Saturday, June 9 Forecast (Day 3)
A risk of severe thunderstorms will develop across parts of eastern Alberta on Saturday.
A significant shortwave trough pivoting about a Gulf of Alaska low will begin to dig into the Pacific Northwest, becoming increasingly negatively tilted over time, with an associated H5 speed max of 60-70 knots nosing into Alberta on Saturday. In response, an elongated, north to south-oriented surface low/trough will develop during the day from central sections into the far south.
Low level moisture will be largely scoured out of southern and parts of central Alberta in low level westerly flow during the day on Friday, with the resultant moist axis residing from southcentral Saskatchewan into portions of eastcentral and northeastern Alberta by Saturday morning. As the surface trough begins to deepen during the day, low level moisture return will occur in earnest from Montana and into the warm sector across much of southeast and eastern Alberta. Meanwhile, an increasing southerly component in flow aloft will advect the northernmost portions of an elevated mixed layer plume over southeastern Alberta, steepening mid-level lapse rates over the region – which will be further steepened by ascent downstream of a prominent perturbation approaching from the south during the day. The combination of steepening mid-level lapse rates atop surging low level moisture, as well as strong diabatic heating, will lead to the development of moderate instability, characterized by MLCAPE values of 1000-1500J/kg throughout much of southeast and eastern Alberta.
NWP guidance suggests a leading disturbance will push through flow aloft on Saturday morning, supporting elevated thunderstorms that likely push north along the QE2 corridor between Calgary and Edmonton. This feature will have to be monitored closely, due to associated cloud cover and convective overturning in central Alberta that could limit later destabilization, as well as in the possibility of its generation of an outflow boundary that could become collocated with the surface trough in the afternoon. Should the convective cluster persist into the afternoon, it could become rooted to the boundary layer, resulting in a possibility of thunderstorms becoming severe over central Alberta.
Elsewhere, low level convergence along the elongated low/trough should overcome the capping inversion, and thunderstorms will begin to initiate by mid-afternoon. With 0-6km shear values of 30-40 knots, a few supercells – with damaging wind gusts and up to 4-6cm hail – will likely initially develop. However, a fairly rapid progression to upscale growth seems likely, due both to the orientation of the low level initiating boundary to deep shear vectors, as well as the rapid congealing of cold pools owing to strong evaporational cooling potential in deep, well-mixed boundary layers. As such, a combination of broken and continuous linear segments will push east through the late afternoon and evening into western Saskatchewan. A non-zero tornado threat will exist as well, especially to the north of the elongated low where favourable low level shear profiles will be further enhanced in the proximity of a low level jet stream. Should any storms remain discrete and survive until the evening transition – especially near the moist axis near Lloydminster, and conditional to ample destabilization – a tornado threat may develop into the evening. However, the overwhelming signal appears to favour high-based thunderstorms with an early large hail and damaging wind threat.
Otherwise, scattered thunderstorms are likely along the northern foothills, with a few strong to marginally severe storms across the far north in the late afternoon.