|Storms on Friday, 4/13/2012|
Unfortunately, Joplin, Missouri, so hard-hit just a year ago, is getting pummeled again with severe weather today and forecast into the weekend.
Here's the NOAA detailed description.
My simplistic view of the spring storm system in the midwest is that cold air from Canada collides with warm air from the Gulf of Mexico, leading to our spring thunderstorms and tornado season. The scenario for this weekend seems much more complicated. An "impressive upper-level low" is moving out across the four-corners region (where New Mexico, Arizona, Colorado and Utah come together) at the same time as a powerful mid-level jet stream moves northeastward into the southern and central plains. This means that there is abundant moisture available in the lower layers of the atmosphere, and strong shearing winds at low levels to spin up parts of the storms into tornadoes. The third ingredient in place is a "cap" to hold the warm moist air near the ground until it "explosively" breaks through the cap in severe storms. The "cap" in this instance is provided by warm dry desert air coming in from the desert southwest four-corner region. The geography then of Canada to the north, the Gulf of Mexico to the south, and the dry desert to the southwest makes the U.S. the tornado capital of the world.
The tornado outbreak from April 25-28, 2011 was the largest tornado outbreak recorded, with 358 tornadoes. In 1974, 148 tornadoes occurred in the U.S. and Canada (see this post on "tornado alley" in Canada.) That earlier outbreak has the distinction of severe tornadoes with 6 F5 and 24 F4 tornadoes.
Other posts on tornadoes: multivortex tornado, Monster Alabama tornado, February 2012 tornado, brutal weather April, 2011.