Temperatures in the 40s this time of year used to be known as the "January thaw," a blessed period when Minnesotans might finally wash their union suits.

But this year, the long johns are still pretty clean.

Mere days after a freakishly warm December -- capped by a record high of 52 the day after Christmas that had some runners out in shorts -- mercury in the Twin Cities is expected to bounce back above 40 degrees Thursday, Friday and Monday. That's about 20 degrees above normal. Even Monday night's low of 11 was three degrees above the normal.

December was the 10th-warmest on record in the Twin Cities; the last six months of 2011 were the second-warmest such period in 138 years of record-keeping.

In the short term, a northward drift of the wall-like jet stream, to run across southern Canada, is allowing southerly warmth to drift northward.

Combined with extremely dry and generally snow-free conditions, it's been tough for skiers, snowmobilers and ice anglers but a benefit for anyone who manages plowing and heating bills. Heating degree days -- a measure of heating demand -- were running more than 20 percent below normal for the season in the Twin Cities through Monday.

Similarly, the natural world has been weighing half-full against half-empty. Without snow cover, browsing animals such as deer have been finding plenty to eat, while small mammals and rodents have been scrambling to hide from predators, said Matt Schuth, a naturalist at the Minnesota Landscape Arboretum. University of Minnesota Extension entomologist Jeff Hahn said the warm weather might help insects survive the winter, although that would include pests such as the emerald ash borer and the Japanese beetle.

Shawn Bernick, director of research and technical support at Rainbow Treecare, said winter warmth can damage some species, but he's more concerned about the possible effects of the ongoing drought and lack of snow. Many trees went into winter short on moisture, but the current lack of snow cover could lead to root damage as well, Bernick said.

Snow and cold, if they ever arrive, could change that picture in several ways. Snow would insulate and provide much-needed moisture for plants and hiding places for small animals. Although some stubborn herbs and non-native plants have continued to bloom and some bulbs in warm spots have sent leaves spiking upward, cold would prevent too much untimely blooming. But intense cold without snow could generate deep frost that could damage roots on trees, shrubs and other perennials. On the third hand, deep cold could also kill off overwintering insect pests.

"It's hard to judge exactly what this combination of things is going to bring," Hahn said.

For what it's worth, the National Climate Prediction Center, which had been signaling strong chances of a colder- and wetter-than-normal December-January-February for Minnesota, is now suggesting the first half of January will continue to be warmer but wetter than normal, which could mean snow or rain.

Staff Writer Mary Jane Smetanka contributed to this report.

Bill McAuliffe • 612-673-7646