Any sudden acceleration in rate of spread or intensification of the fire. Unlike blowup, a flare-up is of relatively short duration and does not radically change existing control plans.
• (n.) A sudden burst of anger or passion; an angry dispute. • (n.) A sudden bursting into flame; a flaring.
Flare'-up` noun A sudden burst of anger or passion; an angry dispute. [ Colloq.]Flare'-up` noun A sudden bursting into flame; a flaring. 
Flare-up is British slang for a sudden quarrel or fight.
Any sudden acceleration in rate of spread or intensification of the fire. Unlike blowup, a flare-up is of relatively short duration and does not radically change existing control plans.
[v] - ignite quickly and suddenly
flare 1 break open verb erupt or intensify suddenly; `Unrest erupted in the country`; `Tempers flared at the meeting`; `The crowd irrupted into a burst of patriotism` verb ignite quickly and suddenly, especially after having died down; `the fire flared up and died down once again`