881 FXCA62 TJSJ 272031 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 431 PM AST Thu Jul 27 2023 .SYNOPSIS...A relatively fair weather pattern with limited shower activity will prevail tonight. Showers with isolated thunderstorm activity generated by a weak tropical wave will likely cause urban and small stream flooding and localized flash flooding across Puerto Rico, with lesser impacts across the U.S. Virgin Islands late Friday into Saturday. Excessive heat hazard risks are likely each afternoon, with heat indices peaking at 102 degrees or higher. Moderate rip current risk and locally-hazardous marine conditions will prevail during the next few days. && .SHORT TERM...Tonight through Saturday... Limited to elevated excessive heat and limited and wind hazard risk levels are likely each day. Elevated excessive rainfall and lightning hazard risk levels are likely on Friday and Saturday. Afternoon convection development was observed over Puerto Rico, clustering over portions of the San Juan metro area and far western sections of the islands. So far, a Flood Advisory has been issued for Bayamon, Toa Baja, and Toa Alta, where radar-estimated rainfall totals peaked around an inch and a half. Preliminary rainfall totals also indicated about one to two inches of rain fell in rural areas of Lares, Las Marias, Mayaguez, and Anasco. Daytime high temperatures peaked into the mid 80s to the lower 80s across lower elevations and urban areas, with heat indices as high as 110 degrees. Winds were generally easterly, peaking at 10-20 mph with gusts up to 25-30 mph. In close agreement with current satellite-derived total precipitable water (PWAT), the most recent model guidance suggests that a drier air mass will dominate the local weather tonight into Friday, with PWAT falling as low as 1.3-1.4 inches by Friday morning. Although this drier air mass should support fair weather conditions tonight, clusters of low-level clouds and passing trade wind showers will likely move from the waters over the local islands and eastern Puerto Rico. By Friday and continuing through the weekend, a weak tropical wave and associated moisture field will move over the forecast area, yielding above climatological PWAT of 1.9 inches or higher between Friday evening and Saturday evening. Another area of drier air trailing the tropical wave will invade the local islands on Saturday night, causing another PWAT drop and generally fair weather conditions. Despite moisture variations, a seasonal shower pattern distribution with the potential for limited excessive rainfall risks will prevail each day. However, a significant increase in moisture will heighten such pattern distribution, with increased shower and thunderstorm activity yielding elevated excessive rainfall hazard risks. Steered by persistent east-southeasterly winds generated by a broad surface high pressure across the central Atlantic, the focus of the activity will shift from the northwestern quadrant of Puerto Rico in the afternoon to the local islands and eastern Puerto Rico at nighttime, where urban and small stream flooding is likely. Daily temperature variations will range from the lower 90 degrees across coastal and other urban areas in the daytime to about 60 degrees across higher elevations at nighttime. Southeasterly winds in a high moisture environment could yield 102 degrees or higher heat indices. Some areas may even experience heat indices of 108 degrees or higher. .LONG TERM...Sunday through Thursday... From previous discussion...issued at 505 AM AST Thu Jul 27 2023 On Sunday, rapid improvement is expected as a drier air mass accompanied by a low to moderate concentration of Saharan dust, will filter in and spread across the region. This drier airmass along with the increasing tradewinds will aid in eroding moisture pooling and thus limit widespread shower activity across the islands and coastal waters at least until early Monday. Based on latest model guidance both suggest the driest period to be Sunday into Monday with more seasonal weather conditions expected dominated by the Atlantic high pressure ridge and the moderate easterly trades. For the rest of the period both model guidance continued to suggest inconsistencies and uncertainty in the overall weather pattern with the approach of a tropical wave which The National Hurricane Center continued to monitor now located a couple of hundred miles southwest of the Cabo Verde Islands. Although inconsistent, both model guidance (GFS and ECMWF), suggest a chance of further development as the tropical wave moves westward into Central Tropical Atlantic. While the GFS,now suggest increasing moisture transport and northeast winds across the region by late Monday through Tuesday, the ECMWF model remain less aggressive and suggest drier condtions with the feature moving even farther northeast of the region during the same time period. All in all, expect a gradual increase in tropical moisture and humidity by the latter part of the period. Given the discrepancies in guidance for the forecast period, confidence remains low for now due to the disrupted weather pattern. The expected wind flow for now should become more east to northeast during the early part of the period, then become more south to southeast by the end of the period. Will therefore remain on the conservative side and continue with the typical summertime weather steered by the Atlantic high pressure ridge and local steering flow along with a slight but gradual increase in advective moisture through Wednesday followed by a drying trend on Thursday. The overall weather pattern for now is for typical passing morning showers becoming more frequent Monday through Tuesday, followed by afternoon convection with isolated diurnally driven thunderstorm activity each day especially Monday through Tuesday when urban and small stream flooding will be possible. Please continue to monitor how the aforementioned feature unfolds and stay tuned for additional info and updates issued by the local weather office WFO San Juan and the National Hurricane Center. && .AVIATION...SHRA/ISO TSTMS over western PR should end around sunset. Otherwise, mainly VFR conditions are expected to prevail across all terminals through the forecast period. However, isolated thunderstorms are expected to develop once again over west/northwest PR between 28/16z-22z. ESE will continue at 12-16 kt, becoming light and variable overnight. && .MARINE...Recent CariCOOS buoy network observations around Puerto Rico and the U.S. Virgin Islands indicated steady seas between 2 and 4 feet. Across the local waters, seas should remain 5 feet or below and favorable for small craft tonight. However, small craft operators should exercise caution across coastal waters of northern Puerto Rico, offshore Caribbean waters, and the Mona Passage due to locally wind-driven choppy seas. Increasing showers with thunderstorm activity and higher winds from a weak tropical wave could yield locally hazardous marine conditions by Friday into Saturday. A moderate rip current risk will return for most north-facing beaches of Puerto Rico and spread to other areas and the local islands during the next few days. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...DSR/CAM LONG TERM....ICP AVIATION...DSR/CAM PUBLIC...MMC/RC