Frost this time of year (outside mountains) is not highly abnormal, but it is late. From RAH:
High pressure will bring dry and cooler conditions for Thursday and
Friday. Thicknesses will be lowest on Thursday morning and skies
will be clear, so leaned toward the colder side of model blends and
close to
GFS/
ECMWF ensembles for temperatures, with forecast lows in
the mid-to-upper-30s. Thus some patchy
frost will be possible,
especially in the usual cooler spots. Highs Thursday will be 5-10
degrees below
normal (lower-60s north to upper-60s south).
Temperatures will start to moderate on Friday as the high pressure
system moves offshore, with forecast lows in the upper-30s to lower-
40s and highs in the lower-to-mid-70s.