This issue with XM/Sirius comes up every summer, especially with all the thunderstorm data the company transmits - which slows down the traffic. First, thunderstorms create an electronic storm when it discharges so much static electricity. Result: possible loss of updates (5 minute refresh).
Second, there have been some solar storms lately, and those do affect the satellites that XM/Sirius use because they're in outer space, not protected by the ionosphere. NOAA actually issues "solar weather" alerts on their website:
Lastly, like any company, there are outages. They have to gather weather, radar, and lightning data from ground sources and beam it up to the satellites in packets, which is hosed back down to Earth so you can use it if you have the right login/activation account. When the humongous "derecho" thunderstorm hit the Washington area last week, it knocked out the internet hub in Reston, Virginia.
Otherwise the stuff works OK. Nothing substitutes for using your eyes to see a storm coming, and yes radar helps during times of low visibility.