There's a lot of water vapor to work with in the atmosphere over that region at the moment. Here's the satellite water vapor image today:
|GOES-E water vapor image from 1615Z, July 28, 2011.|
The more moisture there is in the atmosphere, the greater the potential for heavy rainfall. This seems like a very obvious statement. To that end, one parameter that meteorologists use to evaluate the potential for heavy rainfall is called the precipitable water value (often abbreviated as PWAT or PWTR). It's a pretty simple index to derive--basically, all precipitable water does is look at the profile of moisture content over a given location. It calculates how deep the water on the ground would be if all of the water vapor over a given location were to immediately condense out and fall as rain. It's saying, if all the water in the air over your head immediately condensed into rain and fell out, how much rain would fall. Clearly this is never fully realized, as the atmosphere never dumps out all of its water vapor content at once. But, in general, the higher the precipitable water values, the higher the likelihood for heavy rain.
Most soundings that you see will compute the precipitable water value for that sounding location. The HOOT website soundings are no different. Here's this morning's 12Z sounding from Davenport, Iowa, which is in the middle of that moisture plume.
|KDVN sounding from 12Z, July 28, 2011.|
But we've already established that this is a record-breaking month for rainfall in this region. Just how anomalously moist is the atmosphere within this plume of moisture? The Rapid City, SD forecast office of the National Weather Service has put together a really nice webpage with climatological values of precipitable water for each sounding site. Here's their yearly graph of climatological precipitable water values for Davenport:
|Monthly climatological precipitable water values for KDVN.|
Furthermore, the dashed green line shows a value that is two standard deviations above normal. This value is often used by forecasters to try and separate extreme values from more climatologically normal (but still high) values. Any time you're more than two standard deviations above normal, it's considered to be a more "extreme" case with a high potential for heavy rainfall. 2.29 inches is definitely above the two-standard-deviations line. All of this points to the potential for very heavy rain.
Unisys publishes an interpolated precipitable water map for the US after every sounding release time. So, here's their map of precipitable water across the US this morning at 12Z:
|Interpolated precipitable water at 12Z, July 28, 2011.|
Also notable is the large area of very high precipitable water values right on the Gulf Coast. They cite 2.4 inches at a point near Lake Charles, Louisiana. There, too, precipitable water values are in the 99th percentile and thunderstorms with heavy rain are forecast for the next few days.
Of course, a very potent rainmaker looks to be bearing down on the Texas gulf coast in the next day or two. Tropical Storm Don has formed in the Gulf of Mexico and is moving northwestward.
|NHC predictied 5-day path cone of TS Don as of 10AM CDT, July 28, 2011.|
|US Drought Monitor from the Climate Prediction Center/UNL, as of July 19, 2011.|