I'll attempt to answer some of your points although I'm new to the hobby myself. I have 14 orchids of various types. Is your orchid a phalaenopsis? I think the northwest exposure is probably OK because these orchids don't need particularly bright light. I've read that the oldest leaves do turn yellow and eventually fall off so I wouldn't worry too much about that. Phals can be watered more than other types because they don't have pseudobulbs to store water like other orchids. You might want to give fertilizer in a weak dose (1/4 to 1/2 the normal amount) each time you water. I believe buds or flowers dropping might be caused by a cold draft. Hope I haven't told you anything completely wrong. Good luck!

So here's my take. There could be something wrong or it could be just a freak occurrence. Like Andrea mentioned this could have been caused by a draft, or the opposite by being overexposed to heat (not sure what kind of heating you have in your house). If you fertilized recently that could also be the culprit, I've heard of early flower demise & bud blast from fertilizing during bloom, however I've never seen it myself. I don't know how much fertilizer you give the orchid when you water, but don't forget that orchids really do only need a minuscule amount.

That said, there's also a chance that your orchid is slowly going downhill. You say that nothing has really changed over the course of the year, but you have to take into account the changing levels of light especially in winter. To give us a better idea of growing conditions, how close is the orchid to the window? How much light does the window really get? For reference, I grow my phals in a southern window in the winter. I imagine that in zone 4 in a northwest window your phal gets about 1 hr of sunlight per day, if that? That's probably not enough light to sustain 20-30 blooms for an extended period of time. There's also a chance that the medium that the orchid is growing in is breaking down (I assume you have never re-potted the orchid) and suffocating the roots that are inside. Once your plant is done blooming, it would probably recommend re-potting it, unless you already did so when you bought it.

Fortunately, they days are getting longer and your plant should be getting more and more light. Overall it sounds like you're on the right track, so keep up the great work. If possible, I would consider moving the phal to a brighter window next winter so that it gets a little more light, that way it will be able to keep up it's wonderful display.
Best of luck,
~Jem

Dittoes to what Jem has said. Here's my experience: whenever a blooming phal starts dropping flowers and buds, it is not getting enough water. As Jem said, this can happen because the roots have started to go downhill (poor roots = plant not getting enough water, even if medium is sopping wet).

You need to get a look at the roots to find out what the problem is. Put it back into fresh medium in a pot just barely big enough to contain the roots.

Again, as Jem said, for best growth and blooming it needs more light. Phals are "low light" because unlike sunflowers they will shrivel up outdoors at high noon in summer, not because they love to grow in the dark. A northwest window probably has the bare minimum of light needed, but you have to adjust water and fertilizer down to balance the lower light.

Thanks for the advice. It is very close to a large window where I also grow an african violet with great success. I think your suggestions about the roots may be the problem. I have never repotted it. Should I do it now because literally half the buds have now turned yellow. Also one of the stems has turned light brown. When it is finished flowering, should I cut all the stems back before I repot? I really don't want to loose this lovely plant.

Yes, cut the stems when it has finished. It will get a rest and a chance to recoup its strength. Good luck.