Ludwig, the problem with RH outside is it changes with every degree of temperature change.
If the weather guy sez "Today's high temp will be 65 degrees with an RH of 70%" the minute the temperature drops to say 60 degrees or goes over like to say 70 (well I know it's a stretch to accuse the weather guys of ever being wrong BUT...) the RH changes accordingly. He's giving you a predicted RH based on what his predicted temperature will be. So, if he's wrong on the predicted temp., his predicted RH will be off as well.
If you want ot bring your hygro outside along with one of those wetbulb set ups and the corresponding charts (which I think is the most accurate way to determine the RH outside) then okay but I wouldn't rely on the weatherman.