Well, if you haven't been paying close attention, the NWS will be upgrading the WSR-88D radars to dual polarization over the next couple of years. As a result, each radar will essentially be inop for roughly two weeks (12 days). I've made the latest deployment schedule available here. No, you won't likely see a NOTAM either and don't count on FSS to know about it. Back in 2008 I wrote this IFR magazine article that explains this upgrade. For now, it won't have much of an effect on the NEXRAD images we can get online.
For XM-delivered satellite weather, the best approach is to be sure you always look at the radar coverage map that is also available from the XM broadcast. For example, here is an XM weather coverage map when the Portland radar was undergoing its upgrade back in September. You can see that Portland, Oregon does not have coverage.
CFI & former NWS meteorologist
Administrator of AvWxWorkshops.com
Most pilots are weatherwise, but some are otherwise™
Edited 2 time(s). Last edit at 01/23/2012 06:47PM by Scott Dennstaedt.