The remnants of Tropical Storm Cindy are expected to affect ON and QC tonight and tomorrow as its moisture is absorbed by a LP system moving through ON and QC.
Starting tonight, heavy showers and thunderstorms will overspread southern Ontario then move into Quebec before morning. Marginal elevated instability will limit severe weather to torrential downpours resulting from PWATs to 2″ and freezing levels near 600 mb. Local rainfall amounts to 50 mm are possible with training of storms.
Sometime between the morning and mid-afternoon, the first wave of convection is expected to clear out of southern Quebec. As little as three hours of sunshine will result in temperatures surging above 25 C combined with dewpoints over 22 C will result in CAPE in excess of 2000 j/kg. That, combined with effective shear near 35-40 kt will result in favourable conditions for the development of supercellular structures. However, weak shear in the first kilometre AGL will considerably reduce the tornado threat.
This 12Z NAM sounding from the south shore of Montreal at 11 am tomorrow indicates what could be in the cards for us. The torrential downpour threat will continue, but other threats including the 90+ km/h H7-H8 winds mixing down to the surface and large hail in some of the stronger supercell cores will be present.
Fast storm motion will reduce rainfall totals from individual storms but training is very possible due to storm density with strong forcing and upper-level support. In fact, the 12Z 3K NAM makes a mockery of tomorrow’s provincial holiday as the afternoon storms produce over 100 mm of rain in parts of the Eastern Townships:
The greater Montreal area east to the Eastern Townships will likely be the focal point for both rounds of storms.
There is still some uncertainty regarding the degree of destabilization but I believe that it will eventually happen during the day, whether it’s at 11 am or 6 pm; even if clouds were to remain in place all day, there would be some instability to allow for the formation of low-topped storms with very-high precipitation efficiency, which means that severe thunderstorms in some way, shape, or form tomorrow are almost inevitable.