MET's series starts mid-today and thins toward the tail, and the parser
graded whatever samples a day happened to have: the current day was built
from the remaining afternoon hours (seen live on prod: an 'overnight low'
of 114.8F graded at the ~100th percentile) and the final day can collapse
to one sample with tmin == tmax. A day now needs its samples to span at
least 18 hours before it is emitted — full hourly days and the 0/6/12/18
6-hourly far-term shape both pass; partial today and the degenerate tail
are dropped instead of being served as false extremes.
