More severe weather due after a brief lull
By Doyle Rice, USA TODAY

Updated 2h 33m ago |

May, which typically has the most tornadoes in the USA, will have a hard time equaling April's savagery. However, after a relatively quiet first week of May, the atmosphere appears to be gearing up for another round of violent weather this week in parts of the country.

April's ferocity was one for the record books: There were more tornadoes in April 2011 than in any month in U.S. history. The National Oceanic and Atmospheric Administration estimates there were more than 600 tornadoes in April, shattering the record of 543 in May 2003.

With an estimated 327 deaths, the tornado outbreak April 25-28 was the third-deadliest on record, behind 1925 with 747 and 1932 with 332.

This week, there will be several days with a severe-weather threat from the southern Plains to parts of the Ohio and Mississippi valleys, AccuWeather meteorologist Alex Sosnowski said.

The reason, Weather Channel meteorologist Chris Dolce said, is a combination of an upper-level low-pressure area from the West and an increasingly warm, humid air mass in the Plains and South.

The threat should intensify Wednesday, meteorologist Brian Fortier of The Weather Channel said, primarily over the central and southern Plains. Severe thunderstorms, including tornadoes, are possible.

Why is May prime time for tornadoes in the USA? Not only is it hotter and more humid in the South as the sun climbs higher in the sky, but the warmth and humidity spreads north more often into the Plains and Midwest, Weather Channel meteorologist Jonathan Erdman said.

The key for May, however, is that while it's getting warmer and more humid near the surface, the jet stream remains energetic. This allows for cold, dry air aloft to plow over the top of warm, humid air near the surface.

Erdman says these strong jet-stream disturbances tend to enhance the difference in wind speed and direction at various heights above the surface, which helps sustain the supercell thunderstorms that spawn tornadoes.
USA Today