Weather.gov Current Observation feeds have suddenly begun to fail for all requests from an HTTPClient, and likewise I've observed that many websites across the internet that use AJAX to make calls to weather.gov are also failing.
The result of all calls to weather.gov current observation feeds, e.g. http://w1.weather.gov/xml/current_obs/TAPA.xml, return a 403. Said URL resolves properly in a browser.