Moisture: Surface dewpoints of 17-21 C are expected across southern Manitoba, with 850 mb dewpoints of 12-15 C by early evening. Moisture is expected to be well mixed within the boundary layer, giving a 100-mb mean mixing ratio of approximately 12-13 g/kg.
Instability: The aforementioned well-mixed boundary layer will sit beneath mid-level lapse rates of approximately 7 C/km. Steep low-level lapse rates will be present owing to the well-mixed, and relatively deep boundary layer. Resulting MLCAPE values will be near 2000-2500 J/kg across south-central Manitoba by early evening. Early evening instability will generally be poor in the Red River Valley and points east, with MLCAPE of 1000 J/kg or less. However, by the late evening period (03Z onward), MLCAPE/MUCAPE will increase to near 2000 J/kg in the region.
Wind Shear: A 50 kt 500-mb jet streak pushing into SW Manitoba will be preceded by approximately 40 kt of westerly flow over southern Manitoba. Except near the surface warm front, surface winds will generally be southerly, and as a result will not enhance the effective bulk wind difference (EBWD) in a notable way. The result will be EBWD values of 30-40 kt across southern Manitoba, with locally higher values near the surface warm front. More interestingly, a strong southerly low-level jet of 30-40 kt is expected over south-central Manitoba by late afternoon. This will result in 0-1 km wind shear of 15-25 kt. Effective storm relative helicity values will also be strong, at 250-400 m2/s2 during the evening. Low-level shear and helicity are also expected to benefit from the decoupling of the boundary layer by mid-late evening, as the LLJ increases to 45 kt by 0300 UTC.
Trigger: The primary trigger for deep convection Monday evening will be a cold front moving in from eastern Saskatchewan and a warm front extending eastward from a low pressure system near the MB/SK border. Forcing for ascent will be quite strong, especially over western Manitoba, where a potent shortwave trough will help knock down heights by the afternoon. The combination of the low-level mesoscale ascent with the surface fronts and mid-level ascent from the incoming shortwave should easily be able to trigger storms by late afternoon across western Manitoba.
Discussion: Severe thunderstorms are likely to develop across western Manitoba/central Interlake beginning Monday afternoon. Initial storms will likely be a mix of supercells and multicell clusters, owing to the strong forcing. Any initial cells that are able to maintain relatively unpolluted surface-based inflow will present a tornado risk, owing to the strong low-level shear/helicity and steep low-level lapse rates. All initial severe cells will be capable of large hail and damaging wind gusts. As the evening progresses, some models hint at upscale growth/cold pool mergers leading to the development of a bowing system over the southern interlake, or adjacent regions. Should such a system develop and have a significant line-trailing cold pool, it would tend to move ESE/SE, potentially impacting Winnipeg, the Red River Valley, and southeastern Manitoba. A system will a less pronounced cold pool may take a most easterly track, primarily avoiding the Red River Valley. Latest models suggest a well-organized, cold-pool driven MCS is the less likely outcome. The overall risk is slight for all of southern Manitoba. A future moderate risk is possible should a forward-propagating system become more likely.
