There has been a fire at some location important to the relay of METARs to the world. This relay is down until further notice, so it appears many METAR / Airport / ASOS+AWOS sites will be unavailable in the interim! Will update this NEWS item with any further details I get.
Here's a plot of my monitoring showing the downturn in available sites:
Updated 9 PM: Most sites are back now, but am unsure of when the full restoration will happen. There was a problem with a telco location in Omaha that caused this outage.