I90 from west to west yellowstone

I'd vote 287. 191 is infamous for the black ice that forms in the shaded spots in the Gallatin Canyon. There is no rhyme or reason to where you might encounter an ice patch even several days after a snow and if you are traveling with a big rig, it can be extremely treacherous. If you are going to be staying in West Yellowstone, best call ahead to see what, if anything, is open. Most parks winterized and put the year to bed several weeks ago.

If you are talking today it isn't a good time to travel. After this storm passes will be ok.

Here's the link to current info on YNP, including closing dates. Dunraven Pass is already closed.
If you're just passing by on 191, its fairly flat, but this time of year, heavy snow can happen any time. Its all about altitude.
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