Why does the rear end slide in the snow and no other time?

Its a basic answer. Snow's molecular properties are varied many different directions Depending on the snows condition. It the Snow is whet, Its molecules are more malleable hence why it is the ideal snow for snowmen and snowball fights. When Snow is "dry" its molecular setup is more structured so it doesn't want to change. So when driving over snow the snow is like a layer of separate concrete in between your tires and the asphalt, and since snow is slippery, the car isn't moving always, sometimes it is but mostly it is the snow that is moving on the asphalt that makes you back end slip out because you front wheels have control, you back wheels are just sitting their. All of this is why trucks but chains on their wheels. Since a metal chain's molecular setup is more solid than snow, the chain can break through the snow and give you more traction because you are touch the actual asphalt instead of the slippery, no friction snow. So Over all the basic answer would be snow doesn't give on as much friction as asphalt.

to be honest that guys answer was not basic at all and really is not the problem, the reason your geting the rear end wandering on you is because while you have 4wd there is still more weight at the fron of the car than the back and that is what gives the car better traction up front, now it may be worse if you have an automatic transmission because Subaru automatics instead of having a split of torque 50 percent to the front and 50 percent to the rear wheels like Subaru's manual transmissions, the auto's use a 90 percent front 10 percent rear torque split causeing it to drive more like a FWD car than a full time AWD car. My suggestion in either case is to get some good all season or snow tire if you have only a highway tire on now, and don't get on the gas too much and make sure your E-brake is not locking at all.

As Nick previously answered the automatic Outbacks are even more apt to slide its rear on corners when driving in the snow. I did notice this occurance happening to me in my previous truck a 97 Toyota Tacoma 4x4 and to counteract this problem I added weights in the back of the Tacoma, as with the Outback I just placed an 80lb. bag of sand on the cargo area and it seem to help providing you are not driving the car like a rally race, and yeah a good set of snows like General Altimax Artic which I am currently using helps.

I agree with jtablan I also have the General Altimax Artic tires I live in New England and we get a lot of snow these tires are great

I suggest you look up Ghostwalking. It's a somewhat well know issue with 05 to 09 Outbacks in North America, probably only in the US, and stems from multiple minor issues that add up to the major handling problem you and others experience. No one has been able to reliably force it to happen in a testing area so it's not well researched, but general consensus is it's tends to mostly result from Subaru raising the ride height 2 inches to get past fuel regulation, which led to the rear suspension being out of it's normally designed position and extremely toeing the rear wheels when the rear is compressed. The lack of ability to adjust camber and toe separately also seems to play a role as many have had luck in fixing the issue just adding a camber kit to the rear and having the alignment redone, sometimes adding an extra 200lbs to simulate having cargo. Some also have added stiffer springs to fight the sag when loaded. Others have also seemed to get normal handling by just inserting a fuse in the fwd slot and making the car run in FWD mode. I've also seen a few say the problem could also be uneven tires that haven't been rotated and the alignment being out of spec, but we can't be sure as noone has been able to reliably test it. And as an fyi, Subaru does know about it, but all they could do is recommend the rear tires be lowered by a few psi and the rear alignment be altered as per a TSB or recall. Beyond that we are on our own. If you need more info, just check out a Subaru forum for the Outback. There are many who are in the same boat as you.

My 2015 OB with new Michelin Xice ghost walks all over the place on icy roads. Fresh snow or slush it is a dream but on black ice or highway packed snow it is a sleigh ride. Just did I 80 east and west over Thanksgiving and this Subaru is the worst I have ever had. {owned 5 of them} My Honda Pilot or Ford Explorer or GMC Denali kick the OB butt. I am going to try alignment and soft rear tires.

Black ice is super slippery. Try studded tires.

2015 Outback Ghost walking on black ice worse than any vehicle I have driven except maybe 2011 Avalon. I am convinced it is the alignment but I can not find a solution as I search online. It is irritating to see comments from people who have no idea how serious this issue is suggest things like tires. Subaru needs to get this figured out. It feels like it is toed in and the side with the most traction at any given second jerks the vehicle in that direction. It could be just the rear but it feels to me like the front and rear.

On black ice tires are all important. So is slowing WAY down.

WaveSailor I have experience the same thing. The rear end on my '16 Outback walks all over the place on the highway now that snow is around and cold temps are consistent. Granted it is on all seasons, but this is plain rediculous. My mk7 GTI with all seasons drives the same roads at higher speeds with literally NONE of the problems with the rear end sliding around. My friend has the same problem with her '15 Outback. She's been to a Subaru dealership twice, gotten new tires and had an alignment, still having the problem. Subaru needs to figure out a solution ASAP!

I have a 2015 Outback that has the same problem as described above. I've had the rear control arms replaced with adjustable ones from Moog so a proper alignment could be done. It also has brand new Michelin all season tires. It's a scary ride in the packed down snow conditions as described above. I'm close to trading it in for something else. My wife has a 2010 with none of the above issues. Very upsetting to me that there's no good fix available.

This might help. https://www.cargurus.com/Cars/Discussion-d380_ds990018

Agree - super annoying to have people suggest tires or sandbags. 2012 OB with Blizzaks (1 year of wear). 260 lbs of dog in the back. 300 lbs of people in the front. Fairbanks, AK at 10 to30 deg F (above zero). 35 mph in a 55 zone. Back end swishing around like a poodle skirt at a sock hop. Every other vehicle on the road passing me without problem. I've driven same car, same tires, same road, same weather, same load for 8 years. My husband sometimes fishtails on braking but I always accuse him of braking too hard. Now that it's suddenly happening to me, daily, I question my previous accusations. Looking forward to below 0. Road gets sticky. Hopefully that will help. Then I'm getting a new car.

If you run your Blizzaks during the summer you may have worn them out. Fairbanks gets too warm to run winter tires in the summer.