Here's my guess:
Weather All The Time appears to mean the LDL will be up during programming, including commercials. During studio-based programming, the L-Bar will be used - at least on HD, though it's possible that TWC will do like other channels and start letterboxing the HD feed on the SD channel.
LOT8's during this time will be a full-screen affair like it is now during studio programming.
Longform - this is all speculation. Does the LDL stay up all the time? Will there be something even smaller - perhaps just the Weather Channel logo and the city and temperature scrolling above it, like what's shown in the LDL? Does LOT8 mean the LDL comes up? Will it be a similar setup as to now - sometimes it's just the LDL, other times it's the L-Bar with a studio update? Does the latter get replaced with a full-screen LOT8, since there's (possibly) no studio programming during the night?