Worldbuilding: mountains and weather

One of the advantages of building a fantasy world from scratch is that you can tailor the geography to fit your aesthetics. Want a dramatic coastline with natural sea arches like the Oregon coast? No problem. Amazing river valleys like the Three Gorges region of China? Drop it right in.

I want my carefully designed landscape to make some sort of real-world sense, though.

In the fantasy world I've been overhauling, geography underwent major changes. I had some definite requirements: relatively secluded but large enough to sustain a small kingdom, and a New England-ish climate. Why New England? Because I grew up there and I still love it. After sketching a lot of maps, I settled on a wide valley flanked by high mountain ranges. At the southern end, the mountains funnel it down to a relatively narrow and steep outlet, through which the major river flows.

Champlain Valley as seen from New York (looking eastward.)
Imagine those mountains rugged and snow-capped year round,
that's what I want for this story

The best way to start, IMO, is to look at similar areas in the real world. In my case, the closest thing overall is the northern end of the Great Appalachian Valley -- the area around Lake Champlain in Vermont. But I want the valley to be much wider than that and the mountains much higher than the Appalachians.

Consequences of higher mountains

Setting aside how the mountains formed in the first place -- though it's important and I'll get to it later -- one of the immediate effects of high mountains is the rain shadow on their leeward side. Simply put, when clouds run into mountains, they have to rise to go over them. That pushes them into colder air and their moisture will fall as rain or snow as they rise. When they get to the other side, there's nothing left. Some of the most intense deserts in the world are in rain shadows.

Big question: which side of my mountains is "leeward?"

Weather

In New England, weather generally moves from west to east and sometimes big storms roll up along the coast from the south. So my valley will be leeward of its western mountains, and could be creating a rain shadow on the neighbors with its eastern mountains.

I don't want my mountain valley to be a cold desert, so I need to get some weather in from another direction. So I installed a coastline to the south, outside the valley, like what's south of New England. My little kingdom will still get nor'easters and the occasional tropical storm in the summer.

I also added a northern coastline for more wet air, though that's going to be some cold, wet air. I don't know if my characters will ever get up to that northern coast, but I would guess it will be something like the Hudson Bay coast.

Thumbnail sketch of the weather in my valley: generally dry air comes over the mountains from the west and runs into cold, wet air from the northern coast, carrying it into the eastern mountains and forcing it to drop its precipitation on the way east. Major storms bull their way up from the south -- blizzards about once a winter, the remnants of a tropical storm every other year or so. Hurricane maybe once a decade, and would be a real disaster.

Rivers

The rain on the eastern mountains will result in most of the rivers being on the eastern side of the valley. They'll drain down to the lowest point and then flow either south (toward the mouth of the valley) or north (toward the coast.) I bet there's a lake at that turning point where the water decides to go either north or south.

For reference on the southbound river, I'll use the river that actually does drain most of New England -- the Connecticut. I'll be adding a series of rapids at the mouth of the valley to reduce upriver traffic (seclusion was a prime criteria in building this) and gauging from the size of the Connecticut where I used to drive over it from New Hampshire to Vermont (near Brattleboro), those could be some spectacular rapids.

What kind of geography did you add to your world because you think it's gorgeous? What did you add in order to shape the story? And how did you explain it, if at all?