AccuWeather.com

By Alex Sosnowski, Expert Senior Meteorologist

Mar 13, 2010; 3:00 AM ET

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River flooding over the central Plains is becoming more substantial and widespread this weekend, due mostly to rain that has already fallen and additional melting snow upstream.

Streams continue to run high and more rivers are on the rise in the region this weekend, including the Platte, Grand, Cedar, Des Moines, Iowa, Raccoon and James.

Minor to moderate flooding is now occurring or is anticipated in unprotected areas along these waterways and others into early next week. Major flooding is expected along portions of the Des Moines and Cedar rivers.

Flooding may suddenly become more serious in a few locations, depending on local conditions, chiefly due to ice jams.

The coverage of flood problems is expanding northward and eastward this weekend as mild air and pockets of rain eat into existing areas of snow.

Major flood problems are anticipated in the northeastern U.S. beginning this weekend. The northern Plains may be next.

So far, above-freezing temperatures and rain are gently melting the snow cover over the Upper Midwest. However, not all of the snow will melt this week and the next.

Expectations are that relatively minor flooding issues will occur in most areas in the short term compared to recent years. Gradually melting snow alone, or melting snow with sporadic light rain, would not lead to major flooding. However, concerns for major flooding of the Red (Dakotas), Ohio and Mississippi rivers and others remain for the long term in the region.

Significant rises along these largest of rivers will occur over the next couple of weeks.

Saturated ground, and streams and rivers swollen from the thaw now accelerating, will remain a loaded gun for much of the spring.

The storm affecting the region now will slowly move away. However, sunshine and mild conditions next week will lead to more snowmelt in northern areas, including locations where snow fell or was still falling from the storm over the northern and central High Plains.

Additional storms over the Plains and Midwest this spring have potential to drop heavy rain, and/or heavy, wet snow in the north and heavy rain in the south.

While these phenomena typically occur in the early spring, the problem is with the water locked up in the remaining snow cover and the saturated ground in areas where the thaw has already occurred.