AWS Availability Zones have counter-intuitive IDs

Today, the AWS Service Health Dashboard referred to an Availability Zone in a way I wasn’t familiar with:

3:52 AM PDT We are experiencing instance connectivity issues in a single Availability Zone (euw2-az2) in the EU-WEST-2 Region.

See:

Here “euw2-az2” is the “Zone ID” of the Availability Zone, and the mapping from Zone ID to the more familiar Zone Name is non-obvious. See, for example:

$ aws ec2 describe-availability-zones --region eu-west-2 | jq -r '.AvailabilityZones[] | [ .ZoneName, .ZoneId ] | join(" = ")'
eu-west-2a = euw2-az2
eu-west-2b = euw2-az3
eu-west-2c = euw2-az1

As you can see a,b,c doesn’t map onto 1,2,3.