Discussion: According to NOAA, the U.S. set a record yesterday for most observed gusts >hurricane force (74 mph), just 5 days since the historic tornado outbreak on December 10th. According to NOAA, more than 55 reports of gusts >74 mph were observed in Midwestern States. According to NOAA, the previous record was 53 reports set on August 10, 2020 (period of record 2004-2021).
The high wind was driven by convection (severe thunderstorms and tornadic activity) or interaction with the Rocky Mountains. Yesterday’s 2100 GMT GFS forecast of 10-meter wind indicated SUSTAINED wind in the 45-60 mph range centered on Nebraska (Fig. 1). Near these wind maxima gusts of >100 mph was possible. Well east of the super wind core wind sustained at 30 mph reached near Chicago.
At 850 MB (5,000 feet) where the low-level jet is looked for as a catalyst to severe weather, wind speeds of 80-85 knots (near 100 mph) were observed over Kansas. This observation may be unprecedented. At jet stream level, similar to the Dec. 10 severe weather event, a wind maximum of 160 mph was projected (Fig. 2). The combination of high wind aloft associated with the jet stream running over a low-level high-wind band is the catalyst to drive vertical motion and severe weather. This set of circumstances was extreme.
NOAA/SPC reported 541 severe weather damage reports yesterday which is more than the December 10 tornado outbreak (day) on December 10 (Fig. 3). Nebraska, Kansas and Iowa were hit hardest.
The catalyst to this storm, similar to the December 10th tornado outbreak, was a connection of the Central U.S. storm to the tropics. The subtropical jet stream carried an energetic and moist “atmospheric river” into this storm from south of Hawaii (Fig. 4) to propel dramatic thermal contrast with height and latitude to drive the instability that propelled the high intensity weather system.
Fig. 1: GFS OP 10-meter wind forecast valid at 3PM CST on December 15, 2021.
Fig. 2: GFS OP 250-MB wind speed forecast valid at 3PM CST on December 15, 2021.
Fig. 3: NOAA/SPC severe weather/damage reports for December 15, 2021 was 541 which is greater than the 518 observed on December 10 when a major tornado outbreak struck the Midwest. Tornadoes were also prevalent in the December 15 outbreak from eastern Nebraska to western Iowa.
Fig. 4: An energetic subtropical jet stream zooming from south of Hawaii into the Midwest U.S. was the catalyst to an unusually intense severe weather outbreak.