I don't really think their really worried about frost...its the freezing temps.
Highly favorable radiational cooling conditions combined with
very low pwat airmass signals increasing confidence of a
potentially damaging freeze across the northern half of CPA. We
expanded freeze watch to the I-80 corridor and blended minT
toward tabular MOS numbers which will challenge some daily low
temps records. MAV is 21F at KBFD and would not surprised if the
usually colder spots across the northern tier drop into the
upper teens! Needless to say it will be about as cold as it can
be for mid May tonight into early Thursday morning!
The other nuance in the forecast tonight is that despite the
unseasonably cold temps, the incoming airmass may actually be
too dry to get widespread frost formation. Our in-house
probability of frost/forecast builder procedure is quite
bearish on frost so we cut back/capped coverage from widespread
to areas. Locations across the southern half of the area where a
freeze watch is not in effect, could easily see at lest some
patchy frost with near-freezing temps capable of damaging
sensitive vegetation.