From what I see, pollen is usually available earlier in the year than nectar. Nectar foraging is certainly limited by temperature of the nectar, but pollen foraging seems to be less so.
Also, keep in mind that allergy forecasts may be largely for windblown pollen / wind pollinated plants, which are not primary bee forage.
Also, keep in mind that allergy forecasts may be largely for windblown pollen / wind pollinated plants, which are not primary bee forage.