Stuff won't fly out unless its really light. I usually put my weather sensitive stuff in plastic totes. If its really raining or snowing hard, water will find a way to get under the tarp, IMO. Make sure your food is secure. Not for blowing away but for critter control. Crows will raid the shit out of you if you dont.
A pickup is about the worst vehicle for snow unless it is 4x4.
My geo prizm drives better in the snow than my frontier in RWD.