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Theoretical predictions based on temperature dependent rate equations are made of the pulse response of the individual axial modes of high power laser diodes. The temperature transients in the active area of the lasers are simulated with a finite element code and entered into the rate equations which describe the time evolution of both the carrier population and the photon number inside the laser cavity. Experimentally, the laser diodes are triggered by a fast electrical pulse and the modes of gain-guided AlGaAs laser diodes are dispersed with a echelle grating and individually imaged onto a scanning avalanche photodiode (ADP). The results of this study showed that modal instabilities during pulsed operation of the laser diodes can be separated into two types of transients: mode buildup transients and thermally-induced transients. Mode buildup transients, due to feedback from the cavity gradually causing power to concentrate in a few modes, were found to last up to 60 ns depending on laser length, whereas thermally-induced transients were found to last several hundred nanoseconds and are dependent on the overall efficiency of the laser. Our experimental results show that broad-stripe lasers, which minimize current spreading, have much smaller temperature transients as compared to narrow single-stripe lasers.