Source: National Centers For Environmental Information
A large late-season snowstorm dumped heavy snow over a four day period (Friday, April 13 to early Monday, April 16) across most of the southeast two-thirds of Minnesota and most of the central and north part of Wisconsin. Widespread snow totals of 10 to 20 inches were reported to the National Weather Service offices covering the event. Localized snowfall of 24 or more inches fell in Northeast Wisconsin including Green Bay with a narrow band of 30 inches or more just northwest of Green Bay. In Duluth, MN 10 to 16 inches fell in the higher elevations along and inland of the ridge crest. Similar amounts fell in the higher elevations south and east of Superior, WI. In the city of Superior itself and in the lower elevations of Duluth, 6 to 9 inches fell. Officially at the National Weather Service in Duluth, 13.4 inches fell. Personal measurement near the ridge crest in Central Duluth was 10.7. The storm produced winds from the northeast strong enough for the National Weather Service to issue a Blizzard Warning for the City of Superior and locations farther south and east.
A fair amount of convection enhanced the snow during the first part of the event, especially April 13 during the evening and extending into the overnight morning hours April 14. This part of the event stayed south of Duluth-Superior and affected areas from around Minneapolis and St Paul in Minnesota, east to the Fox Valley including Green Bay in Wisconsin. The radar mosaic shows the high intensities associated with the convection and there were a few reports of thunder with the snow.
On Saturday, April 14, The event began as some light snow showers off Lake Superior from around 5:30 AM CST and continued through most of the day and into the early evening. The snow was occurring due to winds off the lake ahead of the storm system that would eventually produce the heavy snow. A brief shot of light snow from the storm system moved through Duluth and Superior from 8:30 to 9:05 PM CST, then the steady snow started between 1 and 2 AM in the overnight morning hours Sunday, April 15. At 5 AM snow intensity increased and moderate to heavy snow fell all day to 8:30 PM. After 8:30 PM, much lighter snow fell through the night into the next daylight morning hours on Monday, April 16.
The following processes appear to have enhanced the snow:
1. Orographic lift due to the higher terrain
2. Convergence at the head of the lake due to high elevations on both sides of the lake funneling air into the head of the lake with frictional convergence occurring as the air crosses from the lake to the land
3. Lake enhancement due to cold unstable air over the lake.
The three following radar image loops show the character of the snow during the event. The first loop is in the daylight morning hours of Sunday, April 15. The second loop is midday. The third loop is early in the evening. Notice in the first two loops how even though elements of snow are moving from east to west, a blob of snow appears nearly stationary, attached over the western tip of Lake Superior. Notice in the third loop very small elements of heavy snow embedded in the larger snow area, especially across Northern Bayfield County and into Northeast and Central Douglas County in Wisconsin. Discerning whether or not these elements are lake-induced is a little tricky since the bands have the same orientation as the larger scale bands of snow associated with the large storm system producing the snow.
The following surface, 925 mb, and 850 mb atmospheric charts are valid for 7 AM CDT Sunday, April 15 (6 AM CST). The charts show winds in the first few thousand feet of the atmosphere. The 925 mb pressure level is roughly around 2500 feet. The 850 mb pressure level is typically between 4500 to 5000 feet. Note that the times along the bottom of the charts are in Greenwich Mean Time (GMT) so they are different from the time labels above each chart. The mean winds in the surface to 850 mb layer of the atmosphere were aimed into the head and north shore of Lake Superior. Later in the afternoon and into the evening, winds shifted into the south shore.
Personal observations noted that the ice crystal habit of the snow was mostly dendritic from after 9 AM to around Noon on Sunday, April 15. During this time, snow accumulated at least an inch an hour. Highly dendritic snowflakes were observed at times throughout the day but not as much as during the late morning hours.
Dendrites because of their feathery structure pack more loosely than other types of ice crystals. Less water content is necessary to get the same accumulation compared to snowflakes composed of other types of ice crystals. A countering factor limiting accumulation a bit would be the strong east to northeast winds occurring with the snow. Winds gusted frequently over 30 mph. A few gusts exceeded 40 mph. Peak wind for the day at the Duluth International Airport was 46 mph from the east. Peak wind the previous day was 49 mph from the east-northeast.
The onset of the snowstorm for the Duluth-Superior region was delayed by a blocking pattern in the higher levels of the atmosphere. The snow stayed to the south Friday and Saturday preventing what was a major storm from being a REALLY BIG major storm. The 500 mb pressure (approximately 18000 feet) atmospheric chart shown below, valid for Friday 7 PM CDT, April 13, shows a trough in Canada extending south to the northern edge of the Western Great Lakes. This trough at the time was holding back the northeast movement of the big low pressure system in the central United States. Again, don't forget that the time and date at the bottom of the chart are different since the times on these weather charts are typically in Greenwich Mean Time.
