One way is just to set up an input object to allow the zip to be entered and then access the weather for that zip from a web site, scrape the forecast from the web site and display in text and image objects.
The demo app from the Ciplets development environment (www.ciplets.com) does this from a UK site, it would be straightforward to adapt this to get the info from a US weather site. If you email Ciplet support they'll create you a simple example Ciplet that does just that!
