Tornadoes and the Nocturnal Transition: Will they Persist?

Climatologically, tornadoes are most frequently observed in the afternoon and early evening hours during spring and summer in association with convection forced by solar heating.  With sunset typically comes the disappearance of solar heating, the collapse and stabilization of the boundary layer, and the demise of tornadoes.  However, occasionally this does not occur- tornadoes are observed before sunset and continue after sunset into the night.  Even more infrequently, tornadoes are not observed before sunset, but first occur after sunset.  This research first assesses the climatology of each of these scenarios over the contiguous United States (CONUS) to determine where, when, and how often each of these tornado scenarios occur.  Environmental characteristics are analyzed through a combination of composite analysis, standardized anomaly analysis, clustering, and objective classification to assess and contrast meteorological conditions and storm types associated with each of these event classes.  Lastly, numerical guidance from global and convection-permitting models via NOAA’s Second Generation Global Ensemble Forecast System Reforecast (GEFS/R) and the National Severe Storms Laboratory’s WRF (NSSL-WRF), respectively, are employed to produce daily gridded probabilistic tornado forecasts in addition to conditional probability forecasts of when tornadoes will occur relative to sunset, provided that they do occur.