Update: Mark Seeley's regular Friday morning chat with Cathy Wurzer on MPR's Morning Edition, Dec. 17 edition, covered the tornadoes, the storm and related climatology. Read and listen on MPR.
Visit the WeatherTalk blog for the original post and for more information from Mark Seeley, University of Minnesota Extension climatologist emeritus. It is a product of the Minnesota Climate Adaptation Partnership. Listen to Seeley's climate and weather observations on Minnesota Public Radio's Morning Edition each Friday around 6:50 a.m.
Special Comments on the Storm Predicted for Dec. 15, 2021
I have never seen such a well-defined bullseye for the moderate risk of severe weather over Minnesota in December (released by the NOAA-SPC)
Minnesota has never documented a tornado anywhere in the state for December, January, or February. Historically the earliest calendar occurrence was March 6, 2017, and latest Nov. 16, 1931
Among our neighboring states climate histories: Wisconsin has documented six December tornadoes, Iowa has documented four December tornadoes, South Dakota and North Dakota have not documented any tornadoes in December-February. Thus, this historical probability for December tornadoes in Minnesota, South Dakota and North Dakota is zero.
From noted scholar Dr. Harold Brooks at the NOAA Severe Storms Center in Oklahoma: the daily probability for tornadoes to occur anywhere in the USA landscape ranges from only 7 percent to 19 percent historically (even the southern states) during the month of December (highlighting how unusual the outbreak was on Dec. 10th across Illinois, Missouri, Arkansas, Tennesee and Kentucky.
Weather Set Up for Wednesday, Dec 15.: Fog in some areas, due to advection of warm, moist air overnight (Tue); rapid melting of snow will feed further moisture into the overlying layer of air, elevating dew points into the 50s F for southern Minnesota areas (record-setting high dew points for this time of year); MSP is forecasted to have a 55°F dewpoint, which would be the highest measured dew point historically for the months of December through February 20. A rapid loss of snow cover may trigger further rapid warming of the surface landscape in some areas, so that 60°F or higher may be measured in many parts of Minnesota. I don’t know that we have ever had a tornado in Minnesota when the dew point was below 50°, but that is just a guess. Our statewide record high temperature for Dec. 15th is 63° F at Kinbrae (Nobles County) back in 1891. Forecasted highs for portions of southeastern Minnesota are 60-65°F for Wednesday.
If the MSP radiosonde (balloon system) from 6 p.m. Wednesday evening reports a precipitable water value (integrated vertical water vapor measurement) of 1.13 inches or greater it would be the highest value ever measured during the winter months of December through February, according to Kenny Blumenfeld of the Minnesota State Climatology Office.
Besides the rise to perhaps record-setting temperatures and dew points in southern Minnesota tomorrow (MSP records for Dec. 15 are: max temp 51°F in 2014; dew point 49°F also in 2014), other features to pay attention to are: (1) record low barometric pressure readings; (2) though cloud tops in the late afternoon and evening will not be extremely high, convection could produce some thunderstorms and lightning; (3)wind velocities will be very high (50-70 mph in some areas), and perhaps sustained for several hours; (4) rapid loss of snow cover due to melting may enhance some local scale heating and instability for thunderstorm cells; (5) wind shear (horizontal) aloft will be conducive perhaps to rotation in the clouds, also enhanced by greater helicity (updraft rotation) in the lower levels of the atmosphere, so tornadoes are not out of the question; (6) there will be a rapid freeze up on most places as temperatures drop 30-40 degrees in less than 12 hours.
Additional dangers may be posed by severe storms occurring after sunset (in the dark) and well into the evening, and the sustained and freezing up of the rain on the surface of roads, sidewalks and highways which will make driving and walking on Thursday and Friday a challenge for many.
Historical perspective: This storm system may indeed be a singularity (one off) in Minnesota history for the month of December considering all of its unusual attributes together. One historical analogy is the storm of March 31, 2014 over southwestern Minnesota where citizens in Yellow Medicine, Lincoln, Lyon and Yellow Medicine counties had to endure both a blizzard warning and a tornado warning simultaneously during the afternoon, another singular weather episode in Minnesota history. Reality check is that both of these unusual storm systems may be tied to our changing climate.