ラジオで天気予報を聞いているとよく"Today's guaranteed high is ○○ degree." という台詞が出てきますが、この"today's guaranteed high"とは何のことですか？ 日本語ではなんというのでしょうか?

天気予報を賭けの対象にしたギャンブルじゃないですかねえ。
そんなシステムが日本にあるとも思えませんが。

誰でも参加できて、最高気温がその値から３度以上違った場合には、
参加者の中の一人が賞金をもらえるようです。

詳細はこちらをご覧ください。
http://en.wikipedia.org/wiki/CFTR_(AM)#Weather_guarantee

A "weather guarantee" is a game played by some radio stations. How it typically works is that a jackpot is seeded with some initial money, and then for each day when temperature prediction is correct, more money is added. If the prediction is incorrect, then a winning person is chosen from among the contestants, and receives the money. The jackpot is then seeded again with the initial amount.

"Correct" usually means within 3 degrees or so. If the weather person says "tomorrow's guaranteed high is 26 degrees" that means that if the daily high temperature is less than 23 degrees or greater than 29, the prediction is bad and someone wins.

Of course the details depend on the radio station.

Res1.2さん、ありがとうございます！ ありがとうございます！ これでスッキリしました。