As we continue to roll along in the month of March, some subtle and not-so-subtle changes are occurring in our atmosphere. What goes on in the air thousands of feet above our heads is often visible. The thick storm clouds or the vivid sunshine are indicators of what the atmosphere is cooking up for us. But there are some invisible things going on as well, and March is the month when our atmosphere switches gears from winter to spring.

One of the main switches is the position of the jet stream. I've talked often in this column about the fast river of air flowing over the United States. The fastest core of winds is typically between 25,000 and 35,000 feet. But the path that the fast core takes across our country changes from season to season. Think of the jet stream as the great divider between warm air to the south and cold air to the north.

In the winter, much of the U.S. is gripped in the cold air, so the main core of jet stream winds typically rides across the middle of the country from west to east.

Here in Wisconsin we are usually north of the main jet stream core. Winter also features a sub-tropical jet stream that can roar across the lower third of the nation from southern California to Texas to Florida. In an El Nino winter, when the eastern Pacific Ocean water tends to be a little warmer, the sub-tropical jet stream is strong and can push powerful storms and flooding conditions through the southern states.

Now in March, the Sun is getting higher in the sky and warmth is beginning to return to the central United States. The main jet stream is beginning to shift to the north. It will carry the brunt of the storm systems across the country as well. Meteorologists constantly watch the shifting position of the jet stream core, but this time of year it is more important than ever to keep an eye on it.

When that jet stream dips down south and then roars back north into the Great Lakes, it can carry a tremendous amount of moisture from the Gulf of Mexico that can bring either flooding spring rainfall or late-season heavy snow.

March is also the month when you will notice more severe thunderstorms popping up across the country. Our severe weather season doesn't get rolling until sometime in April, although just a few years ago a twister touched down on the northeast corner of Mitchell International Airport on March 8, 2000. That was Milwaukee County's earliest tornado of the season and a definite rare event. But twisters start popping up this month in the southern part of "tornado alley". Texas, Oklahoma, Arkansas, and Kansas see their share of strong thunderstorms, fueled in part by the energy from the strong jet stream roaring overhead.

Temperature contrasts are the key. After all, we can still have blizzards in the northern Plains and powerful thunderstorms in the southern Plains this time of year. The temperature contrast increases the strength of the surface low-pressure system that travels along the path of the jet stream. Nature cooks up quite a combination of surface and upper air factors that can combine to produce powerful weather systems.

April and May continue to feature a strong jet stream slicing through the central United States. By June, as warm air takes over most of the lower 48 states, the main core of fast winds migrates north into southern Canada where it usually hangs out for the summer months. Cool air may briefly intrude into the northern third of the U.S. in summer, but summertime cold fronts are poor imitators of the more powerful springtime fronts.

There is just never a dull moment around here when it comes to weather, and transition months like March keep things interesting!