Radar loop, National Weather Service Forecast Office, Duluth, MN
Ending 6:04 PM CDT (23:04 UTC), May 8, 2019
A strong late-season storm dumped 6 to 12 inches of wet, dense snow in the higher elevations of Duluth and the higher elevations south of Superior, WI from late afternoon on May 8 to the early morning daylight hours of May 9. Convection early in the event, including a few rumbles of thunder around 6 PM CDT, helped boost snowfall rates to 1 inch or more per hour in spite of ground temperatures and air temperatures above freezing. The temperature at the Duluth International Airport was consistently 34 degrees or warmer for most of the storm and only briefly dropped to 33 degrees. Snow totals varied greatly due to ground surface types, localized bursts of heavier snow, elevation, and proximity to Lake Superior. Snow only accumulated 1 to 4 inches at the bottom of the hill in Duluth and in Superior, Wisconsin. Precipitation in those areas began initially as rain or a rain-snow mix, then fell as snow for several hours, then changed back to rain during the night. The National Weather Service Forecast Office several miles inland near the airport measured 10.6 inches. The meteorologists in their Area Forecast Discussion issued at 7:18 PM CDT also noted they observed a brief period of thundersnow at the office. Personal observations of thunder, near the ridge crest of central Duluth, were made at 5:45 PM CDT, 6:01 PM CDT, and 6:08 PM CDT.
As a side note, another 2.4 inches of wet snow accumulated at the National Weather Service on May 19. Add in 0.3 inches that fell on May 1st and the total of 13.3 inches brakes the record for the month. The previous snowfall record for May was 8.1 inches set in May 1954. Enough snow fell on May 8th, 8.3 inches, to break the record for the most snow on any one day. The previous record was 5.5 set in 1902.
The most notable feature of the storm on May 8 to 9, other than the thundersnow and broken snowfall records, is that the atmosphere in the first few thousand feet cooled dramatically during the afternoon. This strong cooling occurred in spite of the strength of the May sun's radiation. Some apparent cooling mechanisms are listed below.
Cool air flowing off Lake Superior
Adiabatic cooling due to air rising up the hill (orographic lift)
Sublimation of snow into dry air
Evaporation into dry air of rain from the snow that was melting just above the ground
Partial or complete melting of snow
High intensity of precipitation that increased the rate of cooling from sublimation, evaporation, and melting
Concerning cool air off Lake Superior, the Area Forecast Discussion (AFD) issued by the National Weather Service Forecast Office in Duluth at 3:26 PM CDT Wednesday, May 8, mentioned a water temperature reading of 36 oF. Cooler air off the water mixing with the warmer air over the land, plus thickening clouds to reduce the sun's heating, caused temperatures at the surface to cool prior to the onset of precipitation. The temperature at the Duluth International Airport reached a high of 48 oF then fell back through the 40s during the early to mid-afternoon. Temperatures were around 40 oF at the start of the precipitation. Warmer air from over the lake usually reduces snow potential but in this case it seemed to help, especially when combined with other atmospheric cooling mechanisms.
The following surface and 500 mb pressure maps give an overview of the movement and intensification of the storm. The 500 mb level was chosen to show storm's upper air features.
Surface maps from the Weather Prediction Center's surface map archive
7:00 AM CDT, May 8, 2019 (12:00 UTC, May 8)
1:00 PM CDT, May 8, 2019 (18:00 UTC, May 8)
7:00 PM CDT, May 8, 2019 (00:00 UTC, May 9)
1:00 AM CDT, May 9, 2019 (06:00 UTC, May 9)
7:00 AM CDT, May 9, 2019 (12:00 UTC, May 9)
500 mb maps from the Storm Prediction Center's upper air map archive
7:00 AM CDT, May 8, 2019 (12:00 UTC, May 8)
7:00 PM CDT, May 8, 2019 (00:00 UTC, May 9)
7:00 AM CDT, May 9, 2019 (12:00 UTC, May 9)
Very dry air, especially just above the surface and extending several thousand feet above the ground, provided the condition for sublimation and evaporation of the snow and rain respectively to cool the air so snow could reach the surface. The following two upper air charts are for reference. In the following order and valid for early morning May 8, they are 925 mb at 7:00 AM CDT and 850 mb at 7:00 AM CDT. The altitude of a pressure level vary but 925 mb is approximately 2500 feet above mean sea level. The altitude of 850 mb generally averages a little less than 5000 feet above mean sea level. The tip of the wind barbs mark the location of the upper air stations. The temperatures at the upper left of the stations plots and the dew point depressions at the lower left of the station plots (or below the temperatures) are both in degrees celsius oC). Dew point depression is the difference between the temperature and the dew point.
Upper air maps from the Storm Prediction Center's upper air map archive
925 mb, 7:00 AM CDT, May 8, 2019 (12:00 UTC, May 8)
850 mb, 7:00 AM CDT, May 8, 2019 (12:00 UTC, May 8)
Notice the dew point depressions at 7:00 AM CDT at 925 mb and 850 mb at Minneapolis and International Falls in Minnesota, and at Green Bay, Wisconsin. A rough interpolation between the listed stations, plus considering some other readings at stations in southern Canada and northern Lower Michigan, yields dew point depressions at 925 mb and 850 mb over Duluth of -8 to -10 oC. The surface maps for 7 AM CDT and 1 PM CDT, in the storm evolution section, showed dew points holding steady in the middle to upper 20s degree Fahrenheit (oF). With surface temperatures around 40 (oF) in Duluth at the start of precipitation around 4 PM, enough cooling potential existed to drop temperatures to around 35 (oF) or a little cooler.