Meteorological overview
What were the meteorological conditions that set the stage for such an unusual and impactful event? Information from the 20th-century reanalysis project (\citealt{Slivinski_2019}), a reconstruction of the weather of the 20th century that assimilates surface pressure observations into a modern atmospheric computer model, provides valuable insights into what happened. At the midlevels of the atmosphere (Fig. \ref{626562}), there was a deep trough of low pressure centered near the Four Corners on the morning of November 4th. To the east of this trough were strong winds out of the south-southwest. At the surface, a developing low-pressure system was located over eastern Colorado, with south-southeasterly winds transporting moisture northward across the Great Plains (Fig. \ref{519969}). Dewpoints in eastern Colorado approached 10°C (50°F) to the east of what appears to be a dryline. These findings establish that the ingredients for strong, rotating storms (moisture, instability, lift, and vertical wind shear) were in place on this day. Deep troughs of low pressure and extratropical cyclones are often associated with high-impact weather in Colorado, including heavy snow and severe thunderstorms. Large-scale patterns such as these are also not that uncommon in the fall months. More detailed research would be needed to better understand what allowed this particular weather system to produce significant tornadoes, when broadly similar weather systems often occur in November but do not result in tornadoes, or at least not as far west as Colorado. But, we do have some additional tools we can use to dig in a bit further.