A threat of severe thunderstorms returns to Winnipeg and the Red River Valley today as a cold front advances eastwards into a hot, increasingly humid air mass that has been building into the region over the past few days.
Before thunderstorms develop this afternoon, we’ll see another hot summer day. We’ll see temperatures climb towards the 30°C mark with a fairly breezy wind out of the south at 40-50km/h. A fair amount of smoke will still be lingering aloft, so temperatures may struggle getting those final couple degrees towards the 30 mark.
Thunderstorms will develop this afternoon along the cold front through the western Red River Valley and Interlake and push eastwards. We’ve assessed a slight risk for severe storms from the MIST assessment:
- Moisture: Surface dewpoints will climb into the high teens today and although they are forecast to drop off somewhat through the boundary layer, there should be ample moisture in place for storm development.
- Instability: A capping inversion will keep things fairly quiet through much of the day today. Moderate mid-level lapse rates will develop through the day (possibly enhanced by the EML[1] from Saskatchewan’s overnight convection). Low-level instability will be somewhat limited today by diminished heating due to cloud cover and smoke aloft, however temperatures in the upper 20’s will come very close to breaking the cap.
- Shear: Looks very good. Around 30kt of surface to 6km bulk shear will be in place which will support organization of thunderstorms. 0-2km shear values of around 20-25kt reflect the moderate veering of the winds that will exist through the lower levels of the atmosphere and will support the development of supercell thunderstorms early in the event. As the storms mature, shear becomes more linear and will help support the development of a squall line.
- Trigger: Check! A strong cold front with good convergence will either erode the cap to initiate thunderstorms or provide enough forcing to overcome the cap.
Once thunderstorms develop this afternoon, they will likely quickly organize into discrete supercells capable of all modes of severe weather[2]. More unidirectional shear above 850mb will marginalize the tornado threat as more energy will have to be expended to develop the mesocyclone through the vertical, but with potentially as much as 2500-2700J/kg of MLCAPE, tornado(es) cannot be ruled out given the wind profile.
Storms will then increase in coverage and develop into a line along the cold front. Severe weather will still be possible, although the primary threats will shift to damaging winds and large hail. With strong unidirectional shear profiles through the Interlake, the development of bowing segments giving damaging winds is possible through that region.
The thunderstorms will push off into southeastern Manitoba this evening with a continued severe weather threat before tapering off in northwestern Ontario overnight. Winnipeg will see clear skies and a low around 16°C.
Mixed Weekend
Saturday will be a fairly pleasant day with weak ridging producing mainly sunny skies with light southerly winds and a high near 28°C.
Clouds will push in on Saturday night as we drop to a low around 17 or 18°C. There is a very slight chance of a shower or thundershower in the overnight period.
Sunday will be another warm day with a high close to 30°C under mixed skies. The afternoon will bring another chance for showers or thunderstorms as a trough line works into the province.
