Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
515 FXCA62 TJSJ 160911 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 511 AM AST Mon Sep 16 2024 .SYNOPSIS... The dry airmass that continues to move across the islands will limit the shower activity today. Hot and humid conditions will continue with an elevated to significant heat risk, particularly across lower elevations of the islands. Easterly winds will dominate today through mid-week, then winds will become light and more variable by the second half of the week. && .SHORT TERM...Today through Wednesday... A very quiet night was observed across the islands. A few showers managed to develop across the area, but mostly stayed over the waters. The night was on the warm side too, with minimum temperatures in the low and mid-80s in several areas. A surface high pressure anchored over the central Atlantic will drive the trade winds from the east to east-southeast today. At the mid and upper levels, ridge also dominates. Additionally, the most recent satellite-derived precipitable water show a dry air mass pushing in, with values expected to drop below the climatological value for today. All these factors together translate into stable conditions across the area. In fact, a trade wind cap inversion is expected to develop around 850 mb, trapping all the moisture below these levels. Shower activity will not be favored today, but strong heating may still lead to isolated thunderstorms across western Puerto Rico this afternoon. For Tuesday and Wednesday, the mid level ridge will hold, while the surface high will migrate westward as Tropical Cyclone Gordon advances to the north, far away from the islands. The mid to upper levels will remain dry, with limit the vertical development of the showers (meaning the showers will not be as strong or widespread), but small patches of moisture may reach the region from time to time. The aerosol models also indicate slight concentrations of Saharan dust in the area, so skies may look gray and hazy at times today through Wednesday. In terms of temperatures, above normal values are expected to continue, with 925 mb temperatures nearly at two standard deviation above the climatological mean. Therefore, excessive heat impacts will persist. && .LONG TERM...Thursday through Sunday... 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... (06Z TAFs) Mainly VFR conditions expected to prevail. Very isolated showers across the local waters are not expected to have impacts in operations today. Then, after 18Z, SHRA and isolated TSRA expected in the vicinity of TJBQ, with brief periods of reduced VIS and low ceilings possible. Winds are from the ESE at 8-12 kts, with stronger gusts. && .MARINE... A weak surface high pressure will promote gentle to moderate east to southeast winds while moving west across the Atlantic waters. 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 2 to 4 feet, occasionally up to 5 feet over the offshore Atlantic waters and passages, and between 1 to 3 feet elsewhere during the next 5 to 7 days. && .BEACH FORECAST... Today, the risk of rip currents will remain low at most area beaches, with the exception of St. Croix, where there is a moderate risk. Over the next few days, moderate risk will be prevalent, particularly at the northern-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 from 10 AM this morning to 5 PM AST this afternoon for PRZ001-005-008-010-011. Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ002-003-007-012-013. VI...Excessive Heat Warning from 10 AM this morning to 5 PM AST this afternoon for VIZ002. Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for VIZ001. AM...None. && $$ SHORT TERM/AVIATION...ERG LONG TERM/MARINE/BEACH FORECAST....YZR