A strengthening Alberta clipper system will track from NW to SE out of Canada and into Central Minnesota on Sunday, which should result in the biggest snow event of the season so far for the Twin Cities. All of the ingredients are coming together with this storm, including abundant moisture, favorable upper level dynamics, and very cold air aloft to result in high snow-liquid ratios.
The image below shows the NAM-model projected upper level pattern for Sunday evening, which clearly shows the trough of low pressure moving into southern/central Minnesota (image source: pivotalweather.com). This is a favorable storm track for Minneapolis snow.
Snow will begin to move into the Minneapolis-St. Paul area by mid-afternoon Sunday as the surface low pressure center nears the Canada/North Dakota border, as projected by the NAM Model (image source: College of DuPage).
Look for snowfall to quickly pick up during the evening and overnight hours, falling heavily at times. The heaviest snowfall rates are generally expected between about sunset Sunday evening and 3am Monday. Temperatures will be very cold with this system, which will result in a variety of impacts. With surface temperatures in the single digits, snow will easily accumulate on roadways throughout the event.
In addition, the cold layer of air aloft will result in very efficient snowfall production, meaning high snow to liquid ratios. In other words, it will not take as much liquid precipitation to produce a given amount of snowfall as usual. During a wetter snowfall event, snow to liquid ratios are usually near 10:1 (meaning 1″ of liquid = 10″ of snow), but for this event we are likely to see snow to liquid ratios approaching 20:1.
Bottom line, it looks like we will see widespread snowfall amounts in excess of 6″ across the Twin Cities, with most areas likely ending up in the 6-10″ range. Given the high snow to liquid ratios, this will also be a very dry, powdery snowfall. The image below shows projected snowfall amounts from the NAM Model across the Northern Plains (image source: College of DuPage), which are generally in line with our thinking. Models are actually in strong agreement with this system, which adds a degree of confidence to the forecast.
Snowfall rates should be decreasing by sunrise on Monday, but given the significant amount of overnight snowfall, significant travel impacts can be expected for the Monday morning commute. Winds will not be excessive with this event, but moderate breezes from the SE at 10-15mph Sunday night, and NW at 10-15mph on Monday could result in some areas of blowing snow, given the powdery nature of the snowfall.
Lingering light snow showers will remain possible into early Monday afternoon, before an extreme blast of arctic air arrives behind this system. Some models are projecting temperatures in Minneapolis-St. Paul to fall to 30 below or colder by Wednesday morning, which would tie or break a record low for the day. In addition, gusty winds Tuesday night/Wednesday morning are expected, which will result in excessively low wind chill values.
While snowfall Sunday night will result in plenty of work on Monday morning as far as snow removal goes, school districts and other organizations should prepare for the growing likelihood of dangerously cold temperatures and wind chills for the middle part of the week.