Going from drought to deluge, the southwestern United States is being inundated by flooding rainfall yet again today. One of the terms meteorologists are batting around to describe this record-setting pool of atmospheric moisture has people confused, but it's actually pretty easy to understand.
The term meteorologists are using online and on television is "precipitable water," and it's a commonly-used product that tells us how much moisture is available in the atmosphere. Precipitable water, shortened to "PWAT," tells us how much rain would fall if all of the moisture in a column of the atmosphere condensed and fell as rain.
Higher moisture correlates to higher PWAT values, and lower moisture produces lower PWAT values. This morning's PWAT value for Greensboro, N.C. is 1.85"; this means that there's enough moisture in the column of the atmosphere over Greensboro that if it all condensed and fell as precipitation, it would produce 1.85" of rain. That makes sense, because it's a dreary, rainy day here in the Piedmont.
The American Meteorological Society's website has the mathematical formula used to compute precipitable water if you're interested (nerd), but basically they arrive at the PWAT by factoring in the amount of water vapor from the surface to the top of the atmosphere. PWAT is just for that little column of the atmosphere — since the atmosphere isn't static and air (and water vapor) doesn't stay neat and tidy in one column, thunderstorms are able to produce much more rain than what's indicated by the PWAT values.
Now, how do we know that precipitable water values are at record levels? The National Weather Service office in Rapid City, S.D. keeps a handy website for that. The site keeps climatological records for several upper-air data points, including precipitable water. If we look at this morning's weather balloon soundings from Tucson, Arizona, it'll show us that record amount of moisture sitting over the southwestern United States.
The chart shows the statistical values for PWAT over Tucson for the year, with the monsoon season standing out as the wettest part of the year. That solid green line at the top of the chart shows the record highest PWAT values measured each day during the 62-year period of records in Tucson, and this morning's sounding is a record high for September 8. The PWAT in Tucson was 2.02" this morning — three standard deviations above today's normal of around 0.85".
The record-setting pool of atmospheric moisture is thanks to the remnants of Hurricane Norbert pumping additional moisture into the southwestern U.S. The tropical system and the monsoon worked together to create a Florida-like airmass over the desert southwest, providing thunderstorms the moisture they need to produce major flash flooding.
This morning's rainfall in Phoenix is a testament to the sheer amount of water vapor in the air. Phoenix Sky Harbor International Airport saw about three inches of rain this morning, blowing past the previous rainfall record of 1.33" set on September 8, 1933.
Atmospheric moisture should return closer to normal in a couple of days as the remnants of Norbert continue to weaken.
[Images via WeatherBELL and NWS]