Why is rainfall forecast worse in summer than in cooler periods?

The warm season is characterised by the development of daytime cumulus clouds with occasional rainfall. In the evening and at night the clouds disappear and the next day the cycle repeats itself.

The horizontal extent of precipitation from these clouds ranges from a few hundred metres to a dozen-or-so kilometres. Sometimes they are larger when the clouds merge into larger assemblages. The UM model has a computational grid step of 4 km, so the precipitation field could be smaller than the grid step, or be comparable to the grid step.

Reliability of precipitation forecasts tends to increase in the cool season, where clouds form due to slipping over the frontal area. These are stratus clouds and the width of the precipitation zone reaches up to 300 km, its length much more. Zones of precipitation from such clouds do not have "holes", as is the case with precipitation from cumulus clouds.