Showers and thunderstorms developed over the northern half of PR Sat Sep 14 and produced localized areas of 2 to 4 inches of rain that resulted in urban flooding and a new daily rainfall record at the San Juan (SJU) LMM Int’l Airport. A total of 2.88 inches fell at the SJU airport on Sat Sep 14 with 2.52 inches falling in less than 2 hours between 1145 AM and 130 PM. These showers and thunderstorms developed as a result of upper level diffluence ahead of an upper level low over the central Caribbean Sea aided by surface convergence due to sea breeze interaction and abundant moisture. Figure 1 below shows a four panel display of various water vapor channels from the GOES-Sounder and GOES imager overlaid with ECMWF model height fields and 15/00Z RAOB data. Figure 2 below shows the 12Z Sat Sep 14 Skew-T for SJU. Note the 20-35-kt south southwest flow at 300 and 200 mb on both the four panel display and the 12Z Skew-T respectively.
Animation of GOES-IR imagery (Fig. 3 below) with increased temporal resolution due to Rapid Scan Operations (RSO) in effect shows the development of showers and thunderstorms first over the San Juan area around 1445 UTC and then over north central and northwest PR through 2015 UTC. The imagery shows cold cloud tops ranging from -50C to -70C that were displaced to the north due to 20-35 kt south southwest flow aloft seen on the 12Z SJU RAOB data.
As part of GOES-R Proving Ground activities, WFO SJU continues to evaluate the NESDIS QPE product which uses mainly GOES-IR channel to estimate rainfall rates. Satellite based rainfall estimates for the 24-hr period ending 12 UTC Sun Sep 15 (Fig 4 below) showed up to an inch of rain fell just offshore of San Juan due to the coldest cloud tops being displaced to the north due to 20-35 kt south southwest flow seen on the 12Z upper air data.
Gauge data (Fig. 5 below) and Dual-Pol radar estimates (Fig. 6 below) indicate that in general between 1 to a little over 3 inches fell over parts of northern PR. This clearly indicates the need to continue to improve the GOES-R QPE algorithm to estimate rainfall and also to account for shear and cloud storm motions.