Kevin Myatt is The Roanoke Times' weather columnist.

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What September is lacking in hurricanes for the United States, it has at least partially made up for with springlike severe weather outbreaks.

On Thursday and Friday, there were 57 reports of tornadoes in the central U.S. On Friday alone, there were 41 reports of tornadoes, 79 reports of damaging winds, and 124 reports of hail ³₄ of an inch in diameter or larger.

The region affected stretched from Oklahoma and Kansas to the west, north to near Chicago, south and east to central Tennessee and Alabama. It was triggered by a cold front cutting into warm, humid air from the Gulf of Mexico. The same front moved through our area much more quietly Sunday, though I did encounter some severe thunderstorms along the Virginia-North Carolina border near Kerr Lake to chase and photograph.

The most deadly part of the severe weather Thursday through Saturday in the South and Midwest was flooding, as many of the storms kept moving across the same areas repeatedly, known as a "training effect." At least 12 people were killed in the flooding.

This outbreak came a week after a Sept. 15-16 outbreak of severe weather in the northern Plains that resulted in 21 reports of tornadoes, one of which had F2 (113 to 157 mph ) wind speeds over an 8-mile-long path in suburban Minneapolis and killed a young girl.

We've had high tornado activity in September the past two years, with 297 tornadoes in the U.S. during September 2004 and 131 for the month in 2005, compared with only 32 in September 2003 and an average of about 56 tornadoes each September during the 1990s.

But the high September tornado numbers the past two years were propelled largely by unusual tropical activity. Hurricanes and tropical storms often spin off tornadoes, sometimes prolifically, as they move inland. Most tornadoes spawned by tropical systems are brief and relatively weak spin-ups, though there are some notable exceptions.

A different dynamic has been at work this month, one far more typical of why the United States is believed to have a majority of the world's tornadoes. The outbreaks of this September have followed the textbook, with cool, dry air from Canada clashing with warm, moist air from the Gulf of Mexico between the nation's major mountain ranges, the Rockies and Appalachians.

One factor has been much stronger than it has been in recent September weather patterns -- the position and strength of the jet stream.

The last couple of years, the jet stream, the fast-moving current of air 5 to 8 miles up that moves weather systems, has been sluggish in its typical fall return from Canada, where it usually spends the summer. But this September, it has been a big player, dipping into the continental United States to drive through repeated strong cold fronts.

The enhanced winds of the jet stream aloft have also added more shear -- the change of wind direction and/or wind speed with height -- to the atmosphere in severe weather situations.

On Friday, strong upper-level winds with the jet stream from the west and northwest, on top of strong surface winds from the south pulled toward a strong surface low-pressure area in the northern Plains, gave many developing thunderstorms in Missouri, Illinois, Arkansas and adjacent states enough rotation to make them really dangerous.

We'll have to keep an eye on this vigorous jet stream as autumn moves toward winter. If the jet stream from Canada can continue to be this active, and El Nino can help charge up a southern branch of the jet stream from the subtropical regions, as is common, the winter months could be very active in the interplay of the two jet stream branches. That could mean lots of rain, thunderstorms, snow and ice, across the southern half of the nation, including our area, depending on the particulars of each setup.

For now, the vigorous jet stream is going to continue to drive strong cold fronts through. Another arrives about Thursday, bringing a chance of showers and thunderstorms, then a renewed chill and some gusty winds to the area.

See the latest tidbits on weather, locally and nationally, on Kevin Myatt's Weather Journal blog at blogs.roanoke.com/weatherjournal/