WASHINGTON – April was a historic month for wild weather in the United States, and it wasn't just the killer tornado outbreak that set records, according to scientists with the National Oceanic and Atmospheric Administration.
April included an odd mix of downpours, droughts and wildfires. Six states — Illinois, Indiana, Ohio, Kentucky, Pennsylvania, and West Virginia — set records for the wettest April since 1895. Kentucky, for example, got nearly a foot of rain, which was more than three times its normal for the month, NOAA reported.
Yet the U.S. also had the most acres burned by wildfire for April since 2000. Nearly 95 percent of Texas has a drought categorized as severe or worse, exacerbated by the fifth driest April on record for the Lone Star state.
Add to a record 305 tornadoes from April 25-28, which killed at least 309 people and the most tornadoes ever for all of April: 875. The death toll and total tornado figures are still being finalized.
Much of the southern and eastern United States were near record hot for April, while northwestern states were cooler than normal. Overall, the month was warmer than normal for the nation, but not record-setting.
The odd mix of massive April showers and bone-dry drought can be blamed on the cooling of the central Pacific Ocean, which causes storm tracks to lock in along certain paths, said Mike Halpert, deputy director of NOAA's Climate Prediction Center.
"It's very consistent with La Nina; maybe we've had more extremes," Halpert said. "It's a shift of the jet stream, providing all that moisture and shifting it away from the south, so you've seen a lot of drought in Texas."
U.S. scientists also looked for the fingerprints of global warming and La Nina on last month's deadly tornadoes, but couldn't find evidence to blame those oft-cited weather phenomena.
NOAA research meteorologist Martin Hoerling tracked three major factors that go into tornadoes — air instability, wind shear and water vapor — and found no long-term trends that point to either climate change or La Nina. That doesn't mean those factors aren't to blame, but Hoerling couldn't show it, he said.
Climate models say that because of changes in instability and water vapor, severe thunderstorms and maybe tornadoes should increase in the future. But it may take another 30 years for the predicted slow increase to be statistically noticeable, said NOAA research meteorologist and tornado expert Harold Brooks.
But Kevin Trenberth, climate analysis chief at the National Center for Atmospheric Research in Boulder, said the preliminary study that Hoerling conducted was flawed and too simplified. He said there is evidence of an increase in instability in the atmosphere happening now.
NOAA on April weather: http://www.ncdc.noaa.gov/sotc/national/
The NOAA study on the tornado outbreak: http://www.esrl.noaa.gov/psd/csi/events/2011/tornadoes/index.html