Jurgen, great, assume that the PWS is talking to the WU Cloud through the Raspberry Pi successfully. So can you try the following line, note that I cant test this as I don’t have the network equivalent but 8) …
sudo iptables -t nat -A PREROUTING -p tcp -d rtupdate.wunderground.com --dport 80 -j DNAT --to-destination <Weather Service IP:PORT>
The above is for PWS that are using the Rapid Fire (real-time) interface which most are but if that doesn’t work then you may need the one below instead:
sudo iptables -t nat -A PREROUTING -p tcp -d weatherstation.wunderground.com --dport 80 -j DNAT --to-destination <Weather Service IP:PORT>
This should redirect information heading to the wunderground cloud service and send it toward the local Weather Service instead. You can then test whether the data is getting through by hitting the Weather Service endpoint from a browser to see what watering scale is being reported. It might take 5 mins for data to get into the Weather Service depending on how often your PWS provides updates.
If either of these works then you will still need to make the change persist across a reboot by following most of Step 4 using iptables-save and iptables-restore.