Forecast models from the National Weather Service and private guidance place Miami’s overnight low on May 19 in the upper 70s, with the 78-79°F band holding the highest market share at 27 percent. Warm Atlantic sea-surface temperatures limit radiational cooling, while a southerly flow and moderate moisture keep minimum temperatures from dropping into the lower 70s that climatology suggests for mid-May. Traders weigh these persistent boundary-layer conditions against possible brief clear-sky windows that could allow an extra degree or two of cooling, splitting volume across the 73°F-or-below and 76-77°F bins. Resolution will hinge on the official Miami International Airport ASOS reading at midnight local time, with any shift in steering flow or cloud cover likely to move probabilities in the final 48 hours.