It's September, for most people, this means back to school, harvest and preparing for Halloween! There is the risk for severe weather tomorrow in most of southern and central Manitoba. This is due to an upper-level trough ejecting (seen above), which is bringing unseasonably warm temperatures to parts of the Northern Plains and Prairie Provinces.
Friday September 20th
Threat level: MODERATE
Confidence: HIGH (based on model agreements)
Expected hazards: HAIL // DAMAGING WINDS // NON-ZERO RISK OF A TORNADO // FLASH FLOODING
Locations impacted: Souris, Killarney, Neepawa, east of Riding Mountain Prov Park to Lake Manitoba, MacGregor, Winnipeg, Morden, Morris, Winkler, Carman, Pilot Mound, Portage La Prairie, Turtle Mountain and surrounding communities (in Manitoba)
Timing: 4:00 pm CDT to overnight
Expected watch: severe thunderstorm watches (issued by morning)
Discussion: an upper-level trough, currently situated over N California / Nevada, is set to eject northeast in the next 24hrs. This positively-tilted trough will bring southeasterly winds at the surface for the Northern Plains and Manitoba early in the period, with high surface temperatures and moisture advection expected as far north as central Manitoba by 18z. At the surface, a warm front will advect north throughout the day bringing with it cloud cover. Therefore, Manitoba should be mostly cloud cover throughout the morning and possibly early afternoon hours. Showers and thunderstorms should be ongoing during the morning hours north of the warm front in central Manitoba, possibly elevated in nature due to high CIN in the region. Depending where the Low / warm front sets up in Manitoba, air mass recovery should occur throughout the afternoon as vertical mixing occurs ahead of the cold front and cloud cover erodes, allowing for significant CAPE values to build up in the warm sector and ahead of the cold front. Therefore, as the warm front may initially be the focus for strong to possibly severe thunderstorms, the cold front will become the focus for surface-based supercells in the afternoon-evening hours, especially in North Dakota and parts of southern Manitoba. Due to the nature of the front orientation and perpendicular surface flow along the front and therefore unidirectional shear should lead for primarily a damaging wind and heavy downpours (due to high PWAT values) threat, possibly in the form of a long-lasting squall line, that could last well within the overnight hours for communities along the MB/ND border. Below is a possible scenario depicting a large line of storms from central Manitoba to North Dakota at 7pm CDT.
While we have not included North Dakota in our risk map, the risk is certainly there for them and is outlined by the Storm Prediction Center below. Click the image to read their discussion.