One Last Post (Maybe) on Precipitable Water

I've been blogging the past few days about weather balloons/soundings, and specifically how "precipitable water" is one of the pieces of information that can be gleaned from the resulting data.  

The above image was captured a few moments ago from the New Braunfels, TX (KEWX) NEXRAD.  The image depicts "storm total precipitation", or at least the radar's estimate of the total precipitation, across the area since 6-7-10 (at 9am to be exact).

In my blog posts of the past couple of days, I've noted how there was around 2 inches of "precipitable water" in the skies over south & central Texas.  So if that's the case, how is it that over 3 inches of rain (depicted by the darker orange, red and purple colors on the above image) has fallen across a large part of the region during the last 24-36 hours?  

It's not necessarily a "that's all she wrote" situation with respect to precipitable water.  While it's true that the weather balloon and/or computer forecast model sounding indicate (or estimate/forecast in the case of the computer forecast model) how much precipitable water is present in the atmosphere at that given moment, you have to remember that as long as the wind is blowing from the right direction, additional moisture is being transported back into the same areas, often times as fast as the moisture is being "rained out" of the atmosphere.

While one thunderstorm (or thunderstorm complex) may have "rained out" the 1.9 inches of precipitable water that was indicated at 7 o'clock in the morning on the weather balloon sounding, Southeast winds continued to transport "replacement" moisture back into the same areas, resulting in additional heavy rains as more storms developed over the same areas.  As a result, and as in the above image, several inches of rain may ultimately end up falling over one or more areas.  

Nobody ever said that forecasting the weather was easy.  The bottom line is that we knew the atmosphere was very moist across the area, based in part on the high precipitable water values that we had observed over the previous 48 or so hours.  A forecast of widespread, sometimes heavy, rain was made, and indeed verified.  Some areas (noted by the pink/purple colors on the above image) received over 6 inches of rain before all was said and done!