United States

Paul Pastelok, leader of the AccuWeather.com Long-Range Forecasting Team, released his Spring 2011 Forecast Thursday, and one of his major areas of concern this season is the flooding potential in the Midwest, Great Lakes and Mississippi Valley.

Spring flooding is a concern every year, though the location and severity of it vary.

This year, Pastelok highlights areas from southern Minnesota through the Ohio River Valley and New England as having the greatest risk for flooding.

Rapid snowmelt and heavy rainfall are both major causes of spring flooding, but when both of these are combined, the potential for disastrous conditions increases rapidly.

This image, courtesy of the National Operational Hydrologic Remote Sensing Center, shows estimated snow cover and snow-water equivalent across the U.S. as of March 3, 2011.

Although a significant snowpack is a concern, what is more important than the depth of the snow is the liquid contained in it. Slight thaws and refreezing over time can lower the thickness of the snow, but the amount of liquid can stay virtually unchanged.

"A few inches of heavy snowpack, having been subject to cycles of thawing and even rain, can easily hold as much water as a major snowstorm," said AccuWeather.com Senior Meteorologist Jim Andrews. "Typical fresh-fallen snow has a ratio of about 1 inch of liquid for every 12 inches of snow, but an old snowpack may have a ratio of only 1 inch of liquid to 2 to 3 inches of snow."

This second image shows estimated snow cover and snow-water equivalent on Feb. 25, 2011. Temperature increases and a series of severe storm systems melted much of the snow south of the Dakotas.

Flooding has already become a problem for the Ohio Valley region, largely due to heavy rain that hit the area in late February, but also in part to snowmelt.

A severe storm system affected much of the country at the end of February. During this time, the snowpack in Missouri virtually disappeared. As a result, the water levels of the Mississippi River at St. Louis, a major river junction point, rose considerably.

This graph shows the gauge height of the Mississippi River at St. Louis from Feb. 23 to March 2. The rapid increase that starts at Feb. 28 was the result of the severe storm system, which caused snowmelt and precipitation in the area.

The Mississippi River waterway widens as the water flows southward because of the tributaries that join it along the way. Because of this, the river is most likely to flood farther to the north, and the area north of St. Louis, including the Dakotas and Minnesota, is at risk for flooding this spring.

"If a flooding river meets a non-flooding river, the junction below it will likely be able to handle the excess water, and flooding potential will decrease," said Andrews.

The Red River is flowing fairly high, but below flood stage at this point, which is 18 feet. Major flood stage is 30 feet, and this point has been reached 17 times in recorded history, which goes back to the 1800s.

The river flows northward and forms the eastern border of North Dakota. This river is known for causing flooding problems for the area. In 2009, the river reached a record high crest and caused major flooding in Fargo, N.D.

There is still a deep snowpack in place from the northern Plains into New England, and the potential for more snow to pile on will linger through March or even early April. Rain that spreads through these areas this spring could combine with melting snow to cause significant flooding.

The Ohio River Valley is a bit of a different story for the spring season, as most of the area's snowpack has already melted or been washed out in recent weeks. The only remaining snow cover is confined mainly to the extreme northeastern portion of its catch basin.

In addition, Pastelok has a concern for mesoscale convective systems (MCS) in the Plains and Great Lakes, as well as the Mississippi and Ohio valleys.

An MCS is a complex of severe thunderstorms that becomes highly organized to the point that it can resemble an extratropical cyclone. An MCS can cause substantial rainfall, strong straight-line winds and tornadoes.

"The battle zone setup to the north will continue from the West Coast through the Plains and dipping down into the mid-Mississippi Valley, Tennessee Valley and Ohio Valley," Pastelok said. "Severe weather could get kick-started this year."

An MCS is extremely beneficial to crop areas, especially during a drought, but to areas with rivers and streams nearing capacity, a severe weather event can be disastrous.

Pastelok also said that significant precipitation will be a concern for aggravating flooding in the region through at least May.

For the complete AccuWeather.com Spring 2011 Forecast, be sure to check out the full story.

AccuWeather.com Meteorologist Heather Buchman contributed to the content of this story.