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June Precipitable Water + Rainfall

11 Jun 2025 05:30 AM
Using an offline archive the IEM maintains of NWS forecast model grid point output for airport locations in Iowa, the featured chart takes a fun look comparing precipitable water (PW) and precipitation frequency. PW is a depth measure of the amount of liquid equivalent water that's in a column of atmosphere. Rewording, if all the water (in its various phases) were squeezed out of the atmosphere as liquid, how much rain would fall. So the plot compares this PW value based on the NWS NAM model with the same day calendar day rainfall total for Des Moines during the month of June. Please note that the model value is valid at 7 AM. We find a rather direct and intuitive relationship. As the atmosphere loads up with water, the frequency of observed rainfall increases! The overall climatology is around 40%, so PW values of around 1.25 inches and lower tend to have a higher frequency of dry days vs climatology. The bottom panel compares the 7 AM PW value with the same calendar day precipitation. The one-to-one line nicely shows that the PW value is often an upper bound to precipitation, but it is certainly not a hard limit and things like training thunderstorms can replenish PW during rainfall events. PW values will be on the increase this week, particularly over northern Iowa and support some significant rainfall events.

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Tags:   pwater