Saturday November 7th 2020
Threat level: MEDIUM
Impacts: HIGH (based on population density)
Confidence: LOW (based on model agreements)
Expected hazards: Winter storm, freezing rain, sleet and possible whiteout conditions
Expected snowfall: 15-35cm (impossible to say at the moment)
Time frame: starting Saturday Nov 7th into Sunday night Nov 8th
Locations impacted: Southeastern Saskatchewan // Southern & Central Manitoba
Expected watch: special weather statements then winter storm watches
Most likely areas impacted: areas of Southwestern Manitoba and Southeastern Saskatchewan
Discussion: most models agree a trough will dig into the western part of the United States and British Columbia by the middle to end of the week next week. This will bring several impulses to the continental United States and the Prairie Provinces. The key debate, at the moment, will be cyclogenesis on the lee-side of the Rocky Mountains. Explicitly, whether it is going to occur on the U.S. side of the Rockies or Canadian Rockies. This will depend on the southern extent of the trough. Where cyclogenesis occurs will have a major impact on the resulting outlook. In our outlook above, we have outlined the most likely area at risk of since winter storm conditions and blizzard conditions. To do so, we looked at model trends and did a model blend of low positioning (depicted as a large "L"). Based on this low location, we outlined areas most at risk of experiencing winter weather. The blizzard risk was added for colder areas of the low pressure, mainly in Manitoba and SE Saskatchewan, where precipitation will fall mainly as snow and where temperature gradients // tight pressure gradients will support blowing snow // drifting of snow.
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Above is the GFS ensemble depicting the trough modelled to impact the U.S. and Canada next week. The technical discussion follows below.
Technical Discussion
Below is the ECMWF (European model) ensemble, which shows cyclogenesis or "low" pressure formation at 12z Saturday (Saturday morning). All the "L's" depict a different model output for low pressure location. As you can see below, there are a lot of "L's", which indicate poor model consensus on the placement of the low pressure we are looking at. One thing to note in this model is that no low pressure systems are modelled coming out of Alberta, which would form the elusive Alberta Clipper. All the low pressure systems depicted below would be Colorado Lows and therefore bring a much more significant winter storm to the Prairies.
Any good forecaster looks at different model outputs to make a forecast. This is because different models will have different inputs and therefore different outputs. When different models agree on a similar output, then you have consensus across models and forecast confidence increases. Below is the GFS (American model) ensemble, for the same time as depicted above. You can see similarities in low placement and again no low pressures are depicted coming out of Alberta.
Let us look at one last model, the GDPS (Canadian model). The GDPS is not an ensemble, so it has slightly different parameters than the above models, but it still paints an interesting story. The GDPS depicts an Alberta Clipper sweeping through the Prairies during the week (next week), before the low we are looking at is set to develop. Below, the GDPS has the low of interesting coming out of the higher terrain of Wyoming & Montana and setting up shop in North Dakota.
So far, we have:
ECMWF: Colorado Low forming in the Wyoming/Colorado area
GFS: Colorado Low forming in the Wyoming/Colorado area
GDPS: Low forming in the Wyoming/Montana area
So it doesn't seem fair to have seen precipitation only for the GDPS right? Below is the GFS depiction of precipitation type for Saturday:
Bottom Line
The bottom line is that something is brewing for the Northern Plains and the Prairie Provinces. We cannot narrow down the details until we have better consensus within models such as low positioning, which will dictate where the transition zone is and area of highest precipitation. However, we can say with some confidence that unsettled weather is expected next week across the area, especially across North Dakota, Montana, Manitoba and Saskatchewan. This will need to be monitored closely in the next several days.