After several fairly nice days in a row, unsettled weather will make it’s return to southern Manitoba as a very powerful upper low moves into the Prairies. The unsettled weather will push through in three distinct phases over the next 3 days, varying from just a few showers to the potential for severe thunderstorms. Without further ado, lets get to it.

Today

We’ll see a very mixed day today as multiple weather features make their way over southern Manitoba. We’ll see a decent chance of some showers this morning as some elevated convection moves through the Red River Valley along the nose of an upper-level jet. We’ll then see a mix of sun and cloud through most of the rest of the day as we climb to a temperature around 24°C. The next bout of unsettled weather will move in this afternoon over SW Manitoba and in the late afternoon/evening for the Red River Valley as two distinct features move into the Red River Valley.

Through the afternoon the 850mb low-level jet (LLJ) will be working it’s way eastwards across Southern Manitoba and, while 700mb temperatures are forecast to sit around 6 or 7°C – a little high to get much convection – the LLJ has had a history of being able to get some heavy showers or weak thunderstorms going despite the marginal instability. This first feature will not likely produce any severe weather.

The second feature moving across southern Manitoba will be a surface trough extending from a low pressure centre in central Saskatchewan southeastwards through southern Manitoba. This trough has the potential to produce severe weather in Southern Manitoba, but the threat has to be considered fairly conditional due to two uncertainties:

1. Uncertainty in cloud coverage could result in cooler daytime highs than expected; the warmer it gets today the more likely storms will develop.
2. The warm temperatures at 700mb may be enough to prevent convection from developing; the trough will have fairly strong convergence associated with it, and there may be enough broad, synoptic-scale lift to overcome the warm air aloft.

My gut feeling is that we will see storms develop this afternoon; this trough has had a history of producing thunderstorms the past 3 days in a row. Going with this idea, it’s likely storms will initiate somewhere in SW Manitoba in the mid-to-late afternoon and slowly progress eastwards. The main threats with the storms will be:

1. Heavy Rain: Precipitable water (PWAT) values are expected to sit around 35–40mm this afternoon thanks to quite a deep layer of moisture moving across the province with this system. That’s a lot of water for these storms to work with. The trough line won’t be moving too quickly and, especially near the western escarpment of the RRV, conditions will be somewhat conducive to the development of quasi-stationary storms. Slow or non-existent storm motion could result in rapid accumulations of 2+” of rain in isolated storms.
2. Strong Straight-line Winds/Tornadoes: 0–6km bulk shear values will sit around 40kt this afternoon with favourable veering profiles. There’s certainly enough directional shear for storms to develop into supercells, however there may be a lack of backing in the lowest levels to produce enough low-level shear for tornadoes to become a threat. For that reason, I think the most likely wind-related threat would be the potential for strong straight-line winds. This threat would be secondary to the heavy rainfall threat.

Fortunately, with limited CAPE, a high freezing level and significantly weaker winds in the upper atmosphere than on Monday, hail will not likely pose much of a threat today. The showers and thunderstorms should develop this afternoon over SW Manitoba, push through the Red River Valley this evening and lift northwards into the Interlake, flattening out into an area of rain overnight. We’ll see clearing skies overnight here in Winnipeg as we drop to a low of around 14°C.

Saturday & Sunday

Through the weekend we’ll see the passage of the upper low as it moves from central Saskatchewan into NW Ontario. It will begin to move into the region on Saturday and allow things to broadly destabilize in the afternoon which will produce scattered showers and thundershowers. No severe weather is expected. We’ll see a high near 23°C on Saturday and then dip to a low of 12°C overnight with some cloudy periods. On Sunday, we’ll see more cloud than sun and a high near 21°C with scattered showers throughout southern Manitoba. Skies will clear out on Sunday night as we dip to near 10°C.

Next Week

It looks nice for the start of next week with sunshine returning and temperatures climbing back into the mid-to-upper 20’s. At this time it looks like another system will move through late next week bringing another chance of showers and thunderstorms.