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Area Forecast Discussion (AFD)
676 FXCA62 TJSJ 291702 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 102 PM AST Tue Sep 29 2026 ...New SHORT TERM, AVIATION, MARINE, BEACH FORECAST... .KEY MESSAGES... Issued at 1236 PM AST Tue Sep 29 2026 - Limited heat risk expected for the next few days, as easterly trade winds strengthen, allowing only localized areas across Puerto Rico and the US Virgin Islands to experience brief periods near heat advisory criteria. - Trade wind showers will continue to move across the USVI and eastern sections of PR during the night/early morning hours, followed by afternoon thunderstorms over western PR each day. - Higher moisture content is expected from Thursday through Saturday. - A moderate risk of rip currents is expected to continue along the west, north, and east facing beaches of the islands through the end of the week. && .Short Term(This evening through Thursday)... Issued at 1236 PM AST Tue Sep 29 2026 A couple of synoptic features are currently setting the scene for the observed weather pattern over the CWA. A lingering upper-level trough located to the northeast of the US Virgin Islands is currently promoting a region of enhanced upper-level convergence which is acting to limit convective activity. Furthermore, the steering flow for the duration of the short term forecast will be driven by a building surface high pressure system currently located over the east Atlantic Ocean. This system will lead to the strengthening of the steering flow with light to locally breezy winds expected today and Thursday, and breezy to locally windy conditions expected by Wednesday. This increase in wind flow will strengthen the potential for the development of advective showers and streamers from the US Virgin Islands, Vieques, Culebra, and EL Yunque rainforest during the morning into the afternoons, potentially impacting the windward waters and eastern sectors of Puerto Rico. This means that the activity that develops over the remainder of the short term forecast will be driven by the building surface high pressure and local effects. For today, while mid-to-upper levels will act to inhibit the development of showers and thunderstorms, there are other features that may aid in the eventual development of rainfall activity, particularly across the southwest of Puerto Rico during the afternoon hours. There are lower than average mid-tropospheric temperatures, this favors the potential for deep convective cells if activity does develop over the region. In spite of the drier mid- levels, precipitable water values are expected to increase by the afternoon across southwestern sector of Puerto Rico. Model guidance suggest that these conditions coupled with local effects will lead to the development of showers and thunderstorms over the region. Given the light winds, activity may linger, this means there is a limited risk of urban and small stream flooding across the region. Where rainfall occurs you may expect the ponding of water along roads and poorly drained areas. Similar conditions are expected by Wednesday, with the difference that mid-tropospheric temperatures will increase and become less amenable for the development of deep convective cells. Nonetheless, the steering flow will also increase and shift to be predominantly from the E. This will favor the potential for the development of advective showers and streamers from el Yunque and the smaller islands. Hence, a more active morning can be expected with impacts predominantly focused on the US Virgin Islands, windward waters and eastern Puerto Rico in the morning. The shift in the wind flow will also move the focus of the afternoon convection to the western and western interior regions of Puerto Rico. Rainfall associated impacts are expected to be similar to those for today. A similar pattern is forecasted for Thursday. With activity expected to be focused on the western interior and the northwestern regions of Puerto Rico during the afternoon. This pattern will by similarly set by local effects and breezy-to-locally windy ESE winds. It will be further enhanced by increasing precipitable water values over the west of Puerto Rico during the afternoon. Given the breezy-to-locally windy conditions along with average precipitable water values, the low-level temperatures are expected to range around average values for the season. Thus, lower temperatures to those experienced during the previous days may be expected. Individuals that are sensitive to heat should remain alert as the heat risk remains limited throughout the short term forecast. As always, citizens and visitors are invited to keep updated with the evolving weather conditions. && .Long Term(Friday through next Monday)... Issued at 133 AM AST Tue Sep 29 2026 Unstable conditions are still expected on Friday and Saturday, transitioning to a typical weather pattern Sunday onward. Model guidance continues to suggest a surface high pressure the western Atlantic, promoting an E-SE wind flow throughout the period. As mentioned in the previous discussion, a surface trough may weaken the surface high, with isolated breezy conditions across coastal areas of the islands. Friday and Saturday remain the active days of the long-term, as the latest model guidance suggests that the presence of upper-level troughs should introduce instability across the eastern Caribbean. In terms of moisture, the deterministic guidance of the GFS and ECMWF continues to suggest abundant tropical moisture across the region, with PWAT values between 1.8 and 2.0 inches, mainly in the low and mid levels. Additionally, model sounding plots continue to show skinny profiles, with weak inversion caps that could easily be broken and high CAPEs. Due to the proximity of the troughs, mid-levels might get cooler than expected (500 mb temperatures reaching -8 C) with steeper lapse rates in the mid-levels, allowing cloud growth and instability aloft. Therefore, the most likely scenario should be for isolated to scattered showers across the windward sections during the night and morning hours, with deep convection in the afternoon, particularly across the interior and western/northwestern. Additionally, local island streamers are also expected, affecting the San Juan Metro Area. On Sunday, drier air is expected to filter in the low-to-mid-levels (850 - 500mb), inhibiting deeper convection over the islands, as precipitable water (PWAT) values are expected to drop to seasonal or below-seasonal values. However, local effects and daytime heating driven by an easterly flow should promote afternoon convection mainly over the interior to western half of Puerto Rico. A similar pattern is expected on Monday, as drier air persists in the low-to- mid levels. In general, shower and thunderstorm activity is expected each day, with the highest flooding threat on Friday and Saturday. Rainfall accumulations could enhance the chance of urban and small stream flooding over the aforementioned areas. Meanwhile, Sunday and Monday rainfall activity may lead to ponding of water over roadways and poorly drained areas, including minor flooding. In addition to flooding, thunderstorm activity could produce gusty winds and frequent lightning, without ruling out small hail. Warmer conditions are expected throughout the long term, with seasonal to above-seasonal 925-mb temperatures. Although heat indices should exceed 100 F, a limited risk of excessive heat will persist throughout the forecast period. Individuals sensitive to these conditions should stay hydrated and avoid prolonged outdoor activities. && .AVIATION... (18Z TAFS) Issued at 1236 PM AST Tue Sep 29 2026 Mainly VFR conditions are expected to prevail across all terminals during the next 24 hours. However, streamers developing downwind of the islands may cause brief MVFR conds at TISX/TIST, while locally induced TSRA/SHRA over S-SW PR could lead to tempo MVFR cigs at TJPS. The 29/12z TJSJ sounding indicated ENE winds up to 20 kt blo FL120. && .MARINE... Issued at 1236 PM AST Tue Sep 29 2026 Building high pressure north of the region will maintain choppy conditions across the Atlantic waters and Mona Passage, with winds strengthening tonight into Wednesday and becoming locally fresh. Seas will generally range from 3 to 5 feet, occasionally up to 6 feet across the Atlantic and Caribbean waters, and the local passages. Periods of passing showers will persist, and afternoon showers and isolated thunderstorms are expected to develop mainly near western Puerto Rico over the next few days. Thunderstorms may produce locally higher winds and seas, lightning, and reduced visibility. && .BEACH FORECAST... Issued at 1236 PM AST Tue Sep 29 2026 A moderate risk of rip currents will persist into tonight along the exposed beaches of western, northern, and eastern Puerto Rico, as well as Culebra and the U.S. Virgin Islands. Breaking waves of around 5 feet are expected along portions of the northern coastline, with lower heights elsewhere. Tomorrow, the moderate rip current risk will continue for those areas and expand to exposed beaches of Vieques and southeastern Puerto Rico due to increasing easterly trade winds. This means that rip currents are possible in the surf zone. Afternoon thunderstorms may also pose a lightning threat to beachgoers, especially across southwestern Puerto Rico for the rest of today, and western Puerto Rico over the next few days. A limited heat threat and locally breezy conditions will continue over the next few days. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...RVM AVIATION/KEY MESSAGES...DSR BEACH/MARINE...KML LONG TERM....MNG/JCP
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