From reading Rachio's Weather Service FAQ, it doesn't look like the Aeris service they get weather data from collects from Weatherunderground.
The two workarounds folks have been using are a) the meteobridge device (which doesn't list SKY2 as something it supports), and b) a script that can run on a computer that's always turned on that will crosspost weather data from Weatherunderground over to one of the services that Aeris does collect from.
I think @franz was excited about some changes to the way that Rachio gets weather, but the last I read, it didn't sound like they were going to be incorporated into updates this year.
I ordered a PWS that posts to Weatherunderground. When I get around to installing it, my plan was to run that script that crossposts the weather data on a Raspberry PI computer that I already have on 24/7 doing a few other functions (HomeBridge mostly).