While the official reading for today will be less than one inch (see Airport on the above map), several places topped two inches of rain in just a few hours.  Seeing so many heavy downpours today has many of you asking, "Where did all this water come from?" Let's break down how we look at how much water is in the atmosphere. 

Warning: in-depth science ahead! The green vertical line on the image below shows the "moisture profile" going up through the atmosphere.  Your take away from this should be the air was basically saturated through roughly 9 km (or roughly 30,000 feet). Saturated means the green line and red line are almost touching - green is the dewpoint and red is the temperature. 

The primary ways we look at moisture in the air: 

dewpoint temperature: "The temperature to which a given air parcel must be cooled at constant pressure and constant water vapor content in order for saturation to occur." This is our only moisture value that we actually measure - the other ones listed here are derived through equations.
relative humidity: "The ratio of the vapor pressure to the saturation vapor pressure with respect to water." This is usually given in a percentage and is something you are used to seeing. 
PWAT (precipitable water): "the total atmospheric water vapor contained in a vertical column." 
*definitions from American Meteorological Society Glossary

 

The image above is precipitable water from early Tuesday when we saw so much rain - this is a model, not observed data.  This helps us look at the whole column of air as opposed to the surface/ground or one specific layer of the atmosphere. The map shows a very high amount of water (2.13 at the yellow spot) through the atmosphere which supports the model sounding above it.  

The storms we saw today were not just feeding off low-level moisture.  They drew from a saturated column of air which is why they were able to dump so much water so quickly. 

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