Warmer temperatures allow rapid bacterial growth in flowers. If bacteria numbers exceed a certain minimum while the flower is in good condition, then the flower is lightly wetted, infection is possible. Watch temperatures over time. Blossom wetting alone does not cause fire blight. Rain during cold or cool weather does not lead to infection, or blight would be common everywhere, every year.
Moisture can be from rain, dew or light irrigation wetting.
The water moves the bacteria from the stigma tip into the flower's nectary. The bacteria may then grow rapidly, attack the fruitlet, then move into the tree, attacking the living portion between the wood and the bark. The visible strike appears at this time. The best version of this model is automated, and totals the hourly fire blight degree hour value each hour of the 96 hours leading up to the present, or a wettness period. Most growers do not have this sort of data available.