While the Rev Tent case is designed to be water resistant and keep the tent dry while transporting, intense and prolonged exposure to rain storms while driving can cause moisture to get under the storm flap and through the zippers.  This is not an issue for camping, as the moisture will only make contact with the outside of the tent and the inner tent will be completely dry - it is annoying to set up a tent that may be wet on the outside.   If you need more dry storage on top of your vehicle, consider putting the Rev Tent and other camping gear - pillows, sleeping bags, etc. in one of these roof top travel bags.   Check Amazon as there are many different styles and price points.  

From Amazon:  

23 Cubic Feet Roof Bag – Safety Tested Heavy-Duty Waterproof Rooftop Cargo Carrier for All Cars & SUVs (with or

Without Rack) – Includes Protective Mat & Storage Bag – Military-Grade 840D PVC