Phoenix Rainfall Index

+ Monthly Totals

 

+ Background

Phoenix, Arizona is a vast, sprawling metropolis which covers an area of nearly 2000 sq-mi. Historically, the official rain gauge for the Phoenix Metropolitan Area (PMA) has been situated near the center of the city. Since 1933, it has resided at Phoenix Sky Harbor International Airport (KPHX). While KPHX did a good job of reprenting rainfall of Phoenix when it was smaller, with the vast sprawl of the area now there are times where portions of the PMA receive rain while other, including KPHX, do not. This is especially true during the monsoon. To offer a companion measurement to the official observations at KPHX, the Phoenix Rainfall Index (PRI) was been created.

The PRI, based on rain observed within 40 miles of Downtown Phoenix from the Maricopa County Flood Control District's network of ALERT rain gauges across the PMA, provides an average rainfall amount and what percent of gages measured rain.

The FCD rain gages measure precipitation in millimeters (increments of 0.04"). Therefore, any precipitation less than 1 mm/0.04" will not be measured by the rain gages. Coupled with mathematical rounding, it is very likely there will be days when precipitation falls across the PMA yet the average rainfall is 0.00". When this happens it is reflective of either a very light or very isolated precipitation event. Also, please be aware that the rainfall data are only passed through minimal quality control mechanisms. The most common error will be light precip amounts at individual gages when no rain actually occurred.

To generate the averages, a Monte Carlo simulation is used. With all available gages within 40 miles of Downtown Phoenix, 30 gages are randomly selected and their average and percent with measurable rain are computed. This process is repeated 10,000 times. The average value from all these simulations are then recorded.