Another active day in terms of severe storms is set to take shape across the country tomorrow particularly in southern ON and in AB/W’rn SK. The water vapour image below shows the two upper level lows that will be responsible for these active days, the one currently located off the coast of BC and the other along the WY/MN borders. By late tomorrow afternoon the GFS/GEM/ECMWF have the western low still situated on the Vancouver Island region and the eastern impulse just tracking into IA/WI region. Reason why I am showing the GFS, as just mentioned, there is great agreement on the forecast position of these impulses among the cluster of models indicated and the NAM is the outlier among the group, not by much but enough that it will make some difference down the road.

Investigating the 500 mb wind forecast shows a good balance of divergence and diffluence over central to eastern AB which will lead to cyclogensis in the lee side of the Rockies by the afternoon and strong divergence over southern ON which will continue to push the low up from the US by late afternoon/evening.

Below is my forecast analysis for tomorrow evening/afternoon across the area of interest but in the section below will break the discussion into the two areas of interest. Note that the regions circled in purple are general risk area, and area outlined in pink show tornado risk areas.

Ontario

At the surface there is a low pressure system that will be pushing toward and slide along or just south of the northern shore of Lake Erie. There will be a warm front associated with this low and will follow a similar track just ahead of the low center. In south central ON Lake Breeze (LB) convergence will also be setting up with the Huron/Georgian Bay/Ontario lake breeze fronts converging roughly around an area outline by Kitchener-Minto-Orangeville. The warm front and lake breezes will be the main region of focus for any surface based convection tomorrow afternoon and evening. Within the LB zone SFCAPE values are forecast to reach the 500-1000 j/kg range and MUCAPE values near the warm front in southwestern ON are forecast to reach the 500-1500 j/kg range through the afternoon/evening. There will be ample shear present across the region but will make note that vectors with the LB region a perpendicular to the LB fronts which will lead to more isolated updrafts where as shear vectors in the vicinity of the warm front are more parallel thus leading to more messy convection with multiple slitting cells. However, forecast hodographs across much of the area do show veering with height, and even more so in southwestern ON. All factors combined would suggest that the LB region will likely see a few isolated cells go up, possibly supercell modes, and through southern ON expect multiple modes with embedded rotating updrafts. Greatest tornado threat looks to be along and just south of the warm front from Simcoe to Windsor, and a small risk in the LB region. Important Note: LB region will greatly depend on how much cloud cover persists through the late morning hours to allow for suitable destabilization through the day.

Alberta/Western SK

All factors discussed above will lead to low developing to the northwest of the Edmonton region roughly near the Slave Lake region. Front he region a triple point will setup with the warm front extending east toward Cold Lake, AB, the cold front extending west northwest toward Fort St. John, and a dryline extending south along or just east of the QE 2 corridor to the US border. Within the region east of the dryline and south of the warm front SBCAPE values are forecast to reach the 1000-1500 J/kg range by mid afternoon, bulk shear values expected to range from 40-60 kts, and forecast hodographs indicate veering with height, should all contribute to mainly isolated updrafts and a few supercells. The issue will be how much cloud cover will linger through the morning. If there is sufficient clearing by late morning-mid day, the region just NW of Edmonton would see the largest risk of tornadoes particularly through the late afternoon and evening as the 850 mb LLJ really begins to ramp up. At the moment general consensus appears that initiation will likely take place between 20-22 UTC (2-4 pm MDT).