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498 FXCA62 TJSJ 171740 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 140 PM AST Wed Sep 17 2025 .KEY MESSAGES... * For the rest of today into the evening, showers and thunderstorms will persist over the local waters due to the combined influence of a tropical wave and an upper-level trough just north of the region. * A warming trend is expected to begin on Friday across the islands, with warmer conditions allowing heat indices to reach Heat Advisory criteria in coastal and urban areas. * Across the U.S. Virgin Islands, a showery pattern will persist through the afternoon into the evening, leading to a limited increase in flood potential. * A moderate risk of rip currents is in effect today and tonight for the northern and eastern beaches of Puerto Rico, as well as Culebra and St. Croix. This weekend, Tropical Depression Seven is forecast to move well northeast of the area as a Tropical Storm and may generate swells, further deteriorating marine and beach conditions. && .SHORT TERM...Rest of Today through Friday... Shower activity diminished in the early morning hours, with some remaining thunderstorms observed across the Caribbean waters and around southeastern Puerto Rico. Some showers managed to form in the interior and west. It was very warm too, with highs ranging from the mid-80s to the low 90s across coastal areas of Puerto Rico and in the Virgin Islands. The axis of the wave was analyzed just to the east of the Virgin Islands, where an extend area of above normal precipitable water is observed in satellite-derived products. As this feature continues to move westward, the winds will shift from the northeast, increasing the potential for showers and thunderstorms across the local islands. This afternoon, the focus of the rain should be around western Puerto Rico, and also around the Virgin Islands and eastern Puerto Rico. Although the rain is not expected to be widespread, these areas that receive heavy rain can expect some urban and small stream flooding, isolated mudslides, water surges along rivers, and lightning and gusty winds. Tonight and tomorrow, as the wave moves toward the Dominican Republic, the wind flow is expected to shift from the southeast or south-southeast. The focus of the rain then will be along the southern and eastern half of Puerto Rico, while some showers will also reach portions of the Virgin Islands, Vieques and Culebra. Threats are impacts are expected to be similar, with rainfall accumulations of 1 to 2 inches, and isolated higher amounts. On Friday, the pressure gradient weakens, with a deep layer wind flow from the south, and surface speeds around 10 kts or less. With these winds, 925 mb temperatures will go up, again nearly two standard deviation above the climatological mean. This translate into very hot conditions, with a level of heat that affect anyone without effective cooling or adequate hydration. Lighter south winds should also focus the afternoon showers and thunderstorms along the interior and the northwest quadrant of Puerto Rico. Again, urban and small stream flooding, rapid river rises along rivers, and frequent lightning are the main impact associated with the rain. .LONG TERM...Saturday through Wednesday... //from previous discussion// By Saturday, on of the systems the National Hurricane Center is monitoring, recently formed Tropical Depression Seven, should be located about 550 miles northeast of the area. If the forecast track continues as expected, a col region over our area will promote weak and variable winds at least through Tuesday night. The latest model guidance suggests that precipitable water (PWAT) values will remain within the average range for this time of year, between 1.70 and 1.90 inches on Saturday, decreasing to between 1.25 and 1.50 inches by Sunday and Monday. 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. As a result, an elevated flood risk is in place for Saturday, while the risk will be more limited on Sunday and Monday, particularly across the aforementioned areas. With soils already saturated in parts of Puerto Rico due to recent rainfall and lighter winds expected during this period, showers will likely move slowly, leading to greater rainfall accumulations and an increased risk of flooding. Urban and small stream flooding is likely to persist each day, and some rivers may reach flood stage. In the Virgin Islands, periods of showers and thunderstorms are expected to cause water surges along guts and drainage channels, ponding on roadways, and isolated urban flooding. Additional hazards include landslides, frequent lightning, and gusty winds. By Tuesday night, a wind surge is expected to approach the area, bringing gusty winds and increasing moisture and rain chances for the middle part of the week. Flood risk will likely remain elevated. This wind surge appears to be associated with the second tropical wave the National Hurricane Center is monitoring, which currently has a 10 percent chance of formation in the next 48 hours and a 20 percent chance over the next seven days. At the moment, it seems this system will remain trapped well east of our region while surface high pressure dominates to the north. We will continue to monitor this closely and provide updates as needed. In terms of temperature, 925 mb temperatures are forecast to peak at nearly two standard deviations above normal on Saturday and remain near the 75th percentile from Sunday through early next week. This pattern will elevate the heat risk across the region. Heat Advisories or even Extreme Heat Warnings may be issued. Residents and visitors are strongly advised to stay hydrated, avoid prolonged sun exposure during peak heat hours, and take extra precautions, particularly those without access to adequate cooling or hydration. && .AVIATION... (18Z TAFs) Brief period of MVFR conditions will remain possible in and around TJBQ through 17/22z. Overnight...Passing showers may affect the USVI terminals, nevertheless mainly VFR conditions expected. Easterly winds 10 to 15 knots through 17/22z with sea breeze variations, becoming light and variable overnight. Gusty winds near showers. && .MARINE... A surface high-pressure system over the central and eastern Atlantic will promote moderate to locally fresh easterly winds, becoming light from Thursday night onward. Small craft should exercise caution across the offshore waters and local passages through at least tonight. A tropical wave moving across the region will result in frequent showers and stronger thunderstorms over the local waters, mainly today through Thursday. Additionally, swells generated by Invest AL92, as it moves northeast of the region, could further deteriorate marine conditions this weekend, increasing the risk of hazardous seas. && .BEACH FORECAST... No changes were introduced to the coastal forecast. For the rest of the day, the moderate risk of rip currents will continue across the north and east-facing beaches of Puerto Rico, Culebra, and St. Croix, while a low risk is expected elsewhere. Similar conditions are expected to prevail through most of the workweek, so beachgoers are advised to exercise caution at all times, as a moderate risk means life-threatening rip currents are possible in the surf zone. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...ERG LONG TERM...YZR AVIATION...OMS MARINE/BEACH FORECAST...LIS
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