It's not explained what the vertical coloured dashed lines on the Temperature & Moisture plots mean but I am guessing that they show the first date AIM spotted NLC in the Northern Hemisphere based on the Red Line for 2020 and the earliest line which appears to be for 2013 which ties up with the text for the year with the earliest sighting.
Expect the amount of NLC to develop rapidly over the next 10 days or so and start observing night and morning.
This is definitely the beginning of the current season. However, the data for May 18 already show that the clouds practically disappeared over the northern polar regions. Apparently, the parameters in the polar stratospheres are close to the boundary values.