A tropical wave moved across Puerto Rico on Thu July 18, 2013 and brought historic rains to the San Juan metro area dumping between 8 to 10 inches of rain in a 24-hr period with most of these rains (8.91 inches) falling in a 6-hr period between 11AM to 430PM. These rains had significant impacts across the San Juan area causing significant flash flooding in area streams and rivers and greatly disrupting ground and air traffic. At one time, over 12 flights scheduled to arrive at the San Juan Luis Muñoz Marin Int’l Airport (SJU LMM) were diverted to the Borinquen airport in northwest PR and the Charlotte Amalie airport in St. Thomas. The 9.23 inches of rain that fell at the SJU LMM Int’l Airport was the second highest 24-hr total ever observed in any single day since records began in 1898. To read more about other climate records broken with this event, click here.
This 36-hr animation loop (click above for animation) of total water vapor content product from CIRA starting from 18Z Wed July 17 through 03Z Fri July 19 shows the progression of the tropical wave from east of the Lesser Antilles across PR. Values of 60 mm (2.36 inches) were observed over PR with this wave. These values were near the 99th percentile for mid-July according to a precipitable water climatology study done by the Rapid City SD Forecast Office.
Composite Reflectivity animation loop (click above for animation) from the TDWR from 1239Z through 1609Z shows several clusters of showers and thunderstorms moving over the San Juan area and northeast PR. Just after 1609Z, the TDWR failed likely due to intense lightning activity cutting out power at the radar site.
GOES 4-km infrared imagery animation (click above for animation) shows very cold and intense convection with a large area of tops colder than -70C with tops as cold as -82C at 1710 UTC just north of San Juan.
As part of an evaluation project between WFO SJU and NASA SPoRT, the WFO SJU is evaluating the NESDIS Quantitative Precipitation Estimate (QPE), a satellite-based precipitation estimation product that is derived from both Infrared and microwave data. The image below shows a maximum of 3.14 inches right over the San Juan area during the 24-hr period ending 8AM Fri July 19. Because of the coarse resolution of GOES-IR channel (currently 4km), the satellite is going to miss some of the smaller-scale extremes that will show up in individual gauge reports since the satellite represents an average value over the entire pixel. Even taking that into consideration, satellite based estimates were greatly underestimated for this event. Another factor that may have contributed to the underestimation of precipitation by GOES was the temporal resolution. Rain rates are currently derived every 15 minutes and that didn’t seem to keep up with the rapid evolution and explosiveness of the convection. At one time during the event, 15-minute rain rates were 0.75 inches with 1-hr rain rates of 3.10 inches. However, in the future GOES-R era, both the spatial and temporal resolution will be improved by a factor of 2 with the resolution of the infrared channel at 2km and routine imagery available every 5 minutes.
The 24-hr rainfall total (image below) precipitation ending for the same period described above shows widespread rainfall amounts of 1-3 inches of rain across all of PR. Over the western half of PR, the satellite-based estimates appeared to have overestimated the amount of rain. This is because the satellite can’t see through thick cirrus clouds and incorrectly assigns a rain rate because the cirrus appear to be a cold cloud. This is a well-known limitation of the current algorithm to derive rain rates from current GOES.