Minnesota’s relentless winter is poised to reassert itself Thursday afternoon into Friday, bringing about 6 inches of snow to the Twin Cities metro area and as much as a foot to Duluth.

Meanwhile, continuing snow, rain and cold weather have combined to heighten the flood threats to northwest Minnesota. National Weather Service officials said Wednesday that the chance of a record crest on the Red River at Fargo-Moorhead in the next few weeks is now about 40 percent, up from 15 percent four weeks ago.

Fargo, which has more flood-prone land than its Minnesota neighbor, reopened its volunteer sandbag-filling operation Thursday, with a goal of adding 500,000 sandbags to the 1.8 million it has on hand.

Metro drivers, meantime, will face more winter road challenges Friday morning.

Rain on Thursday is expected to turn to all snow by the afternoon, then come down at the rate of 1 to 2 inches per hour Thursday night — hard enough to accumulate even on warm roads, National Weather Service meteorologist Bill Berghoff said.

Friday traffic, however, is usually lighter than that on other days of the workweek, and the snow should not hang around long with highs in the mid-30s, said T.K. Kramascz, spokesman for the Minnesota Department of Transportation’s metro district.

The amount forecast for the Twin Cities would push this April into the five snowiest. Duluth, meanwhile, is enduring its snowiest February-through-April on record and had 17 inches of snow on the ground through Tuesday.

International Falls residents are trudging through 21 inches, the most at this late date, assistant state climatologist Pete Boulay said.

While April snows, even deep ones, are not extraordinary across Minnesota, nature watchers say the combination of snow and cold appears to have set back some bird ­arrivals by about two weeks.

Robins have clustering in unusual numbers — “piled up knee deep” recently in southeast Minnesota, one observer said.

But Kirk Mona, outreach coordinator for the Warner Nature Center in Marine on St. Croix, said that may simply be a sort of train-station effect, with robins that come south to Minnesota for the winter unable or unwilling to return north yet, while those coming up from the South have gone as far as they’re able. Other species that respond more to light changes than temperature have arrived on time but may be struggling to find food, Mona added.

Pete Harris, naturalist at Wolf Ridge Nature Center near Isabella, said the first day when the temperature remains above freezing is usually March 18, but it hasn’t happened yet this year. Likewise, April 13 usually brings the first mosquito sighting; this year, none yet.

National Weather Service hydrologist Greg Gust said the continuing cold has cast the agency’s flood-predicting models into “uncharted territory,” with wintry conditions unprecedented for this late.

While rivers have usually flooded by now, they remain frozen across the Red River Valley, and the late date has increased the risk of a sudden warm-up and thaw across the region, Gust said. The Red could crest at Fargo-Moorhead the week after next, one of its latest spring melt floods ever.