A fall system will bring the first significant snowfall accumulations of the year for the Northern Plains in the next few days. Winter storm watches are already in effect for parts of Montana, North Dakota, South Dakota and Minnesota. This article is broken down into four sections:
North Dakota impacts
A Colorado Low will develop late Wednesday into Thursday over the western United States. Precipitation will develop along a trough in the Dakotas, with warm air nosing in aloft as the low continues to develop Thursday. A zone of freezing rain looks possible along or just south the ND/SD state line, though there is some uncertainty as to how far north or south this band will be (the warm nose aloft associated with the system). Significant moisture will be present as the low develops, which combined with cold surface temperatures near/below freezing, could bring significant snowfall, mainly from North Dakota to Northern Minnesota. The developing Low will track over the Great Lakes region into Friday, bringing snowfall to locations around Lake Superior. It will also bring a warm sector into eastern Ontario, allowing temperatures to be much above normal for Southern Ontario.
The heaviest precipitation looks to fall in the northern part of South Dakota and adjacent North Dakota, where the rain snow line looks to sit. Some freezing rain looks likely around the Aberdeen area if a sufficient amount of warm air aloft can set up, though there is some models that shows the freezing rain line along the SD/ND state line, or even a bit to the south of Aberdeen.
From the National Weather Service in Bismarck, ND:
LONG TERM...(Wednesday night through Tuesday) Issued at 322 PM CDT Tue Oct 20 2020 As trough deepens upstream, several ingredients come together to support a significant heavy snow event. Between 00Z and 12Z Thu, nearly the entire state is located under coupled jet streaks, combined with strong and moist isentropic lift, strong Q-vector convergence, with the best frontogenetical forcing across the southern part of the state, closest to SD border. The NAM was an outlier with a strong trough and surface low placed farther north, and was generally ignored in favor of the consensus of other model and ensemble input. WPC QPF values resulted in snowfall totals up to 12 inches at some locations, mainly south of Highway 200. A couple of caveats are the potential of mixed precip, including freezing rain. Forecast soundings generally looked cold enough, with the NAM a little more aggressive with a fairly shallow layer above freezing around 800mb for a few hours near the SD border. GEFS plumes also continue to show a little more dispersion that would like to see, but mean QPF for Bismarck around .6 to .8 inches. With all these factors and after collaboration with WPC and neighboring WFO`s we have issued a Winter Storm Watch for most areas south of Highway 200. Another interesting point brought up during coordination with neighbors and WPC is how some of these early season events have overperformed expectations. Winds were also bumped up from NBM guidance which resulted in blowing snow being added to the grids.
...WINTER STORM WATCH REMAINS IN EFFECT FROM LATE WEDNESDAY NIGHT THROUGH LATE THURSDAY NIGHT... * WHAT...Heavy snow possible. Total snow accumulations of 6 to 12 inches possible. Winds could gust as high as 35 mph. There could also be some sleet and freezing rain along the South Dakota border. * WHERE...From Garrison, Harvey and Carrington, through Bismarck and Jamestown, to the South Dakota border, basically along and south of Highway 200. * WHEN...From late Wednesday night through late Thursday night. * IMPACTS...Travel could be very difficult to impossible. Patchy blowing snow could significantly reduce visibility.
North of this line, snow will be the primary hazard with temperatures near to just below freezing. European Ensemble Precipitation estimates range from 0.5” to 0.7”, meaning a solid 10-20cm could occur for southern North Dakota/Northern SD, with lesser amounts as you go south in South Dakota where there will be a transition to freezing rain, then rain over southern parts of the state. However, many other models suggest more precipitation will be possible for the area, perhaps up to or over an inch in places, which would entail a risk for up to 25-30+cm of snow across the southern part of ND and Northern SD, with lesser amounts towards the Canadian Border. A fair amount of uncertainty still exists, and the public should monitor and listen to local weather outlets for updates. Generally, there is a reasonable risk of a widespread event of 15+cm of snow at this time across North Dakota, South Dakota and Minnesota.
The bulk of the snowfall should remain south of the international border with lake-effect snow making a return on the backside of this system. Manitoba could see some pretty substantial snowfall accumulations from lake-effect snow over the weekend.
The low will move through the central Great Lakes region on Friday as it develops more mature frontal characteristics. Warm air in the warm sector will bring much above normal temperatures for Southern Ontario where highs will be in the upper teens to possibly the low 20s.
The cold front will move through later in the day with showers and potentially some thunderstorms associated with it.
For northern Ontario, snow will be likely north of the low's track with a low to moderate impact winter storm possible. The rain-snow line looks to be along a line extending from Wawa towards Timmins. Areas north of this could see 10-15cm of snow, though there is some uncertainty as the low has yet to develop. Heavy rain will be possible south of the rain-snow line were 30-60mm will be possible. Residents in the region should monitor local forecasts for future updates.
Keep updated with watches/warnings issued from Environment Canada here: https://weather.gc.ca/warnings/index_e.html