The upper deck at Kauffman Stadium is one of baseball's largest. Section 420 alone features more than 40 rows of seating and the front offers a wildly different experience than the back.
Covered Seating is Only Guaranteed in the Highest Hy-Vee Level Rows
To guarantee yourself coverage, you'll need to focus on seats in rows RR and above. These are the last ten rows of seats and are anywhere from 20-30 steps up from the concourse entrance. So while you'll be covered, you'll have a bit of a hike to reach your seats.
For covered seating options with less stairs to travel, please see: I need two seats that are fully covered/shaded