Tomorrow afternoon, a substantial H5 cold pool of about -14 C will exist over S QC, combined with dewpoints near 20 C and surface temperatures in the mid 20s, resulting in substantial CAPE of over 1500 j/kg.
H5 winds over 30 kt from the SW overtop light easterlies at the surface will provide sufficient shear for storm organization, with the potential for some transient supercells due to veering.
The main threats should be large hail and damaging winds due to mid-level dryness in addition to the aforementioned factors. Locally heavy rainfall amounts are also possible due to the combination of very slow storm motion and convective organization allowing for the regeneration of storm cells as shown in this 12Z NAM sounding for 11 am tomorrow near Granby:
Pay attention to those hail analogues!
Convective activity should be focused along the stationary front depicted on this WPC map valid for tomorrow morning. Surface convergence combined with decent mid-level lapse rates and upper-level divergence from a jet streak will create a trifecta of factors favouring vigorous updraft development in the St. Lawrence Valley and the Eastern Townships starting early tomorrow afternoon.