Intense storms to impact SE Manitoba; City of Winnipeg under the gun

Today July 5, 2020

Threat level: MODERATE TO HIGH

Confidence: HIGH (based on model agreements)

Expected hazards: DAMAGING DOWNBURST WINDS // LARGE HAIL // FLASH FLOODING // TORNADOES POSSIBLE

Locations impacted: City of Winnipeg, St Andrews, Selkirk, Portage La Prairie, Carman, Steinbach, Stonewall, Elie, Starbuck, Morden, Winkler, Ste. Agathe, Morris and surroundings.

Timing: 1pm CDT to 8pm CDT

Expected watch(s): Severe thunderstorm watches

GEOS-16 Imagery at 16:26UTC showing large storm (denoted by "R") over northern Lake Manitoba

Discussion: ongoing thunderstorms near Sandy Bay will continue to move eastward quickly. Rapid development of supercells expected in the next hour or on southern edge of said thunderstorms in a corridor from Manitou to Woodlands as warm front (red in image below) moves northward. Explosive development expected in uncapped and highly unstable (~3000-4000 J/kg MLCAPE) airmass in south-central and southeastern Manitoba expected along the cold front (blue in image below).

Likely position of warm front (red), cold front (blue) and low pressure at 16Z

Low shear environment with DCAPE in ~1000 J/kg should favour quick storm merger and/or collapsing updrafts, which would make primary threat damaging winds (possibly downbursts/microbursts). Very moist airmass in place should give high risk of flash flooding and pooling of water, especially with any training storms. Storms should impact City of Winnipeg around 6pm time frame and exit the province into Ontario in the 8-9pm time frame. Storm motion should be almost due east with storm merger into squall line around the Hwy 75 corridor in the early-evening hours. Soundings suggest slight risk of tornado from St. Andrews to Winnipeg to Morris with an extremely unstable airmass. However, thinking shear is very poor, therefore risk of tornadoes <2% for today.

NAM 12km generated sounding for City of Winnipeg at 4pm

For the reasons mentioned above, if tornadoes occur today, they will be more likely in the areas outlined below:

Most likely area for tornadic development in the 4-7pm time frame

Please note that we believe the tornado risk to be fairly low (<2%). However, the environment present today in southeast Manitoba is highly volatile for significant supercells and tornadoes may certainly occur in any storms that can sustain its updraft long enough to have low-level rotation.

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From the Prairies and Arctic Storm Prediction Centre:

PASPC Day 1 Thunderstorm Outlook

Manitoba

• Area(s): Most of Manitoba

• Timing: Sunday afternoon, evening, and overnight

• Threats: Wind gusts to 100 km/h, hail 2 to 4 cm, and locally heavy downpours with rainfall totals of 50 mm or more

Widespread thunderstorms and isolated severe thunderstorms are expected Sunday ahead of a cold front moving through the province. Hail to the size of ping pong balls and damaging wind gusts are possible. Heavy rainfall with totals over 50 mm may also occur, especially from the Interlake northwards where thunderstorm activity will be more organized. The front moves into northwestern Ontario in the late evening hours, though thunderstorms will continue toward the Hudson Bay coast into Monday morning.