Hurricane Season runs from June 1-November 30
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Area Forecast Discussion (AFD)
496 FXCA62 TJSJ 180703 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 303 AM AST Thu Jun 18 2026 ...New SHORT TERM, LONG TERM, AVIATION, MARINE, BEACH FORECAST, FIRE WEATHER... .KEY MESSAGES... Issued at 303 AM AST Thu Jun 18 2026 * Periods of moderate to heavy rainfall, with isolated thunderstorms, may result in ponding of water on roads and poorly drained areas, urban and small-stream flooding, and isolated flash flooding today, with the greatest impacts expected from late morning through the afternoon, particularly across Puerto Rico. * Dangerous heat index values are expected each afternoon as hot temperatures combine with the available moisture, increasing the risk of heat exhaustion and heat stroke, especially in coastal and urban areas, poorly ventilated locations, and among vulnerable populations. * Moderate to high concentrations of Saharan dust will spread across Puerto Rico and the U.S. Virgin Islands from early Friday through at least late Saturday night, reducing air quality and visibility while potentially aggravating respiratory conditions among sensitive groups. * A tropical wave will move across the region on Sunday, increasing the risk of heavy rainfall, thunderstorms, urban and small- stream flooding, and isolated flash flooding across portions of Puerto Rico and the U.S. Virgin Islands. * Life-threatening rip currents will remain possible along exposed beaches of Puerto Rico and the U.S. Virgin Islands throughout the forecast period. Beachgoers should exercise caution and follow local beach safety guidance. && .Short Term(Today through Saturday)... Issued at 303 AM AST Thu Jun 18 2026 Most of the islands observed quiet weather conditions amid warmer-than-normal low temperatures. However, some showers associated with an approaching weak tropical wave were noted moving across the eastern third of PR and the southern US Virgin Islands overnight. Winds were mainly from the east to east- southeast at 10 mph or less under the influence of land breeze fluctuations. Although the western and northern sections of PR, as well as the USVI, will observe periods of no rain amid a warmer-than-normal morning, the arrival of a weak tropical wave will increase the chance for showers and isolated thunderstorms, especially from mid-morning onward. In terms of mid and upper-level dynamics, this wave will encounter a hostile environment as a dry air mass is observed at those levels, which could limit the potential to observe widespread activity. However, some guidance indicates the potential to observe low and mid-level vorticity associated with this wave, which, combined with local effects, low-level convergence, topography, and excessive heating, could enhance the activity. For that reason, our forecast calls for the best chance of flooding rains downwind from El Yunque into the San Juan Metropolitan area, as well as along the Cordillera Central and the northwest quadrant. Regardless, the USVI and additional parts of PR should monitor the evolution of weather conditions, as there is a chance of rain and thunderstorms. On the other hand, temperatures wise, the combination of the available moisture, with the warmer than normal low temperatures which will enhance the potential to reach the 90s along the coastal areas during the maximum diurnal heating today in the areas without rain will result in heat indices ranging from the upper 90s to the 110 degrees Fahrenheit, especially across the urban and coastal areas of the USVI and PR. This trend will extend into the short term. Trailing today`s tropical wave, a large Saharan Air Layer (SAL) will move across the region from around midnight Friday through Saturday, bringing hazy skies, reduced visibility, and degraded air quality. Nonetheless, embedded moisture patches within the trade winds will support passing showers across the USVI and PR`s windward sectors. At the same time, daytime heating and local effects may trigger isolated to scattered afternoon convection across the interior and western portions of Puerto Rico. && .Long Term(Sunday through Thursday)... Issued at 303 AM AST Thu Jun 18 2026 A tropical wave is expected to approach and move across the northeastern Caribbean from late Sunday into Monday, bringing a significant increase in moisture across the region. Model guidance indicates precipitable water values rising to around 2 inches, with surface and mid-level relative humidity increasing to above-normal levels for this time of year. Combined with 500 mb temperatures near -7 degrees Celsius, these conditions will support a more unstable atmosphere and enhance the potential for showers and thunderstorms. As a result, periods of moderate to heavy rainfall are expected from late Sunday through late Monday, with the highest coverage likely during the afternoon hours across western and interior Puerto Rico. Locally heavy downpours or frequent showers may lead to ponding of water in urban and poorly drained areas, as well as isolated flooding concerns. By Tuesday into midweek, a drier air mass will gradually filter into the region, improving weather conditions. Although passing trade- wind showers will remain possible across windward areas, a return to a more typical weather pattern is anticipated by midweek. The primary hazards during the long-term period will be ponding of water and flooding, along with isolated thunderstorms. && .AVIATION... (06Z TAFS) Issued at 303 AM AST Thu Jun 18 2026 A weak tropical wave will move by the region today, followed by a dense SAL late-tonight. Mainly VFR conds expected to prevail across all terminals thru the fcst prd. However, SHRA/TSRA will develop across the surrounding waters, the interior and E-NW PR btwn 18/15-22z, resulting in brief MVFR or even IFR conds, especially at JSJ/TJBQ, but IST/ISX could also be impacted. Expect E-ESE winds at 10 or less thru 18/13z, then at 10-20 kt with gusts btwn 20-30kt near SHRA/TSRA or due to sea breeze. && .MARINE... Issued at 303 AM AST Thu Jun 18 2026 A broad surface high pressure building over the Central Atlantic will promote moderate to locally fresh east to east-southeast winds over the next several days, resulting in choppy seas across the local waters and Caribbean passages. Small craft should exercise caution. Shower activity will gradually increase today into tomorrow, Friday, as a tropical wave approaches the Caribbean Basin, leading to localized heavy rainfall, gusty winds, and reduced visibility. A dense layer of Saharan Dust will also gradually filter into the region by this evening onwards, resulting in hazy skies, reduced visibility, and poor air quality through the weekend. Another tropical wave will approach the area late in the weekend. && .BEACH FORECAST... Issued at 303 AM AST Thu Jun 18 2026 Breezy to locally windy conditions will maintain a low to moderate risk of rip currents during the next few days. Areas under a moderate risk of rip currents (life-threatening rip currents are possible in the surf zone) today include, St Croix and later this evening north-central and northwestern PR and Culebra. There will be a low risk of rip currents for most of the rest of the local beaches today. However, life-threatening rip currents may still occur, especially near groins, jetties, reefs, and piers. The moderate risk is forecast to spread across the islands, with most beaches being under a moderate risk to start the next week. Beachgoers should also remain alert for dangerous lightning near isolated thunderstorms. Saharan dust concentrations arriving this afternoon onwards will also result in hazy skies and reduced air quality throughout the weekend. High heat indices will also persist during the forecast period, sensitive groups should take necessary precautions. && .FIRE WEATHER... Issued at 303 AM AST Thu Jun 18 2026 Although a weak tropical wave will move across the islands today, keeping RH values above critical levels, the extensive dry soils and hot daytime temperatures will combine with sustained winds of 10 to 21 mph with stronger gusts to maintain an elevated fire weather risk through late this afternoon. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ KEY MESSAGES/SHORT TERM/AVIATION/FIRE WEATHER...CAM LONG TERM....MMC MARINE/BEACH FORECAST...MRR
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