Home prices are getting out of control in these 18 American cities

Home prices are rising faster than Zillow expects.

The real-estate company forecast that prices would rise 2% year-on-year from last April, but they jumped nearly 5%, according to its home value index.

The story is the same: strong demand from homebuyers coupled with tight inventories is pushing up prices. That's caused bidding wars in some hotbeds like San Francisco.

"The struggle will continue for home shoppers this summer," Zillow chief economist Svenja Gudell said in a report Tuesday.

"In many markets, those looking to buy a home in the bottom or middle of the market will need to be prepared for bidding wars and homes selling for over the asking price."

Using Zillow's data, we selected the 18 markets where the year-on-year home-value change was higher than the national print of 4.9%, and ranked them in ascending order. We also included the percent inventory change in bottom- and middle-tier homes, where supply is tightest, as well as the change for condos.

Here they are: