$Lambda$-Warm Dark Matter (WDM) has been proposed as alternative scenario to $Lambda$ cold dark matter (CDM), motivated by discrepancies at the scale of dwarf galaxies, with less small-scale power and realized by collisionless particles with energies in the range $1-3$ keV. We present a new approach to constrain the viability of such WDM models using star formation histories of the dwarf spheroidal galaxies (dSphs) in the Local Group. We compare their high time-resolution star formation histories (SFHs) obtained with HST-based color magnitude diagrams with the range of possible collapse redshifts of their dark matter halos expected in CDM and in different WDM scenarios. The collapse redshift is inferred after determining a plausible infall mass of the subhalo. This is based on the current mass of individual dwarf inferred from stellar kinematics combined with results of cosmological simulations providing information on the subhalo evolution. Since WDM subhalos close to the filtering mass scale form significantly later than CDM, we show that they are in the first place difficult to reconcile with a truncation of star formation occurring as early as $zgeq 3$. The Ultra-Faint Dwarfs (UFDs) provide the most stringent constraints. Using 6 UFDs with the best determination of the SFHs, we show that we can exclude a 1 keV warm particle to a 2-$sigma$ confidence interval consistently with other methods reported in the literature. For some objects the $2$ keV model is also excluded. We discuss the various caveats of the method, most notably the low number of dwarfs with accurately determined star formation histories and the uncertainties in the determination of the infall mass of the subhalos. Our preliminary analysis serves as a pathfinder for future investigations that will combine upcoming accurate SFHs for more local dSphs with direct analysis of WDM cosmological simulations with baryons.
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