You can use the police officer's mistake about the weather to impeach his credibiility.  Introduce the weather report into evidence to impeach his credibility.  Since he was wrong about the weather in his report, he may have made other mistakes in the paperwork he filed regarding the facts of your case which if there are mistakes in the paperwork, you could use that to further impeach his credibility.  Impeaching the credibility of a witness will help your case by strengthening your defense at trial.