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Abstract The first 2 weeks of December 2021 were exceptionally active for severe convective storms across the central and eastern United States. While previous work has indicated that this was related to the existence of a negative phase of the Pacific–North American pattern, we demonstrate that such a pattern was configured via dynamical linkages between multiple extratropical cyclogenesis events in the western North Pacific, the recurvature of Typhoon Nyatoh, and the subsequent phase evolution of the North Pacific jet. These processes were found to aid in the excitation of Rossby wave packets and the amplification of upper-level flow downstream over the Pacific, ultimately configuring synoptic-scale weather regimes supportive of anomalous high-frequency and high-intensity severe convective weather in the contiguous United States. In addition, abnormally warm Gulf of America/Gulf of Mexico sea surface temperatures, aided by a period of antecedent synoptic-scale subsidence, played a critical role in enhancing convective instability in the surface warm sector. This work underscores the importance of cataloging these events for purposes of examining (and potentially enhancing) predictability. Significance StatementThe first half of December 2021 recorded one of the most active cool-season severe weather periods in the United States, resulting in two billion-dollar convective outbreaks on 10 and 15 December. This study links these extreme events to upstream dynamical processes over the North Pacific, including extratropical cyclogenesis, the recurvature of Typhoon Nyatoh, and the retraction of the North Pacific jet. These processes amplified downstream flow and configured synoptic environments favorable for severe weather across the United States. Additionally, anomalously warm Gulf of America/Gulf of Mexico sea surface temperatures enhanced convective instability. By identifying these key precursors, this work highlights the potential for improved anticipation of extended-range severe weather likelihood—particularly during the cool season—when such events remain rare but highly impactful. 

Abstract A rapidly deepening extratropical cyclone moved across the central Great Plains on 15 December 2021 and resulted in simultaneous extreme weather events. A derecho developed at the cold front and moved from the eastern half of Kansas to Wisconsin. Simultaneously, a nonconvective mesoscale windstorm occurred on the southwest side of the cyclone and moved from western to central Kansas and is the focus of this study. The windstorm downed power lines and triggered a wildfire outbreak covering over 160 000 ac (650 km²) resulting in two fatalities, several injuries, and the loss of hundreds of cattle. Surface wind gusts exceeded 50 kt (26 m s⁻¹) over a large area in western Kansas with a peak gust of 87 kt (45 m s⁻¹) observed at Russell, Kansas, on the southeast flank of the largest wildfire in the region. The extratropical cyclone resembled the Shapiro–Keyser conceptual model with the mesoscale windstorm focused near the cloud head and southern tip of the bent-back front southwest of the cyclone center. The near-surface wind speeds were highest where three airstreams—one along the bent-back front and the other two at higher altitudes to the west of the cyclone—descended and accelerated in a higher horizontal pressure gradient region near the tip of the bent-back front and cloud head. While the nonconvective mesoscale windstorm did not meet the exact definition of a sting jet, it exhibited many of the same characteristics and physical mechanisms that drive sting jets with oceanic Shapiro–Keyser cyclones. 

Possible sources of the observed modulation of the tropical Madden‐Julian oscillation (MJO) by the stratospheric quasi‐biennial oscillation (QBO) and the 11‐year solar cycle are investigated using 41 years of reanalysis data and archived climate model data. Larger upward fluxes of extratropical planetary‐scale waves, leading in some cases to sudden stratospheric warmings (SSWs), are observed in late fall and early winter during the easterly phase of the QBO than during the westerly phase (the “Holton‐Tan effect”). A similar but smaller increase occurs, on average, during solar minima relative to solar maxima. In addition to the warming at high latitudes, extratropical wave forcing events produce cooling and reduced static stability in the tropical lower stratosphere. Here, it is found that if SSWs occur in early winter (before ∼mid‐January), the reduced static stability produces, on average, a statistically significant, lagged strengthening of the MJO. This therefore represents a possible mechanism for producing, or at least enhancing, the observed QBO and solar modulations of the MJO in boreal winter. An initial analysis of archived climate model data shows that at least one model version with realistic QBO and solar forcing and with 4 X CO2 forcings partly simulates both of these characteristics (QBO/solar modulation of early winter wave forcing and lagged strengthening of the MJO following early winter SSWs). However, the modeled MJO is insufficiently sensitive to QBO‐induced static stability reductions, precluding simulation of the QBO‐MJO connection. 

The tropical Madden–Julian oscillation (MJO) excites a northward propagating Rossby wave train that largely determines the extratropical surface weather consequences of the MJO. Previous work has demonstrated a significant influence of the tropospheric El Niño–Southern Oscillation (ENSO) on the characteristics of this wave train. Here, composite analyses of ERA-Interim sea level pressure (SLP) and surface air temperature (SAT) data during the extended northern winter season are performed to investigate the additional role of stratospheric forcings [the quasi-biennial oscillation (QBO) and the 11-yr solar cycle] in modifying the wave train and its consequences. MJO phase composites of 20–100-day filtered data for the two QBO phases show that, similar to the cool phase of ENSO, the easterly phase of the QBO (QBOE) produces a stronger wave train and associated modulation of SLP and SAT anomalies. In particular, during MJO phases 5–7, positive SLP and negative SAT anomalies in the North Atlantic/Eurasian sector are enhanced during QBOE relative to the westerly phase of the QBO (QBOW). The opposite occurs during the earliest MJO phases. SAT anomalies over eastern North America are also more strongly modulated during QBOE. Although less certain because of the short data record, there is some evidence that the minimum phase of the solar cycle (SMIN) produces a similar increased modulation of SLP and SAT anomalies. The strongest modulations of SLP and SAT anomalies are produced when two or more of the forcings are superposed (e.g., QBOE/cool ENSO, SMIN/QBOE, etc.).