Snow fell for an extended period of time from after midnight early Monday morning February 21 to a few hours beyond midnight Wednesday morning February 23. Very cold northeast winds flowing over lake superior created conditions favorable for lake-effect snow. Technically when lake-effect snow is occurring during the influence of a storm system the term to use is lake enhancement. In this case, the northeast flow over Lake Superior was so cold that conditions were already optimum for pure lake effect before the snow from the storm even started. Temperatures in northeast Minnesota and northwest Wisconsin held in the single digits above zero most of the storm, although a few degrees warmer near Lake Superior. Even surface observations at Sky Harbor Airport showed temperatures starting in the teens then falling into the single digits.
The snow began initially after midnight as lake-effect, then more snow spread into Duluth and Superior from the west-southwest after 4 AM CST. The following image shows snow from Lake Superior already affecting in the city of Superior and brushing against Duluth as a larger area of snow approached from the west-southwest.
The lake-effect snow produced bursts of heavy snow within the larger snow area and resulted in localized snowfall accumulation rates of an inch or more per hour. The main band of snow affecting Duluth and Superior was most persistent along the immediate shoreline of Duluth and extending southwest into Gary New Duluth and also into Superior. Farther inland some enhancement from the lake got as far as the National Weather Service Office and the Duluth International Airport but was not nearly as persistent. The radar animation from early in the storm on Monday February 21 and valid from 5:15 AM to 6:34 AM CST shows two primary lake snow bands. One band was affecting Duluth and Superior. The other band extended across extreme northern Ashland County into northeast Bayfield County. During the day, these bands meandered a bit but for the most part affected the same areas resulting in some impressive snow totals. Some other bands of snow moved onshore from the lake but were more transient. Winds shifted from northeast to north late in the evening and pushed the snow bands out of Duluth and Superior. Lake-effect snow still continued to affect areas east of Superior into the overnight morning hours of Tuesday February 22. Steady light snow fell through the rest of the snowstorm's duration.
Snow totals reported to the National Weather Service in Duluth ranged from 8 to 10 inches inland from Duluth to as much as 20 inches from the harbor southwest to Gary New Duluth. Snow totals reported from Superior showed generally 12 to 15 inches with some isolated higher amounts. Also note that lake-effect snow produced excessive accumulations farther east in Bayfield and Ashland counties. Numerous reports indicate 20 to 30 inches fell. The town of Washburn was the big winner with up to 33.5 inches reported. The total at the National Weather Service was 10.4 inches. At this author's location near the ridge crest in Central Duluth 12.1 inches fell; good storm!
The first four weather maps show the development and movement of the surface low pressure system. The next four maps show the movement of the upper level support trough using an analysis of the 500 mb pressure level. Note how the surface low is far to the southeast of the geographic location of the upper system and well to the south of Duluth and the city of Superior. The far southern track placed Lake Superior deep into the cold air. Very cold air flowing over the lake created strong lapse rates resulting in unstable air to support lake-effect. The last map is 850 mb (or several thousand feet above the surface) and is valid for 6 AM CST which is shortly after the snow had started. This map helps further emphasize the depth and the strength of the cold air. Isotherms denoted by the dashed blue lines of the image show temperatures at 850 mb of approximately -16 to -14 oC. Note that the temperatures are listed colder to warmer.
