Doubt clearing cache again would help. That's the Android operating system cache.
HTC Weather doesn't maintain its own cache so there's not a separate one to clear.
The usual suspect here is Accuweather, I've seen this complaint on every HTC by someone at some time.
Whenever it's hit me, I've popped over to their website to find their database screwed up again.
They'll hop hemispheres with coordinates, or countries with similar city names, the works.
One of the lower level HTC guys actually managed to confirm the source of the problem and that was the last we heard of that a few years ago.
I used to post "solutions" on this until I formed the personal opinion that mostly all we've ever done is screw around until Accuweather responded properly and then concluded that screwing around fixed something.
Best bet, if they still allow it, is to add your city manually and try to use that for a few days or a week or so, see if it improves.
I went to 3D Clock and Weather by Droid27 on my phone last year. I still get the HTC Clock but I get Google weather instead. That's far more reliable. But this isn't an option for anyone who wants to rock stock Sense.