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In this alternate scenario, only the Rocky Mountains and the Appalachians stand firm in the United States. Even so, some differences apply.enter image description here

The Appalachian Mountains (right) have grown to as tall as the Sierra Nevada--14,505 feet above sea level. The Rocky Mountains (left) have grown to the height of Denali, 20,310 feet above sea level. Marked in orange is a plateau varying in elevation above sea level from 3300 to 16,000 feet.

For the sake of scope, let's just narrow down to the Midwest, dominated by the prairies of the Great Plains. With a closer Rockies, both ranges being dramatically taller and a plateau blocking off the warm, dry air crucial for the creation of Tornado Alley, would the Midwest still be prairie?

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Two of the primary airflows affecting the weather of the midwestern plains are cold air masses coming down from the arctic in the winter, causing periodic blizzards in otherwise subtropical Central Texas, and warm wet air masses moving up from the Gulf along the Mississippi River, bringing heavy summer thunderstorms to the upper plains.

Minneapolis is at the same latitude as Astana, Kazakhstan, but Kazakhstan has the mountains of Afghanistan and Iran between it and the warm wet air of the Arabian Sea. As a result, while both places are cold in the winter (Minneapolis -14C January lows, Astana -18C) and warm in summer (Minneapolis 29C July high, Astana 27C), Minneapolis is much wetter (Minneapolis 319mm rain in summer, 777 year round; Astana 118mm rain in summer, 320mm year round)

So the two major systems are not affected by your changes, so the Midwest would be pretty much the same as they are. Most Atlantic storm systems track the coast and don't cross the Appalachians to bring rain to the Ohio valley anyways, so that isn't changed much either.

There is one thing that would significantly change though. The high mountains and plateaus of the Rockies would strengthen the North American Monsoon. Especially if the highest parts of the plateau were in Arizona/New Mexico/Chihuahua, you will see a massive low pressure zone develop in summer much like the Tibetan plateau that drives the Indian monsoon. Now you sill aren't as high as the Himalayas or as big as the Tibetan Plateau, so you won't get quite that much rain, but it would be closer.

I suspect you would see westward winds from the Gulf flowing across Texas to NM/AZ in the summer months, Jun-Sep. To imagine the climate in those months, look farther south in the tropics to Veracruz, Mexico. Texas would probably start seeing 4 summer months of 100% humidity, and heavy rainfall...a Houston year in only 4 months, plus normal rainfall patterns the rest of the year.

So Texas is now a rainforest? Not quite. You still have yearly freezes since nothing is blocking canadian air coming south. So tropical trees can't survive. Again we can look to Mexico for a solution. The mountains above Veracruz get monsoon rains but cold winters due to altitude. As a result they have a (mostly) unique pine-oak forest.

As for changes in fauna, I'm sure Black bears and Peccaries would be more common; both oak and pine provide great food seeds. White-tail deer and Elk would be there too, along with Bison along river floodplains. And with a new forest covering Texas, the America's top forest predator would be common too, the Jaguar.

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  • $\begingroup$ I didn't ask this at the risk of being dismissed as "too broad" but will these changes also put northeastern Nebraska (42.4649° N, 96.4131° W) out of the Tornado Alley zone? $\endgroup$ – JohnWDailey Jul 29 '16 at 17:46
  • $\begingroup$ This is all a guess, but...Thunderstorms are one of the primary drivers of tornadoes, and they are still coming up from the Gulf as before. Warm dry air will be more prevalent with higher mountains due to the way that katabatic winds work. The Canadian shield taiga/tundra are still there and still cold. I think you still have all the ingredients for tornadoes. Tornado alley is not much changed. $\endgroup$ – kingledion Jul 29 '16 at 19:29
  • $\begingroup$ Then what WILL I do to drive Tornado Alley off my neighborhood? $\endgroup$ – JohnWDailey Jul 29 '16 at 20:40
  • $\begingroup$ Turn the Gulf of Mexico into land all the way out to the Bahamas and Hispanola. In that case, the whole gulf along with Kansas to North Mexico would be a giant sandy desert (like the Sahara, also huge and at the same latitude, roughly). The upper Midwest between the Rockies and Great Lakes would be a cold dry steppe with no tornadoes, the Ohio Valley would be a dry oak savanna and the river would dump its water into a giant desert lake like the Aral Sea. The Florida coast and bahamas, where the desert met the sea would be a wet/dry subtropical savannah like Maputo, Mozambique. $\endgroup$ – kingledion Jul 29 '16 at 21:07
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    $\begingroup$ Because I am Nebraskan. $\endgroup$ – JohnWDailey Jul 29 '16 at 21:30

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