It's really only a horseshoe.
When the SE Pacific is warm (which is "normal"), the Montana weather is “normal,” which is to say it has the patterns to which we are accustomed for more recent decades. We don’t know why the SE Pacific is sometimes warm, sometimes cool, an oscillation that’s only partly predictable, nor do we know what feeds its temperature. We do know that it is part of the Ring of Fire, which is the circle around the edge of the Pacific Ocean.
This ocean is newer than the Atlantic and its edge is probably the edge of the plate tectonic that defines it from underneath, except that the middle of the sea is an upwelling which pushes back on the land masses riding on adjacent continents, even parting land that was once connected. This is why peonies, grizzly bears and American Indian DNA are on both coasts, Asian and American. It is also why Valier, Montana, is an irrigation town.
From National Geographic. Links are still there.
“The Ring of Fire is the result of plate tectonics. Tectonic plates are huge slabs of the Earth’s crust, which fit together like pieces of a puzzle. The plates are not fixed but are constantly moving atop a layer of solid and molten rock called the mantle. Sometimes these plates collide, move apart, or slide next to each other. Most tectonic activity in the Ring of Fire occurs in these geologically active zones.
“A convergent plate boundary is formed by tectonic plates crashing into each other. Convergent boundaries are often subduction zones, where the heavier plate slips under the lighter plate, creating a deep trench. This subduction changes the dense mantle material into buoyant magma, which rises through the crust to the Earth’s surface. Over millions of years, the rising magma creates a series of active volcanoes known as a volcanic arc.”
When the west coast of the Americas is struck hard by tectonic plates, the continents riding on top are jammed together. If the pressure is hard enough, breaks between may be marked by volcanic mountains like the Cascades to relieve the pressure underneath. Or, further inland, the land may rumple up into cordillera like the broad Rockies. On the east side of the rumples, the major effect -- mostly in the deposition of water taken up in the steady band of moving wind that crosses the Pacific -- travels inland, and has moisture scraped from underneath in order to lighten the current enough to rise over mountains.
On the west side of the Rocky Mountain cordillera the land is wet: trees, streams, and at bottom Flathead lake. In the days of the melting glaciers, ten thousand years ago, so much water accumulated in this valley that the whole space was like a huge vat, but then the south toe found a way out and broke through, circling south and then cutting towards the west. Next as precursor of the Snake River and the Columbia River, the water cut through the volcanic rock of the Cascades, creating a fabulous gorge and giving rise to the epic myth and opera called “The Bridge of the Gods.”
On the east side of the Rockies the land was too dry for trees but the mountains themselves acted as a great frozen body of water which melted gradually every spring and flowed out in a thousand small streams managed by beavers until it was absorbed by grass and the remnants drained out the several massive coulees that remained ten thousands years after the melting of the huge northern continental glaciers. Eventually the last of the moving water joined the Missouri and then Mississippi Rivers and drained south to the Gulf of Mexico.
It may be that this Gulf, a mammoth scallop off the bottom of the mid-continent, was created by the impact of an unimaginable asteroid. Something cracked the tectonic plate under the northern continent and left the Western half on a bit of a long tilt, so that the water would run along the Mississippi. Once there was a shallow sea there, but it all ran out that river crack.
(Wikipedia) “The Western Interior Seaway (also called the Cretaceous Seaway, the Niobraran Sea, the North American Inland Sea, and the Western Interior Sea) was a large inland sea that existed during the mid- to late Cretaceous period as well as the very early Paleogene, splitting the continent of North America into two landmasses, Laramidia to the west and Appalachia to the east. The ancient sea stretched from the Gulf of Mexico and through the middle of the modern-day countries of the United States and Canada, meeting with the Arctic Ocean to the north. At its largest, it was 2,500 feet (760 m) deep, 600 miles (970 km) wide and over 2,000 miles (3,200 km) long.
“The Seaway was created as the Farallon tectonic plate subducted under the North American Plate during the Cretaceous. As plate convergence proceeded, the younger and more buoyant lithosphere of the Farallon Plate subducted at a shallow angle, in what is known as a "flat slab". This shallowly subducting slab exerted traction on the base of the lithosphere, pulling it down and producing dynamic topography at the surface that caused the opening of the Western Interior Seaway. This depression and the high eustatic sea levels existing during the Cretaceous allowed waters from the Arctic Ocean in the north and the Gulf of Mexico in the south to meet and flood the central lowlands, forming a sea that transgressed (grew) and regressed (receded) over the course of the Cretaceous.”
The Cretaceous period is of great significance to humans. This is when many fossils were deposited, including the “buffalo stones” or iniskim which formed when the sea existed: they are the interior of tentacled creatures that grew in series, the pattern like a row of bison. This is also when much oil was created, as well as coal from the peaty swamps. But it underlay the later grass which was the carpet on which the bison fed, themselves the food of the people.
Under and through the whole remains the angle of the land, the key to irrigation, for it is this that makes ice-held water provide melting that sustains the system through the year, renewing every winter. This is the engine that fills Swift Dam and sustains a system of human-dug canals that water miles and miles of grain that feed the world, replacing grass.