Weeping dissectums will always be at risk to damage from snowloads by virtue of their low, spreading growth habit. The dense branch and twig system that makes these trees so attractive in leaf and that seems to collect and hold the leaves when they fall also collects snow when it falls. Other than constructing some sort of overhead covering, there is really no way to protect them aside from being very attentive to the conditions and gently shaking the snow off the tree as it accumulates.

Snow is not so common here and never in such accumulations as you experience but when it falls here, it is very wet and very heavy and will easily damage plants just by its weight if allowed to remain in place. I am out in my garden regularly during any snowfall shaking the snow off vulnerable plants.

I have no solution for you...But I would say building a cover(s)..as GG in passing stated ( an likely not seriously)..... is not a good or practical idea unless you really know what you are doing and have lots of time and money to spend...as far as heavy covering, sturdiness and anchored properly .... one or two branches occasionally damaged is perferablre to the whole tree being damaged when it comes crashing down...although I really don't think it is a practicle idea anyway especially if you need three ;>00000. Putting up snow fence around the trees may be unsightly but if it's a drifting area being made worse by wind blown snow that may be a big help. This is another reason for placing Jm's in the most protected area of your yard that you may have... David

I prune with the idea that a heavy snowload will happen. In the case of the weeping varieties, I make the umbrella thinner by taking out the twiggy under branches so snow won't accumulate as much. I never shorten branches. If a branch is getting superceded by the branches above it, off it goes. The earlier I see this happening, the better.

As for upright Japanese Maples, I still prune for snow load. That means taking out any wide growing branch with a lot of side branches. I also take out some smaller interior branches that will eventually be crowded out anyway. It's easy to see the losers. I leave the ones that will give me good support for the eventual canopy. All others go as the tree gets larger. It's a gradual process because I don't want the tree trunk exposed to too much sun when young.

Most nurseries, and customers, prune them to look like nice little full grown trees rather than pruning them to be well grown big trees with branches than can eventually handle a heavy snow load. I prune for good structure to handle the elements rather than play catch-up to disasters.
Most of the methods mentioned above just work for one year.....maybe. It will be harder the next, and so on.

When I said methods mentioned above, I meant previous posts.
I reread it and even I got confused. ;-)