While the training touted cases where Version 2 (V2) of LightningCast had a clear lead time advantage over Version 1 (V1), many of the new cells I tracked today showed the opposite, with V1 showing higher probabilities sooner than V2.

It seemed like many of the cells were producing lightning very quickly after achieving even modest returns at the -10C level. In this case, the significant instability in the region today led to very rapid vertical cloud growth, which likely outpaced significant radar returns aloft. And while it is purely an educated guess on my part, that might have something to do with the weight given to the -10C reflectivity – if it needs a certain reflectivity threshold to really boost the probability, then the V2 product would be artificially slowing the increase in probabilities. Then in addition to waiting for the -10C reflectivities, you have to add in processing and dissemination lag time.

The V1 product, being based entirely on satellite data, was able to key in on just the rapid vertical growth and boost probabilities based on that alone, and not have to wait for the -10C returns to show up in MRMS.

Fig 1: Loop comparing LightningCast V1 (left) to LightningCast V2 (right)

Fig 2: Comparison of LightningCast V1 (left) and V2 (right) at 2041Z on 19 May 2025, showing V1 being first to have a 30% contour over the cell of interest in NW Arkansas.

Fig 3: Comparison of LightningCast V1 (left) and V2 (right) at 2046Z on 19 May 2025, showing both having a 50% at the same time, though V1 is larger in area.

Fig 4: Comparison of LightningCast V1 (left) and V2 (right) at 2051Z on 19 May 2025, showing V1 being first to have a 70% contour, while V2 still only has a small 50% area.

Fig 5: Comparison of LightningCast V1 (left) and V2 (right) at 2056Z on 19 May 2025, with the first GLM Flash Extent Density return (blue square) noted at 2059Z. Note that V1 had a 70% contour nearly coincident with the GLM Flash square, while V2’s highest return was still only 50%, and well displaced from where the lightning actually happened.

While I don’t doubt that the -10C reflectivity can help in many scenarios, on days like today it didn’t seem to help much, if at all, and in many cases the satellite-only V1 seemed to do a bit better. As I said, it’s my hypothesis that this is due to the rapid vertical development outstripping the production of -10C returns. Additionally, I would be curious if testing out MRMS Vertically Integrated Ice (VII) instead of -10C reflectivity would produce better results (if it hasn’t been tried already)

– Marko Ramius