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In general relativistic magneto-hydrodynamic (GRMHD) simulations, accretedmagnetic flux on the black hole horizon episodically decays, during whichmagnetic reconnection heats up the plasma near the horizon, potentiallypowering high-energy flares like those observed in M87* and Sgr A*. We studythe mm observational counterparts of such flaring episodes. The change in 230GHz flux during the expected high energy flares depends primarily on theefficiency of accelerating $\gamma \gtrsim 100$ ($T_e \gtrsim 10^{11}$ K)electrons. For models in which the electrons are heated to $T_e \sim 10^{11}$ Kduring flares, the hot plasma produced by reconnection significantly enhances230 GHz emission and increases the size of the 230 GHz image. By contrast, formodels in which the electrons are heated to higher temperatures (which we argueare better motivated), the reconnection-heated plasma is too hot to producesignificant 230 GHz synchrotron emission, and the 230 GHz flux decreases duringhigh energy flares. We do not find a significant change in the mm polarizationduring flares as long as the emission is Faraday thin. We also presentexpectations for the ring-shaped image as observed by the Event HorizonTelescope during flares, as well as multi-wavelength synchrotron spectra. Ourresults highlight several limitations of standard post-processing prescriptionsfor the electron temperature in GRMHD simulations. We also discuss theimplications of our results for current and future observations of flares inSgr A*, M87*, and related systems. Appendices contain detailed convergencestudies with respect to resolution and plasma magnetization.

Millimeter Observational Signatures of Flares in Magnetically Arrested Black Hole Accretion Models