Radar loop, National Weather Service Forecast Office, Duluth, MN
Ending 5:31 PM CST (23:31 UTC), November 30, 2019

Brief summary

Snow totals

Wind gusts

Blatnik Bridge   59 MPH   0545 PM   11/30   46.75N/92.10W
Duluth International Airport   54 MPH   0526 PM   11/30   46.85N/92.20W

Weather observations

Radar

Water Vapor Satellite Images

Storm evolution at the surface

Storm evolution at 925 mb

Storm evolution at 850 mb

Storm evolution at 700 mb

Storm evolution at 500 mb

Storm evolution at 250 mb (jet stream level)

Why did so much snow fall in a short period of time?

To repeat, the vast majority of the snow fell in an 18 hour period. Multiple atmospheric and local-scale processes converged at the western tip of Lake Superior, including the Twin Ports region of Duluth, MN, and Superior, WI, to produce an impressive blitz of heavy snow. Most of the processes were strongly emphasized in the Area Forecast Discussions issued by the National Weather Forecast Office in Duluth prior to and near the onset of the event. Here is a list of these processes followed by a discussion of each. References are made to the radar, satellite, surface map, and upper air map images that are displayed prior to this section of the storm summary.

1. A Persistent deformation zone on the north side of a vertically stacked low pressure system.

2. A deep layer of east winds into the higher terrain of the North Shore of Lake Superior but then into the higher terrain near the South Shore late in the event as the winds became more north.

3. A Long fetch of east winds near the surface of Lake Superior to pick up additional moisture and funnel it into the head of the lake, plus the formation of lake-effect snow bands feeding on the moisture.

4. Apparent convective elements embedded in the larger area of snow.

5. A dendritic ice crystal growth zone.

Persistent deformation zone

The 500 mb map valid 00 UTC December 1, 2019 (6 PM CST November 30) shows the western Lake Superior region under an area of diffluent winds. The wind barbs show stronger winds east and west of the region suggesting that the air is accelerating away, supporting the idea of divergence that would induce vertical motion beneath the area of divergence. The water vapor satellite images show the center of the arc of the enhanced moisture band, associated with the deformation zone, pivoting over Northeast Minnesota and Northwest Wisconsin as the upper-level low pressure system moves east

Deep layer of east winds

At 00 UTC December 1, 2019 (6 PM CST November 30), the weather maps for the surface, 925 mb, 850 mb, 700 mb, and 500 mb all show east winds into the head of Lake Superior and into the higher terrain of Duluth, MN, and the North Shore. These east winds were driving high amounts of moisture into the area and producing a deep layer of lift into the higher terrain. This orographic lift and resultant cooling helped to condense out more of that moisture to enhance the snowfall. Late in the storm by 6 AM CST December 1, the winds shifted to the north-northeast to produce orographic lift up the higher terrain near the south shore.

Long fetch of east winds near the surface of Lake Superior

The long fetch of east winds over Lake Superior in the lower several thousand feet of air from the surface to 850 mb added additional moisture from the lake, not that any more was needed. The lake moisture contributed to snow production through the process of orographic lift and supported the formation of lake-effect snow bands. As noted at the end of the radar section by the individual radar image valid 1:59 AM CST November 30, 2019, snow bands were already moving onto the North Shore from Duluth, MN northeast to Silver Bay, MN. As the widespread heavy snow associated with the storm system moved north, the stronger reflectivities masked the visible evidence of the lake-effect snow bands. The lake contribution to moisture shifted more to the south shore by 6 AM CST December 1 when winds became more north as the surface low pressure system passed to the south across Southern Wisconsin and Northern Illinois.

Apparent convective elements

Dark green cellular structures with some speckles of yellow (35 dBZ) are evident on the radar loops, especially the first three, that cover the afternoon and the very early evening portion of the storm. The structures are less evident after that period of time. The temperatures at 700 mb at 12 UTC November 30, 2019 (6 AM CST) show a northward push of warm air toward northeast Minnesota and Northwest Wisconsin on the east side of the closed low pressure system. The tongue of warm air shifted east by 00 UTC December 1, 2019 (6 PM CST November 30). The warm air may have helped destabilize the atmosphere. Once the warm air moved to the east, the radar echos lost most of their cellular structure. A more detailed analysis is needed to determine the extent of convective potential.

Dendritic ice crystal growth zone

Dendrites are important to snowfall rates because their feathery tree branch type structure provides lots of surface area for the crystals to grow rapidly. They can also produce higher snow to liquid ratios when they are well-formed because their feathery structure allows them to accumulate more loosely than other ice crystals. The favorable temperature range for dendritic crystal growth is -10 to -20 ^oC, especially -12 to -18 ^oC, with a peak at -15 to -16 ^oC.

Some interpolation of upper air temperatures between the Minneapolis, MN and International Falls, MN observation sites indicates a dendritic crystal growth zone over Northeast Minnesota and Northwest Wisconsin at 00 UTC December 1, 2019 (6 PM CST November 30). The temperatures at 700 mb at 6 PM CST were -10 at Minneapolis and -8 at International Falls. The temperatures at 500 mb at 6 PM CST were -22 at Minneapolis and -19 at International Falls. Notice that the interpolated temperature over Duluth at 700 mb was very close to -10 which is the warm end of the dendritic crystal growth range. The interpolated temperature at 500 mb was around -20 which is the cold end of the dendritic crystal growth range. A quick and dirty calculation, using geopotential height values at Minneapolis and International Falls for 700 mb and 500 mb, shows that the dendritic crystal growth layer was 2557.5 meters deep or 8391 feet. Of course, only those parts of the layer that were saturated would support crystal growth. Also note that the temperatures used in the calculation might be a little too cool if temperatures from the Green Bay, WI upper air observation site are given at least a little influence on the interpolated values.

Both the moisture and vertical motion for most of this storm were very deep and extended through a range of temperatures beyond what is favorable for dendritic crystal growth. Other ice crystal growth habits and riming also affected the structure of the ice crystals. Strong winds caused increased collisions and fracturing. As a result, well-formed dendrites were not discernable for much of the storm and the snowpack was very dense. Personal observation noticed some increase in dendritic crystal structure during the mid to late overnight morning hours of the second day of the storm.

References

    9:58 PM CST Friday November 29, 2019
    3:36 AM CST Saturday November 30, 2019
    12:23 PM CST Saturday November 30, 2019
    4:01 PM CST Saturday November 30, 2019
    3:21 AM CST Sunday December 1, 2019