It s a great question - one that any mfr can answer, if no one here is able to do so.

I am not a huge proponent of attic fans after reading that unless you have an airtight ceiling (no can lights, or can lights that are well sealed) you may be asking for troubles. The air your pulling out of the attic could be replaced by warm conditioned air from the house, which is a recipe for ice dams - a threat that you are trying to mitigate. But it is your call. :)

Most attics can use the reliable "warm air rises" effect for ventilation in both summer and winter. You need to have lots of high level vents (ridge or gable or nearly flush bug-eye/eyebrow type vents) and low openings (through a soffit or other air passages unblocked by insulation) for it to work effectively. Some areas have a building code requirement for how many square inches of high and low level openings per so many square feet of roof, I know mine does. Check with your city or county's building permit office to find out what's recommended.

Powered fans shouldn't be required and, as bpchill says, if too strong they can cause negative pressure that can suck air out of the house.

Thanks to everyone. Clearly, forced venting an attic is a recipe for more trouble, if there's not enough free air opening to avoid depressurizing the attic. And I'm not thrilled to go off in this direction, but we have a ca. 1700 house with an attic with two layers of pine flooring, with blown fiberglass installed 20+ years ago, but most likely with many areas for air leaks from the heated space. There are no soffit vents, but a 40"x22" louvered vent in each gable, and I opened a double-hung window in the gable opposite the fan, so I feel there is plenty of air intake.

Last year we had air sealing done in the eaves and around both chimneys, but after the 'blizzard' here 3 weeks ago, ice buildup started right away at roof edge. The fan went in 2 days after the storm, and __arrested__ the ice buildup. As I drive around town, now after nearly SIX FEET of snow in 3 weeks, a third to half of the houses have huge icicles off the eaves (6-10 feet long) with 6"-12" thick ice at edge, and a number have the dreaded "brown icicles" emerging from clapboards or shingles. Our longest icicles in contrast are maybe 18". And many homeowners with soffit vents + ridge vent, have the situation where the ridge vent essentially does nothing, as it is covered with at least a foot of snow.

A roof-mounted ventilator would let me open both gable vents to provide more cold air intake, but my original concern (OP) was if operating in continuously would prevent snow blockage.

I had a similar problem. I was loosing too much heat to the attic and that caused premature melting on the roof giving ice dams and huge icicles. A couple of years ago when I re-shingled the roof, I had double wide, anti-ice dams sheets installed. That helped tp control wetting of the walls, but the ice dam problem was still there. The main problem was too much heat in the attic. Last fall, I brought in an insulation contractor. He found that inside air was escaping to the attic from the tops of the interior walls. A powder room fan was vented into the attic. He sealed the wall tops, installed a roof vent for the powder room fan, installed insulation dams were the roof meet the supporting walls (to keep the attic insulation from contacting the roof and to keep a clear path from the soffet vent to the attic. After the prep work was done, fiberglass insulation was blown in to a depth of 2 ft to raise the insulation to R50. Four more roof vents were added plus 4 more soffet vents. This winter, there has been no ice dams and no icicles. The snow either blows off or does not melt until the air temperature is high enough to clear the rain gutters; End of problem one. I have to wait to see what happens for problem 2. Problem 2 is rain on a roof full of snow. That happens at this location about once every 5 years.

My fuel use has reduced. I used "therms per degree-day" to make comparisons. I am 10% less than last year and 13% less than 3 years ago. This is a rough comparison because it does not factor in wind. I have a two story house and wind does affect my heat loss. However, the reduction is large enough to say that good attic insulation is important.