ST. LOUIS — The nation’s worst drought in decades leveled off last week after a two-week spell in which overall conditions slightly worsened, but a persistent lack of rain in the Midwest and Plains has barge operators fearing Mississippi River traffic could soon slow to a crawl or even stop altogether.

The weekly U.S. Drought Monitor report released Thursday shows that more than 62 percent of the contiguous U.S. remains in some form of drought as of Tuesday. One-fifth of the lower 48 states still is in extreme or exceptional drought — the two worst classifications.

Among the hardest hit, all of South Dakota, Iowa, Nebraska, Kansas and Oklahoma are listed as being in some form of drought, with more than 90 percent of Nebraska and Oklahoma gripped by extreme or exceptional drought.

Northern farmers who depend on snowpack to moisten the soil before spring planting haven’t had much luck, as Midwest cities in Nebraska, Iowa and elsewhere are recording less snow than ever by this time of year. Chicago and Milwaukee haven’t had measurable snow at all this season, although that could change this weekend, according to the National Weather Service.

While heavy precipitation pummeled portions of the West over the past week, much of the Midwest got left out, fanning worries that an already low Mississippi River levels could drop to a point where barge traffic along the vital commerce corridor will soon be restricted or shut down.

Months of drought have left the Mississippi at exceptionally low levels, a problem worsened last month when the U.S. Army Corps of Engineers reduced the outflow from an upper Missouri River dam, lessening the amount of water that drains into the Mississippi where the rivers converge near St. Louis.

The National Weather Service revised its forecast for the river Wednesday, showing the water level isn’t dropping as quickly as feared. The Coast Guard has said further restrictions on barge traffic — most notably in a 180-mile stretch between St. Louis and Cairo, Ill. — are likely if the river at St. Louis drops by another four feet — a level the weather service believes may come about Dec. 29.

Thursday’s drought update wasn’t encouraging, noting that forecasts for the rest of this week call for a storm system to bring at least an inch of rain to a broad area from parts of Mississippi and Arkansas near the Mississippi River — south of the river stretch were the rainfall is most needed.

Rainfall measuring two to four inches is possible near where the Mississippi and Ohio rivers converge at Cairo, Rich Tinker of the National Oceanic and Atmospheric Administration’s Climate Prediction Center wrote in Thursday’s update.

“Mississippi River flow continued to decline, and it may be necessary to close parts of the river to barge and shipping traffic at some point,” Tinker wrote, noting the U.S. Army Corps of Engineers’ scramble to dredge in a bid to keep as much of the river navigable for as long as possible.

For much of next week, Tinker said, below-normal precipitation is expected across roughly the southwestern quarter of the continental U.S. and in the central Great Plains, with warmer-than-normal conditions holding sway across the northeastern quarter of the contiguous 48 states and the south-central Plains.