Failure to forecast: A case study in nowcasting and forecasting the eruption of a coronal mass ejection and its geomagnetic impacts on Dec 7-10, 2020.

Forecasting solar flares based on while-light images and photospheric magnetograms of sunspots is notoriously challenging, while accurate forecasting of coronal mass ejections (CME) is still in its infancy. That said, the chances of a CME being launched is more likely following a flare. CMEs launched from the western hemisphere and “halo” CMEs are the most likely to be geomagnetically impactful, but forecasting their arrival and impact at Earth depends on how well their velocity is known near the Sun, the solar wind conditions between the Sun and the Earth, the accuracy of theoretical models and on the orientation of the CME magnetic field.  In this presentation, we describe a well observed active region, flare, CME, radio burst and sudden geomagnetic impulse that was observed on December 7-10, 2020 by a slew of instruments (SDO, ACE, DSCOVR, PSP, US and European magnetometers). This was a solar eruption that was not expected, but the CME and resulting geomagnetic impact should have been straight-forward to model and forecast. What can we learn from our failure to forecast this simple event and its impacts at Earth?