Simultaneous broadband SEDs of MJD 57786 (pre-flare state), MJD 57788 (flare), and MJD 57789 (post-flare). Fermi-LAT spectral points are integrated over 3 days around the VHE measurements. VHE data with square black-filled markers are obtained from FACT observations, while X-ray data in black-filled markers are from Swift-BAT. The FACT SED for the pre-flare state was averaged from MJD 57786 to MJD 57787. The full black line is the two-zone model for the MJD 57788 flare state. The black dotted line represents the emission from the quiescent zone while the dashed line is the one from the flaring zone. The model parameters are listed in Table 6. Archival data representing the typical Mrk 421 state from Abdo et al. (2011) are shown in grey.