Upper air data is taken from balloons which measure upper air conditions over a particular location. This includes plots of data on the standard mandatory and height levels, contours of various upper air parameters and a set of sounding plots for most US sites. These data update once every 12 hours at around 9:45 EST.
This is a contour plot of precipitable water in inches. The contour interval is .2 inches. Precipitatable water reflects the amount of water contained in a vertical column above the surface if it were all precipitated out. This is a good indicator of how much rain or snow might fall as a result of a thunderstorm or low pressure system.