Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
748 FXCA62 TJSJ 071538 AAA AFDSJU Area Forecast Discussion...UPDATED National Weather Service San Juan PR 1138 AM AST Thu Nov 7 2024 .UPDATE...The Caribbean became very active during the morning hours with numerous mesocyclone and shearline signatures appearing amongst the thunderstorms indicating a high probability of waterspouts. Temperatures were also able to climb a little higher today than expected and the grids were updated. Currently showers and thunderstorms are moving onshore in Guanica, Yauco and Guayanilla and these are expected to spread further northwest. High temperatures there have probably already been reached. Much of the southern coast of Puerto Rico is expected to have showers and thunderstorms this afternoon and other showers will continue to develop inland in the northeast flow. That flow is expected to turn southeast later tonight and tomorrow. THe U.S. Virgin Islands had some rain this morning, but were largely missed by the heaviest rains. These rains are currently south of Puerto Rico with modest drying trend in store for the USVI. The continuation of the wet period is still expected as per the previous discussion below. && .AVIATION... (12Z) Increasing SHRA/TSRA across the region will result in brief MVFR/IFR conditions with very brief fog at the surface lowering VSBYs at JPS to VLIFR, otherwise BKN-OVC CIGS at FL015-025 and reduced visibilities are expected. Light to calm and variable winds, becoming E-NE and increasing to 10-15 knots between 07/14-22Z. Wind gusts up to 26 kts are possible expected near TSRA. Maximum winds E 23-28 kts btwn FL140-205. && .PREV DISCUSSION... /issued 711 AM AST Thu Nov 7 2024/ SYNOPSIS... Unsettled weather is expected to persist over the next few days, bringing frequent showers and strong thunderstorms. With soils already saturated and elevated streamflows, flooding impacts are likely. Boating and swimming are strongly discouraged today due to hazardous seas, large breaking waves, and life-threatening rip currents, with these conditions expected to persist through Friday. Stay safe and stay informed! For a depiction of the areas at risk, visit the Experimental Graphical Hazard Weather Outlook: https://www.weather.gov/erh/ghwo?wfo=sju. SHORT TERM...Today through Saturday... Overnight radar and satellite analysis indicate an active night. The first pulse, characterized by persistent light to moderate showers and a few thunderstorms, faded around midnight. Since then, showers and thunderstorms have redeveloped across the region, triggered by a low-level induced low pressure that developed overnight from a deepening upper-level trough, as indicated by satellite imagery and TJUA NEXRAD`s Velocity Azimuth Display (VAD) wind profile signature. While surface observation reports recorded rainfall totals of up to half an inch across northern and northeastern Puerto Rico since 8 PM AST, radar-estimated accumulations in Culebra and Saint Thomas, USVI, reached around three-quarters of an inch. Overnight temperatures ranged from the lower 60s in higher elevations to around 80 degrees across coastal observing stations of southern Puerto Rico and the U.S. Virgin Islands. Winds were generally light and variable, with higher gusts near the thunderstorms. counterpart Unsettled weather is anticipated to continue through much of the short-term forecast period. Although the low-level induced low will shift away from the region, its driving feature, an upper-level low, will position itself favorably to the southwest today. In combination with the upper low, a trough of low pressure, currently monitored by the National Hurricane Center for tropical cyclone formation, will move across the area through Friday, supporting an influx of tropical moisture and promoting unstable conditions. Satellite-derived precipitable water values suggest above-normal November levels, fluctuating between 2.1 and 2.4 inches. Consequently, frequent and strong thunderstorms are anticipated over the next 24-48 hours, with high rain chances (70-90%) across all land areas. As the steering flow shifts from northeast to south-southeast following the passage of the surface trough, rainfall will gradually focus from eastern Puerto Rico today to southern Puerto Rico tonight and into Friday, with the U.S. Virgin Islands also experiencing high rain chances throughout this period. A weaker steering flow is also expected to increase the likelihood of higher rainfall accumulations from slow-moving showers. Given these conditions, an elevated to significant risk of excessive rainfall exists today and Friday, raising concerns for urban and small stream flooding, flash flooding, river flooding, and mudslides across the region. Additional hazards include frequent lightning and gusty winds generated by thunderstorms, as well as breezy to locally windy conditions along coastal areas. Despite the increased chance of rain and cloudiness, southerly winds on Friday may lead to warmer temperatures. According to current model guidance, Saturday`s weather pattern is expected to be less stormy. East-southeasterly winds and daytime heating will likely lead to more localized showers with isolated thunderstorms, primarily in the afternoon. Flooding impacts are likely to occur due to periods of moderate to locally heavy rainfall over soils that are already saturated. Thunderstorm activity could increase Saturday night as a tropical wave approaches the region. For updates on excessive rainfall, lightning, and any other hazard risk in the coming days, please visit the Experimental Graphical Hazard Weather Outlook at https://www.weather.gov/erh/ghwo?wfo=sju. LONG TERM....Sunday through Thursday... Some instability and a wet spell will prevail for the long-term period across the region. By Sunday, tropical moisture from a tropical wave now located several hundred miles east of the Lesser Antilles will move south of the area. Although a significant amount of moisture is associated with this tropical wave, so far today, global model guidelines and The Glvez-Davison Index (GDI) suggest a scenario where the bulk of the moisture with showers and scattered thunderstorms will remain just south of the islands only affecting mostly southeastern section of Puerto Rico and the Caribbean waters due to a rapid change in the surface winds from the northeast in response of a broad building high pressure moving eastward into the central Atlantic replacing the surface trough located north of the region generated by the reflection of an upper level low at the southwestern Caribbean. The east-northeast wind flow will drag a slight area of drier air, limiting the showers on Sunday afternoons somewhat for a brief period. East winds return quickly as the board surface ridge moves more into the central Atlantic, inducing the islands again for moisture values above the climatological normals. From Monday to Wednesday, the main weather feature will be a mid to upper-level ridge providing stable conditions for the vertical profile, inhibiting most of the shower development. Surface winds will persist mainly from the east with, veering winds by Wednesday into Thursday due to a low pressure developed over the western Atlantic moving eastward into the Central Atlantic. This surface change will increase the relative humidity at 700-300 MB. There, the islands can expect widespread showers and thunderstorms for the last part of the upcoming workweek. There are some discrepancies in the amount of moisture available from the two global model guidance (ECMWF & GFS). However, both of them show a wet pattern. AVIATION... (06Z TAFS) Increasing SHRA/TSRA across the region will likely result in brief MVFR conditions, with BKN-OVC at FL015-025 and reduced visibility. Light to calm and variable winds, becoming E-NE and increasing to 10-15 knots between 07/14-22Z. Higher gusty winds are expected near TSRA. MARINE... Recent data from the bouy network around the CWA shows seas between 8 to 12 feet. A surface high pressure across the western to central Atlantic and the proximity of a trough of low pressure will promote fresh to locally northeasterly winds for today into tonight. These wind-driven seas will combine with weak pulses of long-period northeasterly swell in the area, resulting in hazardous marine conditions. Small Craft Advisories are in effect across most of the regional and local waters and passages through at least Friday. BEACH FORECAST... Pulses of long-period wave energy arriving at the locally exposed coastal areas and the high seas across the region will result in large breaking waves up to 10 feet across all the northern coastal areas of Puerto Rico, where a High Surf Advisory was issued this morning until 6 PM this evening. For the western section of PR, Culebra, Vieques, and the U.S. Virgin Islands, there is a high rip current risk for the rest of the workweek in some of those points. Please visit the Surf Zone Forecast (SRFSJU) for more related information. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...High Rip Current Risk through Friday afternoon for PRZ001-002- 005-008-010-012. High Surf Advisory until 6 PM AST this evening for PRZ001-002- 005-008. High Rip Current Risk through late tonight for PRZ013. VI...High Rip Current Risk through late tonight for VIZ001-002. AM...Small Craft Advisory until 2 PM AST Friday for AMZ711-712-716- 741. Small Craft Advisory until 6 AM AST Friday for AMZ723-742. Small Craft Advisory until 6 PM AST this evening for AMZ726. && $$ SHORT TERM/AVIATION...WS PUBLIC....GRS UPPER AIR...RVT