The Winter Solstice Storm System of 2021

On December 19, 2021, a cold front moved southward into the Gulf. This system was still in relative infancy, only undergoing frontogenesis the day prior as a colder airmass tried to push through the very warm and stable air that had setup across much of the eastern half of the United States.

As the front continued southward, the system eventually began to hit a wall. After pushing southward through Florida on Sunday (really bringing no change in weather), the subtropical high halted southward progress. However, this change in airmass, paused or otherwise, continued to provide additional instability to keep thunderstorm activity alive. This was further enhanced by unseasonably warm Sea Surface Temperatures (SSTs) and sufficient moisture over the Gulf. When combined with favorable upper-level parameters to support sufficient venting of the rising air, this began to drive a low-level pressure deficit, causing an area of low pressure to form at the surface.

Over the next day, the low would continue to deepen, driving an intensifying storm system as it approached the Florida west coast during the morning December 21.

On the morning of the 21st, Florida found itself with an amplified upper-level pattern in place. Winds at 250 millibars (mb) (approx. 30,000 - 35,000 feet [kft]), were around 50-60 knots (kts). Compared to a typical summer day where the flow is roughly 15-20 kts, this is significant. Given the placement of the faster jet streaks with respect to Florida, this was a favorable setup for upper-level divergence, a large-scale mechanism for ascent.

At 500 mb (approx. 18 kft) a similarly amplified upper-level pattern could be seen. A shortwave trough axis was propagating through the southeast, with more subtle shortwave perturbations working through the flow ahead of the parent feature. One can infer the positive vorticity advection present as the feature progressed eastward. This would further enhance large-scale ascent.

Faster wind speeds also remained present at this level. Compared to the 50-60 kts at 250 mb, there is still 30-40 kts of shear present at this level. Representing roughly the mid-point of the atmospheric column where weather occurs, the shear at this level is important to storm maintenance. Without shear at this level, the updraft would eventually be cutoff by the downdraft falling down in the same space. With shear, the updraft and downdraft can remain separate, creating an environment where storms can last longer.

At 850 mb (approx. 5kft), ample moisture was present to allow for convection. The southwest flow over much of the peninsula would also support a continuous supply of moisture over Florida as the low arrived. The surface low can also be seen at this level.

When compared to the higher levels, the shear was lighter. Only 20-25 kts are present in this image. Given the changes in wind speed through the column, this supports a sheared environment for thunderstorms to develop within.

The 12Z sounding launched from the NWS Tampa Bay Office captures the snapshot of the atmospheric column well. In the mid and upper levels, the shear profile is rather unidirectional. However, towards the surface, this profile becomes more directional rather than speed driven. This change in direction (when clockwise like it was in this case, is called a veering profile) in the lower levels suggest warm air advection, another mechanism that aides in large-scale ascent. When coupled with the ample moisture (this sounding measured a precipitable water [PW] value of 1.73 inches; plenty sufficient), this is an atmospheric state able to support longer-term convection. Indeed, many cells that developed began well out over the Gulf and lasted into central Florida. However, this sounding is lacking in one key ingredient: instability. So, how was the atmosphere changing at that time?

At 12Z, surface-based instability (known as SBCAPE), was limited - except in Southwest Florida. An axis of 1000-2000 Joules per kilogram (J/kg) was working northward along the lifting warm front. The cold sector remained stable until the warm front passed, at best creating an environment that could only support stratiform rain. The warm front never made it to the Nature Coast, thus preventing significant instability in an otherwise favorable environment.

However, the lapse rates (which can be used as a proxy for how quickly the air will rise) remained weak. The atmosphere was not rapidly cooling with height given the more subtle differences in the airmass in the warm versus the cool sector.

Nevertheless, this was enough to sustain weak forcing for ascent in the warm sector.

As already mentioned, there was sufficient shear present to sustain updrafts if storms could take advantage of the instability. At 12Z, 40-50kts of effective shear was analyzed across the region.

Given the low-level veering profile, this led to an environment where helicity (which is a proxy for vertical spin) was 100-200 m2/s2 during the morning hours across the area.

All factors considered, and when used to calculate the potential for supercells (rotating thunderstorms capable of producing tornadoes), there was a strong signal to support the development of such storms. However, one of the limiting factors of this parameter is the fact that it is using MUCAPE instead of SBCAPE or MLCAPE to calculate the probability of such storms developing. Thus, the parameter assumes an updraft has already tapped into the most favorable region of instability. Over the Gulf, where the strongest instability remained, this was not difficult. However, storms near shore and over land had a much more difficult time tapping into this instability. There were only two isolated instances where this was accomplished effectively. So while the signal in this parameter may have looked strong, the reality was less certain. Ultimately a non-zero risk for supercells (and thus tornadoes) existed, but how widespread would they be?

Despite a challenging forecast for the day-of at the storm-scale, there were impacts that were likely to be associated with the storm system that could be confidently messaged in advance. Additionally, the Storm Prediction Center was able to highlight a potential for severe storms several days in advance.

The risks were continually analyzed over the coming days, and a tornado watch, gale warning, and small craft advisory were all issued in advance of the arriving storms.

At 6:25 AM, a short-lived tornado occurred south of Fort Myers in Lee County, FL. This was the only reported tornado of the day.

There were many reports of homes with roof damage, pool enclosures and vehicles damaged, and trees and large branches broken along the path of the tornado. Some of this damage can be seen in the images below.

Along with the tornado, widespread strong, gusty winds moved across southwest Florida and the southern interior south of the low-pressure track. Peak gusts were as high as 77 mph at Punta Gorda - Charlotte County airport, with many reports over 50 mph.

These winds caused some damage like that seen at a mobile home park and new construction site in North Port, FL. as seen in the images below.

The Winter Solstice storm system of 2021 was an atypical, but not unheard of event that had significant impacts across West Central and Southwest Florida. One tornado, multiple tropical storm to hurricane-force wind reports, and hazardous marine conditions occurred. This makes the event a memorable one for many residents and visitors alike who experienced it.