Widespread Damaging Winds December 16, 2021
Each yellow symbol on the image at right is a report of very strong wind or wind damage.
Summary
An unusually strong area of low pressure developed in the Central Plains and quickly tracked northeastward on Wednesday December 15, 2021, reaching far northeastern Minnesota early in the morning on December 16.
This low may have established a new record low pressure for the month of December in this part of the country (this is a preliminary and unofficial record).
The storm developed what it's known as a "sting jet" - a narrow band of very strong winds on its southern side that can be seen on satellite as thin curved band of moisture shaped like a scorpion's stinging tail.
This sting jet was responsible for the highest winds observed outside of the Keweenaw Peninsula. Except for at Houghton, the highest wind gust was measured at 68 mph at Ford Airport in Kingsford/Iron Mountain as this sting jet passed through. Upstream, Rhinelander, Wisconsin gusted to 76 mph as it moved through. Based on these two observations, and the number of power outages reported, it's likely that most of Iron County saw 65-75 mph wind gusts as well. Houghton gusted to 70 mph later in the day, not as the sting jet passed overhead but as a strong west-southwest gradient wind developed behind the low.
This storm resulted in widespread wind damage. There were numerous instances of downed trees including at least two which fell onto homes and at least one which caused injuries to those inside the home. Power outages were widespread as well, with many outages lasting into the following day, especially in Iron County.
Meteorology
Loop of 300, 500, 700, and 850 hPa showing a strong system quickly ejecting northeastward from the Rockies and Central Plains to western Lake Superior.
In the upper and middle levels of the atmosphere, 300 and 500 hPa, the system had a distinct negative tilt, meaning that if you were to draw a line down the center of the trough, it would lean back to the west. Such negative tilt systems are more prone to producing very strong surface storms.
At the surface, a strong area of low pressure developed in response to that strong negative tilt wave. The low raced northeastward from the High Plains to northeastern Minnesota. Watch as the isobars (the thick black lines) bunch closer and closer together, indicating a strengthening surface pressure gradient and strengthening winds.
A loop of the radar with the mean sea level pressure overlaid shows a thin arcing line of higher reflectivity values (yellows and reds) in Iowa and Wisconsin associated with the thunderstorms along the system's cold front.
This arcing line of storms produced a long swath of damaging winds through the Midwest. The amount of area that experienced damaging winds was great enough that this line of storms was classified as a derecho. Derechos usually occur in the hot and humid late summer months; a derecho in December is exceptionally rare.
This line of storms also produced around 100 tornadoes. No tornadoes occurred in Upper Michigan due to a snowpack in place over Upper Michigan and northern Wisconsin from snow storms on December 5 and 10. The warm air cooled as it moved over the extensive snowpack, limiting instability and the threat for tornadoes this far north. The last tornado from this line was in north-central Wisconsin.
For more on the widespread strong winds and tornadoes in the Plains and Midwest, see reviews from NWS Twin Cities , NWS La Crosse , NWS Quad Cities , and NWS Des Moines .
This storm then ended with enough cold air rushing in behind it for lake effect snow.
This is a single image - not a loop - of a couple different parameters meteorologists use to forecast strong winds. This image is valid at 5:00 am EST Thursday December 17 - just about when the strongest winds were moving through Upper Michigan.
In the top left, the blue shading indicates colder air moving in behind the system's cold front. This lead to a bigger difference between surface temperature and temperature aloft. We call this rate of cooling the lapse rate. As the lapse rate approaches or even exceeds 9 C/km, stronger wind from aloft mixes down to the surface more easily. You can see the result of this colder air moving in as a pocket of orange and red shading in the top right where the lapse rate is very high.
In the bottom left we're looking at the 3-hour change in mean sea level pressure. Strong couplets between pressure rises (in shades of yellow and orange) and pressure falls (shades of teal) are locations to watch for strong winds. Notice that the area between pressure falls and pressure rises matches up with the area where the lapse rate is the highest!
The bottom right image shows the magnitude of the pressure gradient (how close together the isobars are to each other). A stronger gradient means stronger winds.
On water vapor satellite, the storm's "sting jet" shows up as a curved band moisture (blue shading) that's shaped like a scorpion's stinging tail.
Many of the highest wind reports from this event were in Iowa from the line of thunderstorms associated with the cold front. But you'll also notice high wind reports in southern Minnesota, central and northern Wisconsin, and eventually the U.P. as that curved tail lifted through those areas.
(The highest wind gust in the area - 77 mph at Stannard Rock Lighthouse - actually occurred several hours after the end of this loop.)
Historical Context
As alluded to earlier, this storm was as strong or stronger than any storm in this area on record for the month of December.
There are several different ways to put that strength into context.
This is a map produced by NOAA's Weather Prediction Center showing the record low mean sea level pressure in each location in mb.
On this map an older convention is used where the leading 9 is omitted, as is the decimal at the end. For example, "853" would mean 985.3 mb.
You can see the number at Duluth was 766, meaning the record mean sea level pressure during the month of December there was 976.6 mb. To the northeast, at Thunder Bay, Ontario, the record low is 978.8 mb.
The analysis shown in the last section showed the low strengthening to around 976 mb in far northeastern Minnesota, right around the record for December!
Maps for each month, as well as for the entire year, can be found here: https://www.wpc.ncep.noaa.gov/research/roth/SLPrecords.html
Another way to look at this is to take the average of 57 different model runs (37 of the American GFS model and 20 of the Canadian GDPS model) and then compare that average value to climatology for a three-week period at the same time of year.
In this image, the bright pink shading is all areas where the average mean sea level pressure of those 57 model runs is lower than anything in the historical record between the days of December 6 and 27, in the years 1979 to 2009. This is an indication of an extremely strong storm.
A similar tool, though a bit more complicated, is the Extreme Forecast Index (EFI) produced by the European model.
Its ensemble consists of 51 individual runs. Here we see that the forecasted EFI (shading) was very high. An EFI of 1 indicates a truly extreme event - one that has never happened before - and anything close to 1 indicates a highly unusual event. In this case, the red and even pink shading indicate a high certainty from the European model that wind gusts would be very unusually high for this time of year, perhaps even higher than anything ever observed before.
Social Media
Tuesday evening's NWS weather story highlighting strong winds and the possibility of lakeshore flooding with the upcoming storm
Wednesday morning's NWS weather story highlighted the very strong winds that were expected
Messaging on Wednesday morning also continued to highlight the lakeshore flooding threat
Wednesday evening update that emphasized the likely impacts likely from this unusually strong wind storm
Update Thursday morning as the strong winds were under way. This update also introduced a new hazard: the transition to lake effect snow behind the storm and the possibility of blizzard conditions in the Keweenaw as the strong winds continued.
Storm Reports
It would have been impossible to document each and every downed tree and power line with this storm. However, here is a partial listing of all the strongest wind gusts and reports of wind damage. Click on the map for details of each report. Note: the times displayed are in GMT. Subtract 5 hours for Eastern Standard Time.
Dec 16, 2021 Wind Reports
