Fig. 1: Solid red lines represent the Significant Tornado Parameter at 11PM CST Friday evening December 10, 2021. Values of +4 to +5 were measured. Values of >1 = risk of stronger than F1 tornadoes.
Discussion: A severe weather meteorological index known as “significant tornado parameter” is used during an operational severe weather event to identify the potential for tornadic activity with intensity >F1. A STP index >1 indicates potential for >F1 intensity tornadoes. Note that at peak intensity, STP was in the 4 to 5 range late evening on December 10th.
STP is a complex index measured by a combination of convective available potential energy (CAPE), effective storm-relative helicity and the effective bulk wind difference over the lower half of the storm depth according to NOAA/SPC.
Preliminary analysis indicates tornado line no. 1 from eastern Missouri to east-central Illinois was caused by path of travel of the low-pressure system driving this storm and a jet stream wind speed maximum of 140-145 knots (at 30,000 feet) creating the vertical motion to produce tornadoes. Tornado line no. 2 formed at the intersection of high wind aloft and a low-level (5,000 feet) jet out of the western Gulf of Mexico. Near line no. 2 the STP was a whopping 4 to 5. The higher STP near line. No. 2 is likely related to axis of the deepest layer of high moisture (subtropical air in lower atmosphere out of the Gulf).
Also contributing to intensity of this event is the entrainment of deep tropical moisture from a massive area of thunderstorms across a warm SSTA region southwest of Baja California during Dec. 9-10. Severe weather episodes of the magnitude of the Dec. 10 outbreak almost always have an attachment to the tropics.
Another unique contributor to this severe weather outbreak is the nearby presence of snow cover combined with the super warm/humid outflow out of the western Gulf of Mexico. Undoubtedly, this dynamic had influence on both the lower and upper atmosphere driving this horrific severe weather event.
Fig. 2: NOAA/SPC severe weather reports for December 10, 2021.
Fig. 3-4: Always an enhancing factor to a major severe weather event is presence of an unusually strong jet stream and strong inflow from the Gulf of Mexico. Last Friday evening the jet stream wind speed reached maximum intensity (140-145 knots) over eastern Kansas while a 50-70 knot “low level jet” surged northward out of the western Gulf of Mexico carrying buoyant subtropical moisture. The combination of high southwest wind aloft intersecting with strong southern humid flow beneath causes the vertical motion/instability to trigger strong tornadoes.
Fig. 5: Contributing to the intensity of the December 10th outbreak was entrainment of deep tropical moisture and energy from a large mass of thunderstorms southwest of Baja California via the southwesterly jet stream. (See link to view loop of the sequence). The tropical convection occurred over a warm patch of SSTA southwest of Baja California.
Fig. 6: Sharp temperature contrast over a short distance especially when subtropical/tropical moisture is involved leads to a very unstable atmosphere.