Surprise ACCAS
Altocumulus castellanus clouds (ACCAS) unexpectedly developed in southern Wisconsin yesterday morning, producing beautiful cloud features and eventually a severe thunderstorm. ACCAS form due to mid-level instability and manifest in towering billows that can often produce pileus clouds as they burst through a more stable layer.
The rooftop cameras of the UW-AOSS building caught this development nicely.
A model sounding from the High-Resolution Rapid Refresh (HRRR) did hint at mid-level instability due to an elevated mixed layer atop a temperature inversion. The model shows a mid-level lapse rate of 8.7 C/km and 155 J/kg of most-unstable CAPE (MUCAPE). However, the model did not forecast convection or precipitation.
There is some evidence that frontogenesis at 700mb may have served as a forcing mechanism for the convection, but it remains uncertain.
The NOAA/CIMSS LightningCast model caught this ACCAS development, and predicted elevated probabilities of lightning 20-45 minutes prior to first flashes (depending on position in the line).
Several cells produced 1″ hail in far southeastern Wisconsin.
The ProbSevere v3 models (PSv3) never exceeded 19% on these cells. However, an experimental image-based AI model produced > 50% probability of severe for several time steps (it predicts the probability of any severe hazards within the next 45 minutes and 20 km). The image-based approach may be able to better exploit spatial context and multi-modal signals from meteorological observations than the tree-based PSv3.
As a meteorologist, it is not always pleasant to be surprised by unexpected weather, but it is always a delight to behold beauty and wonder in the sky.