December 10, 2021 Tornado Outbreak

When talking about severe weather, four ingredients are important to consider. These ingredients are moisture, lift, instability, and vertical wind shear.

The next few slides will detail each variable and the role it played in the event.

This event was driven by a low pressure system situated over northern Missouri that enhanced the ingredients necessary to produce severe weather in the circled area of concern.

As the evening progressed, tornadic supercells formed ahead of the cold front within the area of concern. The positioning of the low pressure system ensured moisture and warm air were transported into the region which fueled these thunderstorms.

Significant moisture transport is characterized in this figure by red shading (amount of moisture) and length of arrows (wind speed).

At 7 pm, deep moisture was being drawn into the bi-state area. This is important because moisture, paired with dry air above it, promotes atmospheric instability. Humid air is less dense than dry air, so when humid air is below dry air it will rise. When there is enough instability coupled with a source of lift, thunderstorms are able to develop.

Moisture transport is also relevant in determining cloud height. Tornadoes are more likely to form when cloud bases are closer to the ground. In this case, cloud heights were around 2500 feet (~750 m) above ground level. This was sufficient for tornadogenesis to occur.

A crucial aspect in the development of tornadic thunderstorms through the evening hours was the presence of increasing winds in the lower atmosphere. This feature aided the lifting of air from the surface into an environment favorable for tornadic storms ahead of the cold front.

These strong winds also increased the risk of long-track tornadoes. Storm motion was fast partially due to these intense 850 mb winds. When fast-moving storms produce tornadoes, these tornadoes have the potential to cover more ground and cause more extensive damage.

Vertical wind shear is the change in wind speed and/or direction with height. This parameter plays a large role in tornadogenesis. Typically, vertical wind shear 0-1 km above the ground is analyzed to assess the potential of tornado development.

In this case, 0-1 km wind shear in excess of 35 knots was measured areawide. This is sufficient for tornadogenesis.

By mid morning of December 10th, confidence had increased of a severe weather outbreak occurring over parts of the Mississippi and lower Ohio River Valleys due to the likelihood of these ingredients being present at the same time.

This prompted the Storm Prediction Center to upgrade the risk to Moderate (level 4 out of 5) with a few strong, long-track tornadoes, damaging wind gusts, and large hail likely across this area during the afternoon and evening hours.

At approximately 7:35 pm CST, a tornado developed one mile north of the intersection of Highway T and Highway 94 in southwestern St. Charles County, MO and began tracking northeastward.

The tornado continued northeast and strengthened as it approached Highway F, northwest of Defiance, MO.

Two homes were completely destroyed. The first home was 110 years old and did not appear to have any anchoring to the concrete foundation. The second home just 100 yards east of the first was swept clean to the subfloor, which was still anchored to the concrete foundation.

One fatality and two injuries occurred at this location. The damage was rated EF-3 with winds of 165 mph.

The tornado continued northeast, crossing the Missouri River and entering St. Louis County causing extensive tree damage along the river, a mere half mile north of a large outlet mall in Chesterfield, MO and 1.5 miles south of the NWS St. Louis Forecast Office in Weldon Spring. During this time, our employees were forced to take shelter while our colleagues in Kansas City assumed our operations until we were safe.

The tornado then crossed Interstate 64 east of Boone Bridge causing minor damage to vehicles that were on the highway. It continued northeast, crossing the Missouri River again, back into St. Charles County near Greens Bottom Road. Here additional tree damage, broken power poles, and structural damage was observed.

The tornado crossed the Missouri River a third and final time, back into St. Louis County traveling across Highway 364 and north of the Creve Coeur Airport. Here, tree damage and other minor damage was noted. The tornado lifted at 8:01 pm before entering the Hollywood Casino Amphitheatre Complex in Maryland Heights, MO.

The tornado began on the south side of I-270 around three-quarters of a mile west of the I-270/I-255 interchange at 8:27 pm CST. Damage in this area consisted of several downed highway signs and a bent light pole. The tornado moved northeast and intensified rapidly after crossing I-255, striking an Amazon warehouse. The west-facing walls of the warehouse collapsed inward, which was followed by multiple structural failures as the tornado moved through the complex, including the collapse of additional walls and the loss of a large segment of the roof. This was the worst damage along the entire tornado track, rated EF-3. Cars in the rear of the building were strewn eastward and power lines and several large high tension towers were also destroyed. There was one injury and 6 fatalities at the Amazon facility.

The tornado weakened as it moved northeast across Sand Road, where it destroyed an unanchored mobile home and an outbuilding, snapped trees, and downed power poles. Damage in this area was rated EF-1.

Further northeast, the tornado crossed South University Drive where it snapped a power pole and downed additional trees. It then crossed into the Sunset Hills Country Club subdivision. Throughout a portion of the subdivision, there were downed trees, snapped branches, and superficial home damage consisting of missing shingles or small portions of roof covering. The tornado weakened further as it moved northeast, and lifted just west of Butler Boulevard at 8:32 pm CST. Noteworthy is debris from the Amazon warehouse was found along the entire track of this storm, tens of miles to the northeast of the warehouse.

The tornado first touched down south of Sorento on New Douglas Road, west of Sorento Reservoir, where it snapped several trees. The tornado moved northeast, passing east of Donnellson crossing Mt. Moriah Avenue and moving over the southern portion of Coffeen Lake.

As the tornado continued northeast, it crossed IL Route 185 where it destroyed an outbuilding, snapped trees, and downed power lines. Near the intersection of School House Avenue and Dads Trail, extensive tree damage was observed. Further northeast, the tornado crossed Fillmore Lake, moving through Bingham, snapping additional trees. The tornado intensified and widened as it moved across the southeast portion of Ramsey State Park and across US Highway 51 north of Ramsey. The tornado continued to track northeast, crossing IL Route 14 around 2 miles west of Herrick, continuing into Shelby County.

Along this portion of the track, power poles were snapped, and the tree damage was extremely impressive, with a wide swath of snapped and twisted trees. This damage has been re-evaluated and given the extensive nature of snapped trees, it has been rated low-end EF-2.

In total, the tornado tracks from this supercell thunderstorm combined for over 70 miles with all three being rated strong tornadoes.

How anomalous is this? Since 2000, there have been only 13 F/EF2+ tornadoes across the entire states of Missouri and Illinois in December. The only other December tornado outbreak on record in our county warning area to have more than 3 tornadoes rated F/EF2 or greater was on December 18, 1957.

A weak tornado touched down just northwest of Wellsville east of Highway 19 where it broke several power poles. It then moved northeast toward the intersection of Morgan School Road and Route A and traveled northeast to Orchard Road.

Minor tree damage and flattened corn was observed. The width of the tornado was around 50 yards, and the damage was rated EF-0. Interestingly, the radar identified debris, likely from the lofted corn stalks.