Either due to the water accumulated between the grapes that causes fungal diseases, or mechanical damage due to hail, yield losses can be very high in this period.
Climate projections show that heavy rainfall events during the summer will tend to increase in Europe, although they will be reduced in certain areas of the South. In the regions where heavy rainfall events increase, great damage in vineyards can be expected.
Regarding hail events, there are high uncertainties, since global and regional climatic models cannot represent directly hail events at a small scale.
Economic damages in vineyards associated to heavy rainfall events can be noticed in greater frequency in certain areas of the European Southern region, as it was the case of the municipality Montilla in 2018 (Córdoba, Spain), in which 500 hectares of vineyards and 500,000 kilos of grapes were lost due to an occasional hail and heavy rainfall event in August.
Source: Baseline Report AgriAdapt and ABC Córdoba
https://agriadapt.eu/wp-content/uploads/2017/04/A1_Baseline-report_Full-version_V3.pdf