I'm not calculating backiside gains, as I figure I'll get something, just don't know what until I do it. I have the Znshine 450W panels and they rate them up to 25% additional from the back. So, I'm just comparing to what I have now which is standard mono panels at 30 degree tilt.
We have a similar situation to you being in south central WA, in that we get snow and then it may melt before the next snow. But we normally freeze overnight, and if you don't clean the panels off and the weather stays cold, you can't clean the panels because they ice up.
We normally get a 2-6' of snow over the winter. Sometimes it melts other times it just keeps piling up, or freeze thaw with freezing rain, so I like to keep the panels clean, because like you say, one they are cleaned off they can stay clean easier than if they have snow on them from overnight.
I haven't compared the pricing to much between the landscape and portrait, as I'm still trying to figure out what racking to use. I'm more interested in the the 6 months around winter gains than summer, as I'm going to have too much power in the summer no matter what angle or orientation the panels are at. That's why I want to do 45 degrees, as also with SE and SW facing arrays the sun is lower on the horizon, so it makes sense to me to angle them up more to take advantage of the lower angle of the sun in those directions. But maybe it doesn't matter and is just splitting hairs, because it tends to be overcast more then anyway. I also am planning on SE/SW as Hedges suggests, for multiple reasons- I can't parallel 2 strings in the same orientation or I'll go over Isc rating of the inverter, wan't to minimize clipping, want power production spread out over the whole day as I don't have any loads for the peak production and to reduce power that needs to be stored.
I'm not sure about increasing albedo, as I figure I can play with that once the arrays are in, but I want to start off with them installed right to maximize potential gains. I'm planning 3-3.5' off the ground for the front edge, for both light and snow shedding, as my current panels are in the same range and I've had to dig out snow from in front of them when we've had lots of snow.
Pvwatts is accurate on a month by month production basis. I was amazed how close the #'s were to my actual production #'s. I'm most concerned with the winter months, as that's what I'm designing for.
The snow on my 30 degree panels will self shed if above freezing and sunny, but it takes awhile and cuts production substantially with the snow bunching up at the bottom of each panel, just depends on the conditions. And if you happen to go into a cold snap with it bunched up like that, you won't be able to remove the frozen snow/block until it warms up again.
Is it worth it for 45 vs 35 degrees, I don't know either. That's one of the things I'm stuck on at this point. I guess if I'm not getting bifacial specific racking then I'm already making comprimises. Iron Ridge has 45 degrees and the windloads required but not the snowloads required, but I figure the snowload is not as important as I keep the panels clean anyway.