Puerto Rico Caribbean Waters from 10nm to 17N Marine Forecast
| Rest Of Today...South Winds 5 To 10 Knots. Seas Around 2 Feet Or Less. Dominant Period 11 Seconds. Isolated Showers. |
| Tonight...West Winds 5 To 10 Knots, Becoming Northwest After Midnight. Seas Around 2 Feet Or Less. Dominant Period 11 Seconds. Isolated Showers. |
| Thursday...West Winds 5 To 10 Knots. Seas 1 To 3 Feet. Dominant Period 11 Seconds. Isolated Showers. |
| Thursday Night...North Winds 10 To 15 Knots. Gusts Up To 25 Knots In The Evening. Seas 3 To 5 Feet With Occasional Seas Up To 6 Feet. Dominant Period 11 Seconds. Scattered Showers. |
| Friday...Northeast Winds 10 To 15 Knots With Gusts Up To 25 Knots. Seas 3 To 5 Feet With Occasional Seas Up To 6 Feet. Dominant Period 11 Seconds. Isolated Showers. |
| Saturday...Southeast Winds 5 To 10 Knots. Seas 2 To 4 Feet With Occasional Seas Around 5 Feet. Dominant Period 12 Seconds. Isolated Showers. |
| Sunday...Southeast Winds 5 To 10 Knots, Becoming East. Seas 3 To 5 Feet With Occasional Seas Up To 6 Feet. Dominant Period 12 Seconds. |
| Area Forecast Discussion National Weather Service San Juan PR 426am AST Sat September 27 2025 .KEY MESSAGES... * An Extreme Heat Warning is in effect for all coastal areas and urban sectors of Puerto Rico, while a Heat Advisory is in effect for Culebra, Vieques, and the U.S. Virgin Islands. Heat indices are forecast to peak between 12pm and 2 PM, reaching up to 114°F. * An increase in moisture is forecast from Monday into Tuesday; however, Saharan dust will also move in, resulting in hazy skies in some isolated areas. * A long period northeasterly swell from Hurricane Humberto will arrive across the northern local waters, increasing seas and breaking waves, and resulting in a high risk of rip currents. Additionally, there is a small craft advisory in effect from this afternoon across the offshore Atlantic waters. .SHORT TERM...Today through Monday... For today, a meandering shallow band of moisture between Major Hurricane Humberto to our northeast and PTC 9 near Cuba will support an unstable and moist airmass over the local area. A weak mid-level ridge will move over and southwest of the region by Sunday. Light to calm winds with a southerly component will continue through the rest of the weekend as TC Humberto moves north of the area. Therefore, diurnally induced afternoon showers and thunderstorms, triggered by daytime heating, weak sea breezes, and orographic lift, are expected this afternoon. Due to the light steering winds, slow-moving afternoon thunderstorms will increase flooding concerns, mainly across PR. At the same time, the USVI should see an increase in showers throughout the morning hours today, with a slight chance of thunderstorms as they drift slowly between the Atlantic waters and the Anegada Passage. Another surge in passing showers is expected by late Monday. Although a dip in PWAT (Precipitable Water) is expected on Sunday, the 850-700mb lapse rates will remain elevated; therefore, strong afternoon convection is possible once again over portions of the Cordillera and western PR. Elsewhere, the mid-level ridge and drop in PWAT (Precipitable Water) are expected to inhibit widespread shower development over the area. Winds will pick up between 10-15 kt on Monday with an easterly component, and a weak Saharan Air Layer will bring minor concentrations of Saharan dust. However, as Humberto moves into the Southwestern Atlantic, it will pull another shallow band of moisture over the islands, which will enhance the diurnal convection cycle over the islands. Temperatures and overall humidity will also remain well above average, as the 925mb temperature and 850mb Theta-e values are expected to continue at normal to above-normal levels, particularly today and on Sunday. Therefore, despite the active afternoon convection, high heat stress will be a primary concern. High temperatures are expected to remain in the low to mid-90s across most coastal areas, with Extreme Heat Warnings/Heat Advisories anticipated each day. Furthermore, swells generated by Humberto will increase the risk of life-threatening rip currents along the north and east-facing beaches of the islands from late today through Sunday night. Long Term Tuesday through Saturday... The long-term forecast remains on track. On Tuesday, the surface pattern will continue under the influence of Hurricane Humberto well north of the region and a strengthening surface high pressure over the central Atlantic, resulting in a weak southeasterly wind flow. Under this setup, the islands will experience brief intrusions of Saharan dust with precipitable water values near 1.6 inches, close to the 25th percentile for the year. This pattern will favor a generally stable day with calm conditions in the morning and localized afternoon activity. Similar weather will persist on Wednesday, while on Thursday precipitable water is forecast to decrease further, with global model guidance suggesting values nearly two standard deviations below normal. Relative humidity between 700–500 mb also shows a drying trend, supporting mostly stable mornings with only localized showers across the interior regions. A more unsettled pattern is expected from Friday into Saturday as global model guidance indicates an increase in instability, with 700–500 mb lapse rates steepening and 500 mb temperatures slightly decreasing to around –6 °C due to an upper-level trough extending from the western Caribbean into the islands. At the surface, a weakening of the pressure gradient is anticipated as a tropical system moves into the northwestern Atlantic, reducing the influence of the surface high pressure. This evolving pattern will induce surface convergence, enhance instability, and result in very weak steering flow. Consequently, a variable pattern is forecast, with long-lasting afternoon showers primarily affecting the interior, but drifting into different sections of the islands due to weak winds. Marine A pulse of a long period northeasterly swell from Major Hurricane Humberto will increase seas across the local waters from this afternoon through the rest of the weekend. Seas are forecast to build to around 7 feet across the offshore Atlantic waters by 2 PM, spreading into the coastal Atlantic waters and the Anegada Passage during the weekend. As a result, a Small Craft Advisory will be in effect starting at noon today and extend to the coastal Atlantic water later toninght into Sunday. Seas are expected to remain between 7 and 8 feet across the Atlantic waters and up to 5 feet across the Caribbean waters. Improving marine conditions are forecast from Monday as the energy from the swell diminish across the area. Beach Forecast A long period northeasterly swell from Major Hurricane Humberto will increase breaking waves starting at 12pm this afternoon. As a result, deteriorating coastal conditions are expected, especially across the northern coastal areas of Puerto Rico, including beaches along the northeast PR. For Culebra, Vieques, the U.S. Virgin Islands, and northwestern Puerto Rico, the risk will remain moderate. However, deteriorating conditions are forecast to spread to more coastal areas, including St. Croix, Culebra, and the northwestern coast of Puerto Rico, during the weekend. Improving coastal conditions are expected beginning on Monday. .SJU WATCHES/WARNINGS/ADVISORIES... PR...Extreme Heat Warning from 10am this morning to 5pm AST this afternoon for PRZ001>005-007-008-010-011. High Rip Current Risk from noon AST today through late Sunday night for PRZ001-002-005-008. Heat Advisory from 10am this morning to 5pm AST this afternoon for PRZ012-013. VI... Heat Advisory from 10am this morning to 5pm AST this afternoon for VIZ001-002. AM...Small Craft Advisory from noon today to midnight AST Sunday night for AMZ711. Small Craft Advisory from 6am Sunday to midnight AST Sunday night for AMZ712. Small Craft Advisory from 6am to 6pm AST Sunday for AMZ716. Small Craft Advisory from noon Sunday to midnight AST Sunday night for AMZ741. |
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