Yet Another Rainstorm

     A major slow-moving Colorado Low is poised to slam parts of southern Manitoba with excessive rainfall Thursday through to early Saturday. This system looks strangely similar to the major 4-day long storm of 2 weekends ago. However, there are a few small differences. The deformation boundary is expected to reach a bit further north and west this time. What this essentially means is that the heavy rains will reach a bit further west and north. Some of the areas that saw little to no rain last time will likely see high amounts this time. The storm is also expected to be a little shorter; lasting 2-3 days instead of 4. But that wont necessarily mean lower rainfall amounts. It will have an ample amount of time to pump in lots of moisture from the deep southern US. The flow of moisture can be seen in the map below.

Precipitable water map from the NAM model for Thursday afternoon showing the flow of moisture straight from the southern US

     Rain, heavy at times, is expected to move into southern Manitoba in the morning tomorrow. It will likely begin to rain in Winnipeg sometime in the morning. However, there is still a little uncertainty in the timing as some models are not bringing it until late afternoon and others early morning. We'll have to wait and see right now.

     Winds will be strong from the east/northeast at 40 km/h with gusts as high as 60 km/h at times. This will create upsloping near the escarpment and Riding Mountains which will give those areas higher rainfall amounts. A general 15-40 mm is expected throughout the day Thursday generally along and south of the Riding Mountains and from the southern Interlake southwards, with as much as 40-50 mm possible along the escarpment and Riding Mountains. 15-30 mm is expected for the Winnipeg region.

     There will be the potential for thunderstorms tomorrow as well. The best risk will remain south of the border, however scattered embedded thunderstorms are still likely close to the US border. Areas to the north of the border will have a smaller chance. I don't think we'll see storms here in Winnipeg, but just remember that the potential is there. Any storm activity will give locally enhanced rainfall amounts.

     On Friday, much of the same. Rain and wind will be the story with an additional 15-30 mm possible, with higher amounts locally along the escarpment. A few embedded storms will be possible in southeastern Manitoba.

     Conditions will improve on Saturday. Scattered showers are still expected in the early morning for southeastern portions of the province before things begin to clear out in the afternoon or evening.

     In total for the entire storm, a general 20-60 mm can be expected in southern Manitoba with locally higher amounts of 75 mm + possible along the escarpment. Here's my personal forecast for some select communities (updated at 11 am May 30):

     Note that amounts in Gimli and Dauphin will be highly dependant on the location of the deformation boundary. Dauphin may get more than forecast if the boundary is further north, but they may get little to nothing if it sets up further south. This is highly visible on the models this morning with a predicted range of 2 mm (NAM) to 94 mm (GFS) for Dauphin. This is a similar situation that Winnipeg experienced during the last storm 2 weekends ago when we were often right on the edge of the heavier rainfall.