The plants that do best with low humidity are the ones grown on the West Coast of the US. However, Zone 5 is rather unusual on the west coast.

Most do better in the snow. They will suffer some self pruning in snow, but not too much.

Too much shade will prevent flower bud formation. They love shade but don't bud up well in too much shade. They typically need at least 2 hours of sun a day from mid spring to mid summer to bud up well.

Another factor is summer heat. Typically plants grown on the West Coast are not suitable for areas with a high summer heat factor.

Probably the best will be the old standards (variety, height, color):

Album Elegans, 6 ft., white
America, 5 ft., red
Boule de Neige, 5 ft., white
Calsap, 4 ft., white with pronounced dark red blotch
Catawbiense Album, 6 ft., white
Catawbiense Boursalt, 6 ft., purple
English Roseum, 6 ft., pink
Mist Maiden, 3 ft., pink
Nova Zembla, 5 ft., red
Purple Gem, 2 ft., purple
Roseum Elegans, 6 ft., lavender

I purposely avoided ones that bloom early and can get frost bitten. These all should be readily available in the trade.

Here is a link that might be useful: Rhododendrons suitable for zone 5 with pictures

Thanks so much. You are going to make me look good! This guy has a gorgeous tudor style home in an old established neighborhood, and told me he plans to spend $50K in the next two years on landscaping.

I intend to help him spend it!

Thank you again.

Just wanted to point out that some varieties of rhododendrons are native to The northeastern US. They grow wild in the woods all around here. Plenty of snow, zone 5, and they actually prefer acid soil.

http://plants.usda.gov/java/profile?symbol=RHMA4

Everyone has them around here. They'll grow in the shade (but won't flower as well) and can handle very heavy snows. They get quite large too.