From AMS Glossary
Flash heat describes a discrete period of abnormal warming, nominally lasting more than an hour but less than a day, occurring on the mesobeta scale. The term was first defined by studying two such events in the Mediterranean basin via simulations with a mesoscale numerical weather prediction model. Flash heat events last longer than heat bursts but are too short to be considered heat waves and are associated with some atypical physical mechanism rather than normal radiative or advection processes.
Mazon, J., D. Pino, and M. Barriendos, 2014: Rapid and sudden advection of warm and dry air in the Mediterranean Basin. Nat. Hazards Earth Syst. Sci., 14, 235–246, doi:10.5194/nhess-14-235-2014.
(Term added 8/19/2014)