Good example of why it’s dubious to infer causality simply from a vertical NAM cross-section. If the mid-stratosphere were weak, this would look like a downward-propagating disturbance… but evidently the troposphere is shifting to -NAM without a -NAM at 10 hPa. https://t.co/v3fn3q1lqZ

— Dr Simon Lee (@SimonLeeWx) December 14, 2021