Update: Some kind — but, alas, anonymous — soul relayed the following in comments:
Straight from John Celenza at WeatherUnderground:
Man I wish we had updates on time.
Basically the folks who control our domain registration made a MAJOR
mistake today and broke DNS resolution of our site by pointing DNS
lookups to the wrong name service.
We'll be up definitely after the 45 hour timeout period when root name
servers will again attempt to lookup and store Wunderground.com.
However, we are working with our registrar to find a MUCH faster