  We study the thermal stability of non-self-gravitating turbulent $\alpha$
discs around supermassive black holes (SMBHs) to test a new type of
high-amplitude active galactic nuclei (AGN) flares. On calculating discs
structures, we compute the critical points of stability curves for discs around
SMBH, which cover a wide range of accretion rates and resemble the shape of a
$\xi$ curve. We find that there are values of the disc parameters that favour
the transition of a disc ring from a recombined cool state to a hot, fully
ionised, advection dominated, geometrically thick state with higher viscosity
parameter $\alpha$. For SMBH with masses $\sim 10^6-10^8 M_\odot$, such a flare
can occur in the geometrically thin and optically thick neutral disc with
convective energy transfer through the disc thickness surrounding a radiatively
inefficient accretion flow. When self-gravity effects are negligible, the
duration of a flare and the associated mass exhibit a positive correlation with
the truncation radius of the geometrically thin disc prior to the flare.
According to our rough estimates, $\sim 4-3000 M_\odot$ can be involved in a
giant flare, i.e. can be accreted or entrained with an outflow lasting 1 to 400
years, if the flare is triggered somewhere between $60$ and $600$ gravitational
radii in a disc around SMBH with $10^7 M_\odot$. The accretion rate on SMBH
peaks at a super-Eddington value about ten times faster. The peak effective
disc temperature at the trigger radius is $\sim 10^5\,$K, but it can be
obscured by an optically thick outflow that reprocesses the emission to longer
wavelengths. Such a transfer of disc state could trigger a massive outburst,
similar to that following a tidal disruption event.
