ground fast.

Console sales have been in decline for the last four years, chiefly because of aging devices, but the first of the new generation of machines has not reignited the sector.

Nintendo's Wii U, launched in November, had sold only 3.45 million units through the end of March, well below the company's initial forecast of 5.5 million. Hopes for Sony's PS4, teased in March, are low key.

"The next wave crest isn't as high as the previous one," said Lewis Ward, research manager at International Data Corp, who calculates that about 250 million Xbox 360, Sony PlayStation 3 and Nintendo Wii units were sold between 2005 and 2012.

"I do think that consoles as a product category have peaked and the next gen devices won't match those totals," he said.

LOW MARGINS

The Xbox itself is not a key financial factor for the world's largest software maker. Its Entertainment & Devices unit is set to break $10 billion in sales for the first time this year, but that's half the sales of its Windows unit, and a lot less profitable, averaging less than 15 percent margin compared to 60 percent or higher for Windows or Office.

The company has more than 46 million members who subscribe to its online gaming and digital entertainment service Xbox Live, but that's still a fraction of the people who pay for its software.

However, the Xbox is still a key weapon in Microsoft's strategic battle with Google Inc, Apple Inc, Amazon.com Inc and others for a central place in consumers' lives.

"This (the new Xbox) is of massive importance to Microsoft. It is a piece of a larger war for the consumer that it is battling. They want to be fully integrated with the consumer whether it's in the living room or mobile," said P.J. McNealy, CEO and founder of Digital World Research. "Arguably the battle against both Apple and Google for dominating a consumer's time share more so than taking on Sony and Nintendo directly."

That means Microsoft will be aiming for many markets at the same time, from the core and casual gamer to the TV watcher and music fan.

To do that, industry watchers expect Microsoft to integrate the new Xbox much more closely to the TV and cable or satellite box, perhaps allowing users to change channel or buy movies with a wave of the hand through its motion-control Kinect sensor. They also expect to hear more about SmartGlass, Microsoft's app that lets you link an Xbox to a tablet or smartphone.

Users can already get Netflix through the Xbox, and Microsoft recently started its own studio to produce exclusive content, meaning the new device is a gateway to much more than games.

"I think they're going to try to have their cake and eat it too - they will try to get casual people for entertainment while keeping the hardcore gamers interested," said McCaffrey at IGN.com. "They want their console on all the time, whether it's a mom watching Amazon video, the son playing a game and the dad watching (Major League Baseball) TV on another app - that's their goal."

(Additional reporting by Edwin Chan; Editing by Lisa Shumaker)

The Patient Protection and Affordable Care Act mandates annual reductions in Medicaid payments to hospitals through fiscal 2020 in exchange for increased insurance coverage options that are expected to reduce levels of uncompensated care. The payment cuts increase each year.

Analysts said the size of the reductions over the next three years was manageable for U.S. for-profit hospital companies and noted the rule does not penalize states that have opted out of an expansion of the Medicaid program to cover more low-income Americans.

"They left the door open for states that have not agreed to Medicaid expansion yet without penalizing them for it," said Jefferies analyst Brian Tanquilut.

However, the trade group representing public hospitals that primarily care for low-income patients urged that the cuts be delayed until the impact of Medicaid expansion on the uninsured becomes clearer.

"The Affordable Care Act's disproportionate share hospital reductions are neither justified nor sustainable," the National Association of Public Hospitals and Health Systems said in a statement.

Monday's proposed rule was issued by the Centers for Medicare and Medicaid Services, the federal agency that administers Medicare, Medicaid and the State Children's Health Insurance Program.

The expansion of Medicaid under the reform law has been accepted by governors in about half of the 50 U.S. states.

CMS said it would seek comments from the public on the rule's provisions through July 12. Once finalized, the rule would take effect on October 1, unless Congress enacts the president's budget proposal to begin Medicaid reductions in fiscal 2015 instead of 2014, CMS said.

(Reporting by Susan Kelly in Chicago; editing by Matthew Lewis)

Shares of the Calgary-based company rose as much as 11 percent to C$3.14 on the Toronto Stock Exchange on Wednesday. Its Nasdaq-listed shares were up 10 percent.

Reolysin was administered intravenously along with chemotherapy drugs carboplatin and paclitaxel to patients who either failed to respond to prior therapies or were not considered for standard first-line cancer treatment.

"These preliminary results are very encouraging and support progressing to subsequent studies," said Chief Executive Brad Thompson.

Oncolytics said it will not proceed with the second stage of the trial at this time as it try the drug in combination with other emerging treatments.

(Reporting by Bhaswati Mukhopadhyay in Bangalore; Editing by Roshni Menon)

The historical bestseller set in Henry VIII's court has already won the Man Booker Prize for Fiction and the Costa Book Award. If she takes the Women's Prize, Mantel will become the first person to win all three of Britain's top literary prizes.

The shortlist for the 30,000-pound ($45,900) literary prize released on Tuesday also included two former winners: fellow Briton Zadie Smith for "NW" and American writer Barbara Kingsolver for "Flight Behaviour".

The other three books were "Where'd You Go Bernadette" by U.S. author Maria Semple, "May We Be Forgiven" from American A.M. Homes and "Life After Life" by Briton Kate Atkinson.

"The shortlist for 2013 represents six tremendous writers at the top of their game," chair of the judging panel Miranda Richardson said.

"Their individual novels are flawlessly presented, they contain a heady mix of ideas and without exception take the reader on a unique and deeply satisfying journey."

"Bring Up the Bodies" is the second book in Mantel's trilogy about Cromwell's rise to power. The other novels on the shortlist included Smith's look at characters in north London, a darkly comic novel about 21st century domesticity from Homes and Kingsolver's exploration of climate change and rural poverty in America's Appalachian region.

Atkinson's tale follows the turbulent events of the last century in Britain through the constantly reincarnating character of Ursula Todd. Semple sets her story in Seattle, where 15-year-old Bee searches for her missing, wealthy and erratic mother Bernadette Fox.

Set up in 1996 to celebrate and promote international fiction by women throughout the world to the widest range of readers possible, the Women's Prize for Fiction is awarded annually for the best novel of the year written by a woman.

Any woman writing in English - whatever her nationality, country of residence, age or subject matter - is eligible.

The award ceremony will take place in London on June 5.

Previous winners besides Smith and Kingsolver include Madeline Miller for "The Song of Achilles" last year, Rose Tremain for "The Road Home" in 2008 and Lionel Shriver for "We Need to Talk About Kevin" in 2005.

($1 = 0.6531 British pounds)

(Reporting by Paul Casciato; Editing by Pravin Char)