We can gauge how rare or anomalous a weather system is by comparing certain meteorological aspects of the event to climatology. This was quite unusual event for mid-December with respect to how warm and unstable the airmass was, how far north and widespread the severe convective weather traveled for early winter, and how widespread and intense the synoptic-scale winds accompanying the cyclone were. In this regard, let’s take a quick look how anomalous the max temperatures, instability (CAPE), wind speeds and mean sea level pressure (MSLP) were in this event.

First, let’s look at the strength of the surface low pressure system. Figure 1 illustrates consecutive forecasts of a model ensemble average forecast MSLP all valid at midnight (06Z) Thursday December 16th. The colored (shaded) areas denote the forecast surface pressure and how it compares to 30 year climatology of surface pressure. What this animation tells us is that the surface pressure at that location and time was forecast to be lower than any surface pressure observed (at that location and time) over the past 30 years!

We also observe that the ensemble forecasts were fairly consistent in forecasting anomalously low pressure near the Duluth, MN area for consecutive forecast cycles. This is valuable information as it helps the forecaster put into historical perspective the model forecast and more importantly, helps to message the potential impacts appropriately so that our core partners and the general public can prepare!

Figure 1 – NAEFS Forecast MSLP Percentiles (relative to climatology) valid 06Z December 16, 2021

Naturally, the intensity of the surface cyclone would then also impact surface sensible weather elements such as temperature and wind. Figure 2 is a forecast that indicated how anomalous or extreme the daytime maximum surface temperatures would be over the western Great Lakes. Areas shaded in the pink colors are near 1 which suggests daytime highs are forecast to be extremely warm (rare) for mid-December. Many locations within the pink shaded areas shattered their daytime record highs. An unseasonably warm airmass over the western Great Lakes also implies potential for more instability.

Figure 3 is the Extreme Forecast Index (EFI) for CAPE (a measure of instability). Again, we see EFI values of 1, indicating CAPE was forecast to be unusually high (rare) over the western Great Lakes for mid-December. This instability is what fed the intense, damaging wind and tornado-producing squall line.

Finally, Figure 4 shows the EFI for forecast wind gusts. Again, we see EFI values very close to 1 (red & pink shades) from the Central Plains to the Great Lakes. In summary, the intense cyclone of December 15-16, 2016 was truly a rare event in terms of the cyclone strength and its impacts!

Figure 2- ECMWF Extreme Forecast Index (EFI) for
Maximum Temperature

A deep 500mb trough associated with strong short-wave upper-level jet energy, organized over the western U.S. early on Wednesday December 15 and quickly moved northeast toward the upper Mississippi Valley by 6 am Thursday morning December 15th (Figure 5). Very strong upper-level divergence and vertical motion developed ahead of the shortwave energy leading to the rapid development of surface low pressure over eastern Colorado by midday Wednesday December 15th. The surface low rapidly intensified as it moved across the Central Plains into the upper Mississippi Valley (Figure 6).  

Take particular note of the explosive and expansive cloud development that initiated over the Central Plains as the system encountered unseasonably deep moisture and instability already in place in that area. Also note how the middle and upper-level clouds quickly wrap cyclonically (counterclockwise) around the rapidly intensifying upper-level circulation (Figure 7).

Figure 5 - RAP 500mb Analysis (animation):
6 am Wednesday 12/15 to 6am Thursday 12/16

Figure 6 - RAP MSLP Analysis (animation):
9am Wednesday 12/15 to 6 am Thursday 12/16

Figure 7 - GOES 16 WV imagery and MSLP (animation):
3am Wednesday 12/15 to 9 am Thursday 12/16

Let’s take a brief closer look at the initial explosive development over the Central Plains during the afternoon hours of Wednesday December 15th. The animation in (Figure 8) highlights the rapid early development of the system as it moved east from the eastern Colorado Plains area. First, note the explosive development of an extensive severe convective squall line with its associated lightning flashes. The squall line raced eastward at 55 to 65 mph and extended from Oklahoma northward to southeast South Dakota by the end of the loop. The squall line continued to race northeastward across Iowa, southeast Minnesota and central Wisconsin before weakening shortly before midnight. A staggering number of hurricane force wind gusts (> 75 mph) were reported with the squall line.  In fact, preliminary results indicate that 45 such reports were received which would be a daily record, even surpassing the previous record (44 reports) on June 6, 2020 (Iowa Derecho).

Keep in mind that this record was broken in the middle of December! Another interesting aspect illustrated in Figure 8 is the extreme wind gusts that occurred behind the initial squall line. Note the cyclonic swirl that moved off the Colorado Plains eastward into Kansas. The “X” denotes the approximate location of the low-level circulation center. Near the southern end or tail of the cyclonic swirl, locally higher wind gusts in excess of 80 mph are noted. These wind gusts likely knocked down power lines that initiated and exacerbated fires in central Kansas. The yellow circle near the end of the animation highlights hot spots associated with fires in that area as well as an 87 mph wind gust!

Figure 8 - GOES 16 Enhanced Visible Imagery (animation):
12 pm to 430 pm 12/15

The already strong upper-level circulation continued to intensify as it approached the western Great Lakes region during the early morning hours of December 16th. Let’s take another look at this circulation as it moved across northern Wisconsin overnight. Figure 9 highlights specially enhanced composite nighttime infrared imagery referred to as the Airmass RGB.

This imagery helps to reveal details in the cloud characteristics and general characteristics of the surrounding airmass. This enhancement provides somewhat of a 3D feel to the imagery. Deep and/or high clouds in the imagery appear whitish with a greenish hue, while lower level clouds appear in muted, darker cream tones. The brownish/amber hues in the imagery indicate drier air, while bluish hues suggest relatively higher moisture. Take particular note of the southern edge or tail of the circulation as it wraps southeast then eastward across northern Wisconsin in Figure 9.

Figure 9 - GOES 16 Enhanced (Airmass RGB) Infrared Imagery (animation):  12 am to 345 am 12/16

As we saw over central Kansas earlier in the day, we do see evidence of enhanced wind gusts near this cloud feature.  The black line on the imagery represents the leading edge of the tail feature as it evolved and  surges eastward across northern Wisconsin. As the cloud tail approached the Rhinelander, Wisconsin area (yellow circle), note the 66 mph wind gust.

In fact, a peak wind gust of 76 mph was reported at the Rhinelander airport shortly before 3 am (Figure 10). This was the highest measured wind gust in Wisconsin during the entire event. Rhinelander is at an elevation of about 1600 ft AGL and therefore is somewhat more susceptible to stronger winds (Figure 11). 

The possible relationship between the 76 mph wind gust near Rhinelander and the aforementioned "tail" cloud feature in the enhanced GOES16 imagery is not well understood. With intense cyclones, excessive wind gusts have been known to occur near the tail of the comma head for various reasons, including a phenomenon referred to as a sting jet. Regardless of the possible cause, it is interesting to note the strong wind gusts observed near this tail feature over northern Wisconsin, and earlier in the day over central Kansas.

Figure 10 - Time series of wind direction, wind speed and wind gust at Rhinelander, WI (RHI)

This event was unique in that extreme wind gusts were associated, not only within localized severe convective storms accompanying a long-lived squall line, but also over a much broader region of non-convective wind gusts accompanying this rare and intense winter cyclone. 

Figure 11 – Topographic Map of Wisconsin 

Green Bay WSR 88D 2-Panel Radar (Reflectivity left & Velocity right):  10-11 pm CST

The radar animation shows a rare severe squall line racing east across parts of central Wisconsin late in the evening of Wednesday December 15th. Several reports of wind gusts of 50-70 mph and scattered tree damage were reported. Some structural damage also occurred where siding and shingles were blown off and/or where trees fell on a few homes. The damage was associated with straight-lines winds that occurred in association with small embedded bowing structures. The velocity animation on the right, denotes small-scale enhanced velocity surges (darker blue shades denoted by the arrows) that were associated with most of the reported wind damage.

Here is a look at all the severe thunderstorm and tornado warnings that were issued during the severe weather event across the Plains and western Great Lakes.

Here is a look at the preliminary severe weather reports for 12/15/2021.