Rain is in the forecast for this entire weekend, but it isn't about to end the drought.

Across southern Minnesota, soil will need about 5 inches of rain or snowmelt in the coming months to be ready to nourish spring plantings.

"And of course the probability for that happening is very, very low," said state climatologist Greg Spoden, noting that the fall and winter months are normally the driest on the calendar.

Across Minnesota, meanwhile, rivers and lakes are gasping. The St. Louis River near Scanlon, which rose to a record height in June and tore apart Jay Cooke State Park, was at what appeared to be its second-lowest height in more than 60 years Wednesday. The Minnesota River at Jordan was down to one-seventh of its historical average for the date. State rivers are contributing so little to the Mississippi that three-fourths of the water in the river downstream from La Crosse is now from the Wisconsin and Chippewa rivers, according to Steve Buan, hydrologist with the North Central River Forecast Center, an NOAA agency based in Chanhassen. And Lake Minnetonka has dropped 1.5 feet since late June.

The low rivers could mean problems later, Buan said. Lack of flow allows logs and other debris to pile up, and can create obstructions when the water rises. Similarly, vegetation can grow in the former stream bed, causing higher and more unpredictable crests when flooding returns, he said. That happened in September 2010, when hydrologists were surprised by a record crest on the Minnesota at Henderson.

For now, low flows have begun to trigger some restrictions on dam operations along the Mississippi and some of its tributaries, so that downstream flows and fish habitat won't be hurt, said Judy Boudreau, hydrologist in the Department of Natural Resource's dam safety program.

Low water has not affected production from Xcel Energy's hydro-power plants in Minnesota and Wisconsin, because demand is also low this time of year, said spokeswoman Mary Sandok. The company also uses surface water to cool steam at its nuclear plant at Monticello and other fossil-fuel burning plants, and cannot discharge water back into water bodies if it would raise the temperature of those water bodies above permitted levels. But with rivers cooling, the discharges have not reached those limits, Sandok said.

Minnesota was the driest of nine Upper Midwest states in September, when the remains of Hurricane Isaac dropped from 5 to 15 inches of rain from Missouri to Ohio, but none came north.

Tuesday morning's rain, which barely wet the ground, was the first measurable rain this month: .03 inch.

The national Climate Prediction Center foresees neither a dry nor a wet tendency over the region through December.

Said Buan: "When in drought, don't predict rain."

End in sight?

Hurricanes often end droughts overnight, but because they rarely visit Minnesota, the end of a dry spell is subtler here.

"It takes several cold fronts to come through and stall," said David Miskus, meteorologist with the national Climate Prediction Center, who helps compile the U.S. Drought Monitor.

The worst drought in recent memory, which extended from October 1986 into 1988, took three months to retreat, with above-normal rains from early August through early November. The Dust Bowl drought of the 1930s was reversed by a return of wet weather over the early months of 1938, including the seventh-wettest May on record in the Twin Cities.

Indeed, droughts often have no clear beginning or end in the Upper Midwest, and tend to blur in the memory.

Just last fall and winter, the Twin Cities area was in a drought similar to the current one, before being doused in late spring and early summer, when the amount of rain was more than 6.5 inches above normal.

So, is the current drought 16 months old -- interrupted by three wet months -- or not even four months old?

And is this weekend the beginning of its end?

The timing would be good, Miskus said. Almost all rain that hits the ground now gets stored for spring, because plants use so little of it in the cool autumn temperatures. Cooler weather also means little evaporation.

Bill McAuliffe • 612-673-7646