Updated: Oct 5
Before we get into the details, we need to state what is happening in the atmosphere as a whole. Arctic air should return to the Northern Plains and Prairie Provinces mid-week next week, almost all models/ensembles are depicting this at this point. This will create a baroclinic zone and a dry slot somewhere in the Midwest on Thursday. The cold Arctic air is depicted above and the aforementioned dry slot is depicted below.
Now that that is out of the way, let's get in the more 'uncertain' stuff. While most models agree with a deepening low and some form of winter precipitation on Thursday through Saturday, the location of this line is quite uncertain at this point, so one can only speculate which areas will receive the brunt of this system. The key feature to watch is where the dry slot settles.
Extra info: Why? Because this dry air aloft can actually steepen lapse-rates and cause local snowfall enhancements. These are often referred to as 'convective snowbands'. Obviously, however, areas directly below the dry slot will see no precipitation. This is why this area is important to monitor on water vapour imagery.
On Wednesday, a sharp temperature gradient is set to separate mid-teens (C) and near-freezing temperatures (seen above). This boundary will set up in parts of southeastern Manitoba and northwestern Ontario, which will quickly go away on Wednesday afternoon/evening. Cold air will return to the Prairies and will set the stage for some winter weather.
Thursday to Friday
Threat level: MODERATE
Confidence: LOW (based on model agreements)
Expected hazards: WINTER STORM (possible) // STRONG WINDS // BLOWING SNOW
Expected snowfall: TBD (prelim 10-15cm)
Locations impacted: extreme southeastern Manitoba
Expected watch: special weather statements, snowfall warnings and/or winter storm watches (localized)
Most likely areas impacted: Trans-Canada Highway from Steinbach, MB to Kenora, ON
It is important to note that while most models agree about some sort of winter weather, it is uncertain which areas will see those conditions. Below is a GFS model trend for the last 24hrs, you can see that this model has no consensus about the positioning of the low and therefore the precipitation.
Discussion from the NWS Grand Forks, ND:
Wednesday-Friday "The next major 500 mb system will impact the northern Plains mid to late next week. One short wave moving thru Alberta into Saskatchewan late Tuesday will continue east into Manitoba Wednesday, while a second stronger 500 mb vort will move southeast into Wyoming. It is how these two interact in the Thursday-Friday period which models vary quite a bit. Wednesday will see a cold front move into E ND (esp DVL basin) and then slow down as it waits for the second 500 mb vort to move into the Plains and develop a surface low in South Dakota or Nebraska. Most models to some degree develop this low north-northeast into Minnesota Thursday night with a developing 500 mb low. But variations abound in this scenario and strength of this system. GFS ensembles show quite a bit of variety but will need to watch this Thursday-Thursday night into Friday system for wintry precipitation potential. Current NBM will have rain/snow which at this time is ok."