Reason 1: Obviously, it has been raining off & on since last week. As you know... every little bit adds up and has done so around here. I posted some pretty impressive totals here for the weekend timeframe. If you add in some of the lighter amounts that were spread out through last week then you have some really high totals. We could see another 1" or 2" from this round.
Reason 2: Along the same lines as reason1... the ground is very saturated. As we have seen over the last 24-36 hours any new rainfall isn't quickly going into the ground. It is forming pools or running off into a creek. The water is running out of options!
Reason 3: Not all of the upcoming rain will just be those pesky showers. There is a possibility of strong thunderstorms in the area. This is the icing on the cake! I have seen just one thunderstorm dump 4" or more in less than one hour. So just imagine if we get that out of one or two storms. That could lead to a whole world of problems with the current saturation level of the ground. It still appears that we'll see 1"-2" in general, but those storms could lead to more!
Nailing down the axis of the heaviest rainfall is the biggest challenge. After the rain really gets going on Wednesday I'll try to re-post!