My PCs wanted to camp out before hand as well. I didn't want to run an encounter that night and I didn't think to make them play as commoners (great idea!)
What I did do was describe the rains and their journey to the farm. Along the way the PCs passed several farms, all of which were abandoned. One showed signs of a struggle but did not have dead bodies etc.
Finally they came to the largest farm yet (some of the PCs were local and recognized that this was one of the wealthier farms) and they found people. The farmers had banded together and holed up at the farm. Most people where in the barn and it primarly consisted of the old, the young and the women. There were some men around who did not travel to the Hambley farm and their focus was on protecting those who where left.
Basically the farmers had already tried to solve the problem themselves. It didn't work and several people died. They are now concerned and waiting for daylight/ it to stop raining before they set out again. The farmers are unfriendly to the PCs unless they are local enough to know the farmers.