I rebooted OS and gathered statistics for another 10 days. After the reboot, weather calls were successful 100% of the time for 2 and a half days (10 calls). After the first call failure, the failure rate was 86%. The longest continuous string of failures was 12.
The return code on the call failures is always BadWeatherData. My limited understanding of this is that the call to the weather server was successful in that the server responded and provided information, but OS could not calculate a weather scale from the data. Either the server is not providing good data or OS is not dealing with the data correctly, but it is not an issue with connectivity to the server.