Originally Posted by LarryJ

Most US regional airlines do not fly lower-than-CAT-II approaches. CAT II gets you down to 100' above touchdown with 1,200' visibility. For most US airports, it's is rare to have weather that is below CAT II minimums.

Alaska and Horizon, with so many operations at SEA and PDX, do maintain CAT III capability.

There are many reasons why an Alaska or Horizon aircraft, that normally could fly a CAT III approach, might not be able to do it today. CAT II and CAT III approaches have more requirements on systems and aircraft can operate with some systems inoperative which allow CAT I approaches but not CAT II or CAT III. Alaska actually maintains CAT III capability in their 737 fleet with both a HUD and autoland while most other airlines do either the HUD or Autoland. They do this because they can still maintain CAT III capability even if one of the two systems in temporarily inoperative. This costs them more money but with ANC, SEA, PDX, etc. it is worth it for them.

So, generally speaking, a Horizon flight will be able to land with lower ceilings and visibility than a Skywest, or other regional, flight.