Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
647 FXCA62 TJSJ 162033 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 433 PM AST Mon Sep 16 2024 .SYNOPSIS... A dry airmass will continue to limit shower activity through at least Tuesday. Hot and humid conditions will promote an elevated to significant heat risk, particularly across lower elevations of the islands. Easterly winds will dominate through mid-week, then winds will become light and more variable by the second half of the week. A moderate rip current risk will persist across some local beaches. && .SHORT TERM...Tonight through Wednesday... An area of drier air has settled into Puerto Rico and the U.S. Virgin Islands and will be with us for at least another 24 to 36 hours. Precipitable water values are generally between 1.2 and 1.3 inches in this area, which is more than 2 standard deviations below normal for the month of September. The combination of the 250 mb high pressure just north of the area and the flow of air around Tropical Depression Gordon is responsible for our fair but very warm weather. This situation will change, but slowly, as an increase in moisture is not due until Tuesday night. Temperatures today ranged from the upper 70s in the highest mountains to the mid 90s on the leeward coasts and southern slopes. Heat indices did reach 102 to 116 degrees, but there was no daytime rain as of 3 PM AST. As moisture increases Tuesday night and Wednesday, 500 mb temperatures will begin to cool from a toasty minus 2 degrees Celsius tonight to something a little closer to normal (minus 5) by late Wednesday to help aid instability and allow some more shower activity in the normal diurnal pattern. Also an upper level low is developing in the western tropical Atlantic and will send a weak trough into the local area later on Wednesday. .LONG TERM...Thursday through Sunday... From previous discussion...issued at 511 AM AST Mon Sep 16 2024 A slight increase in trade wind moisture is expected by Thursday and Friday as winds shift to the east-northeast, bringing bands of shallow moisture from the central and northeast Atlantic. Although the available moisture will be confined to lower levels, it will be sufficient to trigger rain activity due to strong heating and local effects. Local winds will become light with a northeasterly component as the surface high pressure ridge weakens north of the region. Recent guidance suggests that an upper-level trough will deepen and move southward between Puerto Rico and the northern Leeward Islands by Friday. This should destabilize the upper levels and increase the likelihood of early morning and afternoon convection. Overall, winds are expected to be light, so any locally and diurnally induced afternoon activity may produce slow-moving showers and thunderstorms with locally heavy rainfall, leading to an elevated flood threat, especially in the central and west to southwest sections of Puerto Rico. By the weekend, the latest model guidance suggests that the area of convergence associated with the trough will be over the region, which may help to limit rain chances. Daytime heating and local effects will still enhance afternoon convection in some areas, particularly in the central interior and western sections of Puerto Rico. In contrast, the U.S. Virgin Islands is expected to see mostly isolated showers. Warm to hot conditions are likely to continue throughout this period, with temperatures potentially reaching the upper 90s and heat indices remaining elevated. Residents and visitors should take precautions to stay hydrated by drinking plenty of water, wearing light and breathable clothing, and avoiding prolonged exposure to the sun, especially during peak heat hours. && .AVIATION... (18Z TAFs) VFR conditions. Isold SHRA to dvlp aft 16/17Z--mainly in wrn PR and ovr the Cordillera Central. East flow of 10-15 kts with weak sea breeze influences. Maximum winds E-ESE 13-19 kt blo FL120. Aft 16/22Z winds E 5-10kt with land breezes. Isold SHRA with no operational impacts. Aft 17/14Z Winds E 10-15 kt with gusts up to 23 kt in sea breezes. && .MARINE... A weak surface high pressure will promote gentle to moderate east to southeast winds. By midweek, winds will turn lighter and more northeasterly. Today, showers will become isolated over the local waters as drier air moves in. A few thunderstorms are possible off the western coast of Puerto Rico each afternoon. Overall, seas will range between 3 to 5 feet, occasionally up to 6 feet across most local waters, with seas between 1 to 3 feet across most of the coastal waters during the next 5 to 7 days. && .BEACH FORECAST... Tonight and Tuesday, the risk of rip currents will remain low at most area beaches, with the exception of St. Croix, eastern Vieques and Culebra, and southeastern and northeastern Puerto Rico, where there is a moderate risk. Over the next few days, moderate risk will be prevalent, particularly at the northern- facing and a few south- facing beaches of Puerto Rico, Vieques, and St. Croix. For a more detailed forecast, please refer to the Surf Zone Forecast for Puerto Rico and the U.S. Virgin Islands (SRFSJU). && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Excessive Heat Warning until 5 PM AST this afternoon for PRZ001- 005-008-010-011. Heat Advisory until 5 PM AST this afternoon for PRZ002-003-007- 012-013. VI...Excessive Heat Warning until 5 PM AST this afternoon for VIZ002. Heat Advisory until 5 PM AST this afternoon for VIZ001. AM...None. && $$ SHORT TERM...WS/CVB LONG TERM....ICP AVIATION...WS/CVB PUBLIC...MRR/MMC