Well, no one has officially called it, but the AP has finished their tally of the votes, and Kloppenburg has 204 more votes than Prosser. The AP and the other news organizations appear unwilling to call the election, leaving it up to the county canvass boards to determine the official count. The boards and the state have until May 15 to complete and certify the official count, though it shouldn't realistically take that long. We should have official results in a few days. Until then, JoAnne Kloppenburg is the winner, and she has declared victory.
The Prosser campaign said they're preparing for a recount.
Update: (3:15pm) According to the Journal Sentinel, the AP is verifying the numbers from every county and will have a verified and final total later today. That total most likely won't differ much if at all from the state's certified count.