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989 FXCA62 TJSJ 181740 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 140 PM AST Thu Sep 18 2025 .KEY MESSAGES... * Across the U.S. Virgin Islands, a showery pattern will continue through early this evening, enhancing the flood potential. * A warming trend is expected to begin on Friday, bringing increasing temperatures that may cause heat indices to reach Heat Advisory or Extreme Heat Warning criteria through at least early next week, particularly in coastal and urban areas. * Swells from Tropical Storm Gabrielle will move across the local waters during the weekend, increasing the potential for life- threatening rip currents across the north and east facing beaches of the islands. Seas are expected to remain below Small Craft Advisory criteria. && .SHORT TERM...Rest of today through Saturday... An elongated induced low-level trough over the local area produced persistent rainfall across the U.S. Virgin Islands and portions of eastern Puerto Rico during the past 8-12 hours. Observed rainfall amounts up to 6 inches were reported in St. Croix and up to 4 inches in Las Piedras and Humacao. By noon, high temperatures were in the low 80s across most lower elevations of the islands, except in western PR, where temperatures were in the mid-80s to low 90s. Winds were light with a southerly component, and sea breeze variations in northwestern PR. For the rest of the afternoon hours, additional showers and thunderstorms are expected to redevelop across the Caribbean waters, and under southerly steering winds move again over portions of the USVI. Therefore, flooding concerns will continue across the USVI territories, particularly in St. Croix. Across PR, the interaction of the induced trough and the abundant tropical moisture will produce scattered to locally numerous showers, mainly over western PR, while the eastern half of PR will continue to experiment periods of stratiform rain to possible heavy showers from this evening through at least Friday morning. For the rest of the forecast period, the proximity of an upper- level low to our north and the induced low-level trough to our west will continue to promote an unstable weather pattern across the region. Meanwhile, a light southerly wind flow is expected to prevail, and heat advisory to extreme heat warning conditions are expected to return once again across most coastal areas of the islands, and before the onset of afternoon convection each day. .LONG TERM...Sunday through Thursday... For the long-term period, as Tropical Storm Gabrielle moves well northeast of the region, our area will be positioned in a col region. As the pressure gradient weakens, very light east- southeasterly winds are expected to persist on Sunday and continue through at least Monday night. This flow will help maintain a healthy moisture content across the area. According to the latest model guidance, precipitable water values are forecast to fluctuate between 1.80 and 2.20 inches from Sunday through Tuesday, which is within the average to above-average range for this time of year. In terms of temperature, forecast data suggests that 925 mb temperatures will peak at nearly two standard deviations above normal on Sunday and remain near the 75th percentile into early next week. This pattern will increase the risk of heat-related impacts across the region. Heat Advisories or even Extreme Heat Warnings may be issued. Under this scenario, expect morning showers across windward areas of the islands, followed by afternoon showers and isolated thunderstorms across the interior and western sectors of Puerto Rico, fueled by diurnal heating and local effects. Urban and small stream flooding is likely to persist each day, and some rivers may reach flood stage. Saturated soils from recent rainfall combined with light winds during this period will contribute to slower-moving showers, which could lead to higher rainfall totals and an increased risk of flooding. Additional hazards may include landslides, frequent lightning, and gusty winds. On Tuesday night, a wind surge is expected to approach the region, bringing gusty winds along with increased moisture and a higher chance of rainfall through the middle of the week. By Wednesday and Thursday, one of the tropical waves currently being monitored by the National Hurricane Center is expected to approach, further enhancing a wetter pattern across the islands. We will continue to monitor this evolving situation closely and provide updates as necessary. && .AVIATION... (12Z TAFs) SHRA/TSRA in and around the USVI terminals will continue to cause tempo MVFR to brief IFR conds thru at least 18/22z. Across the PR terminals, mostly -RA expected with mtn tops obscd and BKN/OVC lyrs fm FL060-FL100. The 18/12z TJSJ sounding indicated SE winds up to 13 kt blo FL080. && .MARINE... A weak surface high-pressure over the central Atlantic will promote light to locally moderate east to southeast winds today. For the rest of the forecast period, lighter southeasterly winds will prevail as the local pressure gradient weakens. High moisture content from a departing tropical wave and an induced trough will continue to result in frequent showers and strong thunderstorms over the local waters through tonight. A long period northeasterly swell generated by Tropical Storm Gabrielle will reach the local waters and passages during the weekend. However, seas are forecast to remain below small craft advisory criteria. && .BEACH FORECAST... There is a moderate risk of rip currents for the north and east- facing beaches of Puerto Rico, Culebra, and St. Croix, with a low risk elsewhere. While a brief improvement in conditions is expected on Friday, the moderate risk will return Saturday and persist into early next week, driven by swells from Tropical Storm Gabrielle. However, beachgoers should exercise caution at all times, due to the increasing threat of life-threatening rip currents. In addition, afternoon thunderstorms producing cloud to ground lightning are expected each day, mainly across the north and western coastal areas of PR. Remember, if you hear thunder, seek shelter in a safe indoor location immediately. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...DSR LONG TERM....YZR AVIATION...DSR BEACH/MARINE...DSR PUBLIC...LIS/MNG
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