A Comparative Study Between the Precipitable Water and the Effective Precipitable Water as Predictors of Rainfall in Puerto Rico

The Effective Precipitable Water (ePW) represents the portion of the Precipitable Water (PW) which, in theory, can fall as precipitation. Higher values of ePW should produce more precipitation and the relationship should be 1:1, but of course, more variables impact the final amount of rainfall received. This research tries to assess the effectiveness of the ePW in predicting rainfall over the island of Puerto Rico, versus using the PW. The primary research question is whether ePW serves as a superior predictor of rainfall in Puerto Rico when contrasted with PW.

We used 5 years of data, comparing the ePW and PW obtained from the morning sounding (12z, 8 am local time) launched at the National Weather Service WFO in northeastern Puerto Rico against the 24-hour rainfall observed across Puerto Rico following the launch. Initial findings indicate that using the ePW value obtained up to the pressure level where 90% of the total columnar PW is reached generally produces more accurate estimations of expected rainfall. Additionally, this study reveals that both PW and ePW exhibit enhanced correlations in regions of the island that experience more rainfall each day. This observation is supported by the notably higher correlation coefficients between both indices and the total area experiencing daily precipitation exceeding +0.50 inches. Essentially, the extent of the area anticipated to receive +0.50 inches of rain demonstrates a better correlation with both PW and ePW, highlighting their significance as valuable indicators for rainfall forecasting in regions prone to precipitation events each day.

These findings emphasize the importance of considering the dynamically changing vertical extent of the moisture layer in the atmosphere for a more accurate evaluation of its role in generating rainfall. Results also show the potential of ePW as a valuable tool for predicting rainfall in Puerto Rico. However, further research and refinements are necessary to enhance the precision and accuracy of ePW-based rainfall forecasts.