AFDSJU Area Forecast Discussion National Weather Service San Juan PR 456 AM AST Mon Nov 17 2025 ...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST... .KEY MESSAGES... Issued at 428 AM AST Mon Nov 17 2025 * A high risk of rip currents is in effect along the north and east facing beaches of Puerto Rico and St. Croix in the U.S. Virgin Islands through Wednesday. * Slow-moving showers and isolated thunderstorms will increase the flooding and lightning risk from the interior to western portions of Puerto Rico this afternoon. * More frequent showers and isolated thunderstorms are likely late tonight and early Tuesday morning across the U.S. Virgin Islands. && .Short Term(Today through Wednesday)... Issued at 428 AM AST Mon Nov 17 2025 During the overnight hours, mostly calm and stable conditions prevailed across the region. Skies ranged from mostly clear to partly cloudy, and minimum temperatures fell into the low to mid-70s across most coastal areas, with a few locations across the San Juan metropolitan area and the USVI holding in the upper 70s. Winds remained very light to calm under a weak surface pressure gradient. For today, the forecast remains on track. As discussed previously, the forecast area will be positioned under the subsident side of a mid- to upper-level trough, while a stationary frontal boundary lingers well to the northeast of the islands. Global model climatology comparisons from the 17/00Z runs show a gradual warming of the 500 mb temperatures as the upper trough continues to shift eastward, with values rising toward -5 to -6 degrees Celsius. Although this indicates some erosion of upper-level dynamics, the temperatures remain cool enough to support moderate instability, an environment reflected by 700-500 mb lapse rates that remain near or slightly above climatological mid-range values. A reduction in moisture is also expected, with precipitable water values between 1.3 and 1.4 inches, which is slightly below normal for mid-November. These conditions should support a quiet morning with only a few brief showers moving in from the waters and occasionally brushing coastal sectors. By the afternoon, the combination of diurnal heating, sea-breeze convergence, and local effects will likely generate scattered showers and possibly a few short-lived thunderstorms across the interior and western/northwestern portions of Puerto Rico. High-res models continue to suggest convective development despite the marginal moisture and weakening instability aloft, and under a southeasterly steering flow, some showers may also develop downwind of El Yunque and drift toward the San Juan metropolitan area. Given the relatively dry mid-levels and the short duration of convection, the flooding risk remains low for today. Moisture levels are forecast to recover slightly to near-normal values on Tuesday, with winds gradually shifting from southeasterly to more easterly. A surface trough located near the Leeward Islands may approach the region; however, most associated shower and thunderstorm activity is expected to remain south across the Caribbean waters. Even so, afternoon convection driven by local effects and sea-breeze interactions may still develop over Puerto Rico, but rainfall accumulations should be limited and the flood risk low. By Wednesday, conditions become more favorable for convection as another polar trough deepens and moves eastward across the western Atlantic into the northeastern Caribbean. The region is expected to lie beneath the more favorable, divergent side of this upper-level feature during the afternoon and evening hours. Model climatology comparisons show 500 mb temperatures dropping significantly to around -8 to -9 degrees Celsius, along with steepening 700-500 mb lapse rates and a notable decrease in 250 mb height fields and 1000- 500 mb thickness values, all indicative of more favorable upper- level support and increased instability. These changes should enhance the potential for more widespread showers and isolated thunderstorms. Northeasterly steering winds may slightly limit surface heating, but convective development may still be sustained by stronger upper-level forcing, allowing showers and thunderstorms to continue or redevelop through the evening and into the overnight hours across the Atlantic waters, with occasional movement inland. Under these conditions, the flood risk may increase on Wednesday, especially in areas that experience repeated showers. && .Long Term(Thursday through Monday)... Issued at 428 AM AST Mon Nov 17 2025 A gradual improvement in the weather conditions is still anticipated by the end of the workweek into the upcoming weekend. Thursday may be the wettest day of the period due to lingering moisture across the region and the influence of the polar trough. From the deterministic guidance of the GFS and ECMWF, seasonal Precipitable Water values (1.5 - 1.6 inches) and colder 500 mb temperatures (around -8.5 degrees Celsius) are likely. Combined with local effects and daytime heating , afternoon convection may develop over the mountain ranges and southwestern Puerto Rico. Although rainfall accumulations may not reach significant flooding criteria, ponding of water over roadways, urban areas, and poorly drained areas may lead to isolated urban and small stream flooding over the aforementioned areas. By Friday, a surface high pressure system should build over the western Atlantic, promoting mostly NE winds. Weather conditions should improve over the weekend as drier air filters into the region with a mid level ridge dominating the weather pattern. From the latest model guidance, PWAT values are likely to decrease and remain below climatological normal (between 1.2 - 1.4 inches), while 500 mb temperatures will get warmer than normal by the end of the period. Patches of moisture will move occasionally across the CWA, bringing light to moderate passing showers over windward sections of the islands in the late night into the morning hours. Although afternoon convection is expected, shower activity will likely be limited under this weather pattern. Therefore, the flooding and lightning threat is not expected for the rest of the forecast period. Model guidance continues to suggest a fall in 925 mb temperatures, being Thursday cooler than normal. Although the latest GFS solution is tending to warmer temperatures for Saturday, temperatures should remain seasonal, presenting no heat threat. Hence, the heat risk should remain low for the rest of the period. && .AVIATION... (06Z TAFS) Issued at 428 AM AST Mon Nov 17 2025 VFR conds will prevail through 17/15Z with SKC to FEW/SCT and light/VRB winds. Brief -SHRA over local waters may brush coastal terminals but with no significant impacts. Aft 17/14Z, winds increase from the ESE-SE at 08-13 kt with sea-breeze influences. Aftn convection over the interior and wrn PR may bring VCSH/VCTS to TJSJ and TJBQ, with brief MVFR and mtn obscurations possible. PROB30 TSRA was added for TJBQ/TJSJ during peak heating. Convection diminishes aft 17/22Z with light winds returning. && .MARINE... Issued at 428 AM AST Mon Nov 17 2025 A fading pre frontal trough north of the area will keep light to gentle southeasterly winds today, becoming moderate from the east late tonight due a surface high pressure over the Central Atlantic. Pulses of a north northeasterly swell will spread across the Atlantic waters and local passages through Wednesday. Another approaching pre frontal trough will promote winds from the northeast later in the workweek. && .BEACH FORECAST... Issued at 428 AM AST Mon Nov 17 2025 As mentioned in previous discussions, conditions were expected to deteriorate due to pulses of north-northeasterly swells arriving over the region. Current buoy observations show an increase in period (between 10 - 12 seconds, up to 15 seconds for the San Juan buoy), which results in breaking waves around 6 feet, promoting hazardous beach conditions. Hence, a high risk of rip currents is in effect for the northern and eastern beaches of Puerto Rico, Culebra, and St. Croix in the U.S. Virgin Islands. Residents and visitors planning on visiting the beach are urged to exercise caution, as life- threatening rip currents are present along the surf zone. Heed the advice of the flag warning system and swim near a lifeguard. Afternoon convection may produce gusty winds and lightning, beachgoers should remain weather alert and seek shelter whenever they hear thunder. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...High Rip Current Risk through Wednesday afternoon for PRZ001-002- 005-008-012. VI...High Rip Current Risk through Wednesday afternoon for VIZ002. AM...None. && $$