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

Storm discussion outline

• Brief summary
• Snow totals
• Wind gusts
• 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?
• Persistent deformation zone
• Deep layer of east winds
• The formation of lake-effect snow bands
• Apparent convective elements
• Deep dendritic ice crystal growth zone
• References

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

KDLH 011255Z 03011G21KT 1/2SM R09/3500V5500FT -SN BLSN VV009 M04/M06 A2970 RMK AO2 PK WND 05026/1202 SLP077 SNINCR 1/33 P0000 T10391056 $

KDLH 011155Z 03016G27KT 1/2SM R09/5000VP6000FT -SN BLSN VV013 M03/M05 A2967 RMK AO2 PK WND 03027/1147 SFC VIS 3/4 SLP068 SNINCR 1/32 P0007 60041 70136 T10331050 11017 21033 53014 $

KDLH 011055Z 04016G32KT 1/2SM R09/4000V4500FT -SN BLSN VV008 M03/M04 A2966 RMK AO2 PK WND 06032/1029 SLP065 SNINCR 1/31 P0006 T10281044 $

KDLH 010955Z 06018G31KT 1/2SM R09/3000V4000FT SN BLSN VV006 M02/M03 A2962 RMK AO2 PK WND 07033/0859 SLP052 SNINCR 2/30 P0007 T10171028 $

KDLH 010855Z 07018G30KT 1/2SM R09/4000V5500FT SN BLSN VV006 M02/M03 A2962 RMK AO2 PK WND 07037/0759 SLP052 SNINCR 2/28 P0007 60021 T10171028 56007 $

KDLH 010755Z 08023G38KT 1/2SM R09/4000V4500FT SN BLSN VV006 M02/M03 A2963 RMK AO2 PK WND 07038/0755 SLP052 SNINCR 2/26 P0008 T10171028 $

KDLH 010655Z 07024G36KT 1/2SM R09/3500V4500FT SN BLSN VV006 M02/M03 A2963 RMK AO2 PK WND 09042/0629 SLP054 SNINCR 2/24 P0006 T10171033 $

KDLH 010555Z 08023G36KT 1/4SM R09/4500V6000FT +SN BLSN VV006 M02/M03 A2964 RMK AO2 PK WND 08036/0547 SLP059 SNINCR 2/22 P0011 60054 T10171028 11017 21022 410111022 58002 $

KDLH 010455Z 08022G36KT 1/4SM R09/2600V3500FT +SN BLSN VV004 M02/M03 A2966 RMK AO2 PK WND 08038/0425 SLP064 SNINCR 2/20 P0008 T10171033 $

KDLH 010355Z 08020G34KT 1/4SM R09/2400V3000FT +SN BLSN VV004 M02/M03 A2965 RMK AO2 PK WND 08042/0337 SLP062 SNINCR 1/18 P0010 T10171033 $

KDLH 010255Z 08022G37KT 1/4SM R09/1800V2200FT +SN BLSN VV003 M02/M03 A2965 RMK AO2 PK WND 07044/0234 SLP062 SNINCR 1/17 P0013 60025 T10171033 56010 $

KDLH 010155Z 08026G44KT 1/4SM R09/1800V2200FT +SN BLSN VV003 M02/M03 A2966 RMK AO2 PK WND 09047/0137 SLP066 SNINCR 1/16 P0006 T10171028 $

KDLH 010055Z 08029G44KT 1/4SM R09/2600V3000FT +SN BLSN VV004 M02/M03 A2967 RMK AO2 PK WND 09044/0026 SLP067 SNINCR 1/15 P0006 T10171028 $

KDLH 302355Z 08027G38KT 1/4SM R09/3000V3500FT +SN BLSN VV005 M02/M03 A2968 RMK AO2 PK WND 07047/2326 SLP072 SNINCR 1/14 P0005 60035 T10171028 11011 21017 56021 $

KDLH 302255Z 09025G37KT 1/8SM R09/3500V5000FT +SN BLSN VV005 M02/M03 A2971 RMK AO2 PK WND 09041/2237 SLP083 SNINCR 1/13 P0014 T10171033 $

KDLH 302155Z 08021G37KT 1/16SM R09/1400V2000FT +SN BLSN VV004 M02/M03 A2972 RMK AO2 PK WND 08041/2134 SLP082 SNINCR 1/12 P0009 T10171028 $

KDLH 302055Z 08024G36KT 1/8SM R09/2000V2800FT +SN BLSN VV005 M02/M03 A2975 RMK AO2 PK WND 10036/2055 TWR VIS 1/2 SLP094 SNINCR 1/11 P0004 60007 T10171028 56024

KDLH 301955Z 08019G33KT 1/4SM R09/4000V5000FT +SN BLSN OVC009 M01/M03 A2977 RMK AO2 PK WND 06035/1933 TWR VIS 1/2 SLP102 SNINCR 1/10 P0003 T10111033

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. The formation of lake-effect snow bands feeding on the moisture from lake.

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 additional moisture from a long fetch off of the lake and producing deep layer lift of air up the higher terrain. This orographic lift and resultant cooling helped to condense out the moisture from the lake and from the storm itself to enhance the snowfall. Later in the storm, by 6 AM CST December 1, the winds shifted to the north-northeast to produce lift up the higher terrain of the south shore.

The formation of lake-effect snow bands

A seen at the end of the radar section in the image valid at 1:59 AM CST November 30, 2019, snow bands were already moving into the region of North Shore from Duluth northeast to Silver Bay. As widespread heavy snow associated directly with the storm system moved north, the stronger reflectivities masked the some of the visible evidence of the snow bands. As with the orographic enhancement, the lake effect shifted more to the south shore when winds took on a more north component as the surface low pressure center 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.

Deep 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