Really? Define "waste of money".
Sodding post edit function timed out on me so here's the rest of my response:
Joking aside, I live in the wettest town in Australia so I know a little bit about rainfall. We average over 4000mm per year with a record peak of over 7000mm. We can sometimes get over 500mm in 24 hours. 500mm is roughly London's annual rainfall, if you want a bit of perspective.
I get my weather info from the BoM Web site and a few natural indicators. When the farmer next door mows his lawn during the wet season it means it will rain within two hours. If the tree frogs start croaking during the day it means it will rain within 15 minutes. And so on.
Weather is about tried and tested observation, not apps with sensors that don't like the rain.