FYI, the NWS seems to be starting to transition back to the 403 MHz band for radiosondes, at least for some sites. This page shows several moved in late 2016:
http://www.nws.noaa.gov/os/notification/tin16-23radiosonde.htmAnd another this year:
http://www.nws.noaa.gov/os/notification/scn17-66upperair.htmThat explained why I did not hear the Sterling radiosonde this morning, even though it was forecast to pass right overhead, it was one 401.0 MHz, not on 1680 MHz where I was listening

Interestingly enough, I did hear another radiosonde on 402.3 MHz. I am not sure who it was from, and while it was 4800 baud, it did not seem to be the LMS-6 data format.
There's another good path forecast tonight, so I will check 401.0 MHz.
Radiosondes are typically launched around 0000z and 1200z each day from a number of sites:
http://www.ua.nws.noaa.gov/nws_upper.htm