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Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
260 FXCA62 TJSJ 100815 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 415 AM AST Wed Jul 10 2024 .SYNOPSIS... Moisture from a past tropical wave and the proximity of a TUTT low to the north will keep the potential for showers and t-storms across the forecast area through tonight. A Saharan Air Layer (SAL) over the islands will maintain hazy skies and decreased air quality through this afternoon, but another round of Saharan dust is anticipated by Friday afternoon. An elevated heat risk will also likely persist. Another tropical wave is forecast to reach the islands by Friday night. && .SHORT TERM...Today through Friday... An overnight radar and satellite imagery analysis indicated increased showers with isolated thunderstorms over the local waters, with some generating measurable rainfall amounts to around half an inch in portions of the San Juan metropolitan area and a quarter of an inch in Vieques and the northern U.S. Virgin Islands. Although slightly lower than previous days, reports again highlighted warm overnight low temperatures above 80 degrees Fahrenheit in coastal areas of eastern and southern Puerto Rico and the local islands. In contrast, higher elevations have reported low temperatures in the upper 60s. Winds were mainly from the east-northeast at 5-10 mph but light to calm and variable further inland. Due to lingering moisture from a departing tropical wave and the nearby presence of a TUTT low to the north, showers and isolated thunderstorms are expected to persist today and likely continue into late tonight. Precipitable water values remain above average, ranging from 2.0 to 2.3 inches, and temperatures at the 500 MB level are lower than previous days and within normal thresholds, creating favorable conditions for deep convection. The bulk of the shower and thunderstorm activity will transition from local waters in the morning to inland regions during the afternoon. Notably, areas downwind of mountains toward the west-southwest of Puerto Rico are expected to be most affected, driven by east-northeasterly winds. The expected activity will likely produce frequent lightning, strong gusty winds, and intense downpours, prompting concerns about flooding. Currently, west-southwestern Puerto Rico faces a moderate risk of excessive rainfall, while other areas have a limited risk. Possible effects include water ponding on roads and areas with poor drainage and urban and small stream flooding. Localized flash floods from the heaviest and longest-lasting rainfall are also possible. While convective activity is expected to decrease on Thursday and Friday due to the arrival of a drier-than-normal airmass and the establishment of mid-level ridging over the region, a surge of tropical moisture linked to the leading edge of a tropical wave will reach the northeastern Caribbean by Friday night. Meanwhile, showers and possible thunderstorms are anticipated each day, primarily driven by diurnal heating and local effects in the afternoon. Warmer-than-usual conditions will persist in the coming days. Daytime highs will range from the lower 90s in lower elevations to around 80 degrees in higher elevations, with heat indices in many coastal and urban regions soaring above 108 degrees Fahrenheit. Since these conditions present significant health risks, Heat Advisories have been issued for most coastal areas of Puerto Rico today. It is crucial for residents and visitors, especially those participating in outdoor activities, to take necessary precautions to stay safe. && .LONG TERM...Saturday through Tuesday... A tropical wave will approach the region on Saturday, bringing an increase in moisture to the region. This wave does not look particularly strong, so a significant rainfall event is not anticipated. In general, the weather will be driven by a low to mid level ridge centered over the central Atlantic. Once the wave departs later in Saturday, dry air at the mid levels will filter in. This air mass will contain Saharan dust, so skies will be hazy through early next workweek. On Monday, a Tropical Upper-Tropospheric Trough (TUTT) will approach the region. This feature will also be evident in the mid and low levels of the atmosphere, with winds first from the northeast on Monday, and then shifting from the southeast on Tuesday. These days, enough instability will be available to generate active afternoons across the interior and western Puerto Rico. Additionally, the TUTT will weaken the pressure gradient, with a steering flow below 10 knots anticipated. Trade winds will carry some showers across portions of Puerto Rico and the Virgin Islands too, but the higher risk of flooding and mudslides will be for the interior and west of Puerto Rico. In terms of temperatures, 925 mb temperatures are expected to be above normal, so the users are advised to remain hydrated, and to take frequent breaks from the sun. This level of heat affects most individuals sensitive to heat, specifically those without effective cooling or adequate hydration. Impacts possible in some health systems and in heat sensitive industries are possible. && .AVIATION... (06Z TAFS) SHRA/TSRA may lead to MVFR to brief IFR conditions, with reduced visibility and lower ceilings, primarily affecting TJSJ, TJBQ, and USVI terminals throughout the forecast period and TJPS between 10/17- 23Z. The remaining terminals may experience occasional VCSH/VCTS. Light to calm and variable winds will shift to E-ENE and increase to 14-18 knots between 10/13-23Z, with higher gusts and sea breeze variations. Stronger wind gusts are expected near SHRA/TSRA. && .MARINE... A surface high pressure anchored across the Atlantic Ocean will promote light to moderate easterly winds today, then increasing to moderate to fresh for the latter part of the week. Residual moisture from a departing tropical wave will keep the potential for showers and thunderstorms across the local waters today and tonight, with some improvement tomorrow due to an approaching drier airmass. Moisture from another tropical wave is expected to reach the northeastern Caribbean by Friday night leading to showers and possible thunderstorms. && .BEACH FORECAST... There is a moderate risk of rip current for beaches of Saint Croix today, low risk of rip currents elsewhere. Similar conditions are expected to persist Thursday, then increase to moderate for most beaches at the end of the week through the weekend. For more details about the latest forecast please refer to the Surf Zone Forecast (SRFSJU) issued by the NWS San Juan Office. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ001-003-005-007-008-010-011. VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for VIZ002. AM...None. && $$ SHORT TERM/AVIATION...ICP LONG TERM...ERG/GRS MARINE/BEACH FORECAST...YZR