Precipitation started as rain in the early afternoon on March 10 then changed to snow in the middle of the afternoon, at least on top of the hill in Duluth. The snow occasionally fell with some large 1 to nearly 2 inch diameter snowflakes. The snow became heavier with increasing strong winds late in the afternoon followed by an all out blitz during the evening. Snow accumulated an inch per hour for about five hours from 5 PM to between 9 PM and 10 PM CST. SNINCR remarks were edited into the weather observations for the Duluth International Airport for each hour during that time. A break in precipitation occurred around 10 PM between the dry slot along the south edge of the northward moving precipitation and wrap around precipitation that was approaching the area from the west. The wrap around precipitation produced another quick inch of snowfall in an hour from midnight to 1 AM. Winds during the late afternoon and evening gusted frequently over 40 mph with several gusts in excess of 50 mph. Peak gust in the weather observations was 55 mph from the east. The observations also included two PRESFR (pressure falling rapidly) remarks as the center of the low pressure system at the surface approached from the southwest. Much lighter snow continued through the rest of the night and into the early daylight morning hours.
The National Weather Service located near the airport reported 7.1 inches of dense, soggy snow. Personal measurement was 6.9 inches. Duluth remained near the edge of a mixed precipitation zone for most of the storms duration. Snow occasionally became mixed with snow pellets and briefly with sleet. Reports given to the National Weather Service from locations north and west of Duluth, and thus deeper into the cold air, showed accumulation totals of 8 to 11 inches.
The following series of National Weather Service radar mosaic images provided by the National Centers for Environmental Information (NCEI) showed a strong radar signature as the most intense precipitation moved through the area. Some of the strong reflectivity values (yellow to red) were due to water coated snowflakes and mixed precipitation. Also notice the cellular convective structure of the precipitation approaching from the south, especially on the animated radar image located at the beginning of the storm discussion. If you look closely at the bottom of the animation you can see the remains of a severe thunderstorm warning that was issued by the Twin Cities/Chanhassen National Weather Service office. The fourth and fifth of the non animated images show the brief break that occurred mid to late evening, then another shot of heavy snow associated with a wrap-around/comma head feature.
7 PM CST March 10, 2021 (0100 UTC March 11) - NCEI
8 PM CST March 10, 2021 (0200 UTC March 11) - NCEI
9 PM CST March 10, 2021 (0300 UTC March 11) - NCEI
10 PM CST March 10, 2021 (0400 UTC March 11) - NCEI
12:55 AM CST March 11, 2021 (0655 UTC March 11) - NCEI
The upper-level water vapor (band 8) satellite image below is supplied by the National Environmental Satellite, Data, and Information Service (NESDIS). The image from early in the evening on March 10 showed an enhanced region of moisture was moving over the western tip of Lake Superior from the south, followed by a strong dry slot. The enhancement correlated with moderate to heavy precipitation occurring in East Central and Northeast Minnesota and Northwest Wisconsin. Farther to the southwest, satellite showed a swirl associated with the upper level part of the storm. That feature produced the wrap-around snow that fell from midnight through the early morning of March 11.
6:36 PM CST March 10, 2021 (0036 UTC March 11) - NESDIS
The movement and development of the surface low pressure system is shown by the series of high resolution surface maps from the NOAA Weather Prediction Center. The rapid movement helped trigger the "pressure falling rapidly" (PRESFR) reports in the surface observations from The Duluth International Airport.
3 PM CST March 10, 2021 (2100 UTC March 10) - WPC
6 PM CST March 10, 2021 (0000 UTC March 11) - WPC
9 PM CST March 10, 2021 (0300 UTC March 11) - WPC
12 AM CST March 11, 2021 (0600 UTC March 11) - WPC
3 AM CST March 11, 2021 (0900 UTC March 11) - WPC
The two upper air maps from the Storm Prediction Center of the National Weather Service are valid for approximately 6 PM CST March 10 (0000 UTC March 11). The first map is the 300 mb pressure showing the jet stream flow. The second map is 500 mb and is representative of the upper level component of the storm system. At 300 mb the Duluth area was under the influence of a coupled jet structure with the right rear quadrant of a jet streak to the north and the left front quadrant of a jet streak to the south. Both of the identified quadrants of the upper level jets are favorable for upper level divergence and consequently upward vertical motion of the air below the altitude of the jets. Strong vertical motion was implied over Duluth as it was beneath the combined influence of both quadrants. The 500 mb map shows that the upper level trough of the storm was just entering Minnesota from the southwest.
6 PM CST March 10, 2021 (0000 UTC March 11) - SPC
6 PM CST March 10, 2021 (0000 UTC March 11) - SPC
The following four maps show that the precipitation type was a close call in terms of getting snow. All of the maps are valid for 9 PM CST March 10, 2021 or same as 0300 UTC March 11. The first map is a repeat from the surface map sequence. The next three maps are model initializations valid at 0300 UTC, same time as the surface map, for the High Resolution Rapid Refresh (HRRR) model available from the NOAA Global Systems Laboratory. The three maps, in order, show temperatures in degrees oC for 925 mb, 850 mb, and 700 mb. Winds are also plotted.
The temperatures from the surface up through 925 mb were slightly below freezing (0 oC). Temperatures at 700 mb were also below freezing. In the middle, temperatures at 850 mb were right at freezing with the 0 oC contour impinging on Duluth from the southeast. A warm layer between two cold layers makes for a favorable temperature profile to produce wet snow, snow pellets and sleet. The warm middle layer and the bottom cold layer both being close to either side of freezing was particularly favorable for wet snow.
Surface, 9 PM CST March 10, 2021 (0300 UTC March 11) - SPC
HRRR-NCEP 0-hour forecast for temperature at 925 mb valid 0300 UTC March 11 (9 PM CST March 10)
HRRR-NCEP 0-hour forecast for temperature at 850 mb valid 0300 UTC March 11 (9 PM CST March 10)
HRRR-NCEP 0-hour forecast for temperature at 700 mb valid 0300 UTC March 11 (9 PM CST March 10)
Here is a list of METARs from the Duluth International Airport (KDLH) for true "weather geeks" who like to dig into the details. The observations run from approximately 3 PM March 10 to 8 AM March 11 CST but are listed in reverse chronological time, meaning that the earliest observation is at the end of the list and the latest observation is at the top of the list. The observation times are in "Z" for Zulu, the same as UTC.
Click HERE for a consolidated list of METAR abbreviations and acronyms if you see code that you do not recognize.
KDLH 110755Z 03011KT 3/4SM R09/6000VP6000FT -SN VV009 M02/M03 A2946 RMK AO2 PLE12 SLP991 P0007 T10171028 $ KDLH 110748Z 03011KT 3/4SM R09/5500VP6000FT -SN BLSN VV009 M02/M03 A2945 RMK AO2 PLE12 P0007 T10171028 $ KDLH 110734Z 04010KT 3/4SM R09/P6000FT -SN BLSN VV010 M02/M03 A2945 RMK AO2 PLE12 P0006 T10171028 $ KDLH 110718Z 05010G22KT 3/4SM R09/P6000FT -SN BLSN VV009 M02/M03 A2945 RMK AO2 PLE12 P0005 T10171028 $ KDLH 110709Z 05014G22KT 3/4SM R09/P6000FT -PLSN BLSN OVC009 M02/M03 A2945 RMK AO2 P0003 T10171028 $ KDLH 110655Z COR 06018G25KT 1/2SM R09/P6000FT PLSN BLSN BKN008 OVC011 M02/M03 A2944 RMK AO2 PK WND 08031/0619 PLB29 SLP985 SNINCR 1/7 P0007 T10171028 $ KDLH 110645Z 07012G20KT 1/2SM R09/4500VP6000FT -PLSN BLSN OVC007 M01/M03 A2944 RMK AO2 PK WND 08031/0619 PLB29 P0004 T10111028 $ KDLH 110630Z 07015G30KT 1SM R09/6000VP6000FT -PLSN BLSN OVC007 M02/M03 A2943 RMK AO2 PK WND 08031/0619 PLB29 P0002 T10171028 $ KDLH 110622Z 08019G31KT 3/4SM R09/6000VP6000FT -SN BLSN VV006 M01/M03 A2943 RMK AO2 PK WND 08031/0619 P0002 T10111028 $ KDLH 110555Z 08020G33KT 3/4SM R09/5000V6000FT -SN BLSN OVC003 M01/M02 A2941 RMK AO2 PK WND 08048/0501 SLP975 P0000 60038 T10111022 10006 21011 400891011 55011 $ KDLH 110455Z 09023G39KT 1/2SM R09/3500V5500FT -SN BLSN VV003 M01/M02 A2941 RMK AO2 PK WND 10045/0435 SNE0357B15 SLP974 P0000 T10061017 $ KDLH 110436Z 09031G45KT 3/4SM R09/5000VP6000FT -SN BLSN OVC003 M01/M02 A2943 RMK AO2 PK WND 10045/0435 SNE0357B15 P0000 T10061017 $ KDLH 110424Z 08031G42KT 1SM R09/P6000FT -SN BLSN OVC004 M01/M02 A2943 RMK AO2 PK WND 08042/0419 SNE0357B15 P0000 T10061017 $ KDLH 110355Z 08026G46KT 1SM R09/P6000FT -SN BLSN OVC006 00/M01 A2944 RMK AO2 PK WND 08046/0346 SLP985 SNINCR 1/5 P0009 T00001011 $ KDLH 110352Z 08031G46KT 1SM R09/P6000FT -SN BLSN OVC006 00/M01 A2944 RMK AO2 PK WND 08046/0346 PRESFR SNINCR 1/5 P0009 $ KDLH 110255Z 09024G36KT 1/2SM R09/3000V4500FT SN FG VV005 00/M01 A2955 RMK AO2 PK WND 08036/0246 SLP023 SNINCR 1/4 P0012 60029 T00001011 56021 KDLH 110155Z 08020G30KT 1/2SM R09/3500V4500FT SN FG VV005 01/M01 A2957 RMK AO2 PK WND 08034/0118 SLP030 SNINCR 1/3 P0010 T00061011 KDLH 110134Z 08016G28KT 1/2SM R09/4000V5500FT SN FG OVC005 01/M01 A2959 RMK AO2 PK WND 08034/0118 TWR VIS 2 P0004 T00061011 KDLH 110126Z 08017G34KT 1SM R09/4500VP6000FT -SN BR OVC005 01/M01 A2959 RMK AO2 PK WND 08034/0118 TWR VIS 2 P0002 T00061011 KDLH 110110Z 07014G26KT 2SM -SN BR OVC006 01/M01 A2960 RMK AO2 PK WND 07028/0059 SFC VIS 3 P0000 T00061011 KDLH 110055Z 07021G28KT 1 1/2SM -SN BR BKN006 OVC011 01/M01 A2959 RMK AO2 PK WND 07028/0054 TWR VIS 2 SLP037 SNINCR 1/2 P0007 T00061011 KDLH 110031Z 07014G24KT 1SM R09/P6000FT -SN BR BKN005 OVC009 01/M01 A2961 RMK AO2 PK WND 07026/0004 TWR VIS 2 P0005 T00061011 KDLH 102355Z 08017G25KT 1/2SM R09/4000FT SN FG VV006 01/M01 A2962 RMK AO2 PK WND 07026/2341 TWR VIS 2 SLP044 SNINCR 1/1 P0011 60052 T00061006 10050 20006 58023 KDLH 102310Z 06013G23KT 3/4SM R09/4000V4500FT -SN BR BKN006 OVC009 01/M01 A2964 RMK AO2 TWR VIS 2 P0003 T00111006 KDLH 102255Z 06016G23KT 1 1/2SM R09/4500VP6000FT -SN BR BKN007 OVC010 01/M01 A2964 RMK AO2 TWR VIS 2 SLP051 P0014 T00111006 KDLH 102236Z 06016G22KT 1SM R09/5000VP6000FT -SN BR OVC007 01/M01 A2964 RMK AO2 TWR VIS 2 P0012 T00111006 KDLH 102218Z 07012G20KT 2SM R09/3500VP6000FT -SN BR OVC005 01/M01 A2966 RMK AO2 P0008 T00111006 KDLH 102202Z 06017G22KT 1/2SM R09/4000V5500FT SN FG OVC005 01/M01 A2964 RMK AO2 TWR VIS 2 PRESFR P0002 T00111006 KDLH 102155Z 06013KT 1SM R09/4500VP6000FT -SN BR OVC006 01/M01 A2967 RMK AO2 TWR VIS 2 RAE42SNB22 SLP062 P0010 T00111006 $ KDLH 102151Z 07012KT 1SM R09/5000VP6000FT -SN BR OVC006 01/M01 A2967 RMK AO2 TWR VIS 2 RAE42SNB22 P0009 $ KDLH 102140Z 07012KT 4SM -RASN BR BKN006 OVC026 01/M01 A2967 RMK AO2 SNB22 P0008 T00111006 $ KDLH 102129Z 07013KT 3SM -RASN BR SCT007 OVC025 01/M01 A2968 RMK AO2 VIS 1 1/2 RWY 27 SNB22 P0007 T00111006 $ KDLH 102055Z 10006KT 3SM RA BR OVC024 02/01 A2969 RMK AO2 VIS 2 RWY 27 SLP066 P0017 60017 T00220006 55011 $