The Flow Hive 2 is designed with inbuilt spirit levels which help you to set up your hive with a 3º slope towards the back. This creates the optimal honey harvesting angle, which will allow honey to flow easily from the frames into your collection jars.
The entrance landing board has been designed with this slope in mind and is manufactured to include a downward slope which should encourage water to run out of the hive rather than into it.
In particularly heavy downfalls, it is possible for water to enter the hive, luckily the base tray is designed to capture this water well away from the bees so that they will not be impacted by pooling water in the hive. From time to time, it's good to check your tray and empty any water that may have collected after particularly heavy rain.
If you are concerned about the slope, it is possible to have your hive set to level and then adjust the hive before harvesting time, (make sure you wear protective gear to adjust the level of the hive).