Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
970 FXCA62 TJSJ 170834 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 434 AM AST Mon Jun 17 2024 .SYNOPSIS... Surface high pressure over the central and northeast Atlantic and an elongated area of low pressure west of the region, will induce a weak trough over the area to promote light east southeast winds through Tuesday. High pressure will build north and east of region thereafter promoting an increase in the easterly winds. The upper ridge will continue to erode as a trough deepens west of the region. Moist and unstable condtions will continue through mid week, with showers and isolated thunderstorm likely each day, especially over the central interior and northwestern quadrant of Puerto Rico. Fewer showers will be possible around the U.S. Virgin Islands during the day. Dangerously hot and humid conditions will persist with another round of slight to moderate concentration of Saharan dust forecast to accompany and trail a tropical wave expected to reach the area by Wednesday. && .SHORT TERM...Today through Wednesday... A variable weather pattern was observed during the early morning hours today, with a few pockets of showers streaming across the southeastern sections of Puerto Rico, Vieques, and the local waters. According to estimates from the Doppler Radar, rainfall accumulations were minimal across eastern sections of Puerto Rico. Overnight temperatures were in the upper 70s to lower 80s across the coastal areas and in the mid-60s to lower 70s across the interior. For the rest of today into the short-term period, the islands are forecast to observe a deteriorating weather pattern as an induced surface trough from a deep upper-level trough moves just north of the region. The instability provided by this feature and the abundant moisture present in the area with PWAT above 2 inches, as suggested by derived imagery from the GOES Satellite, indicate another variable day in the morning, followed by an active afternoon with high chances of thunderstorm activity across the interior sections of the islands. As these surface features move and interact with the broad surface high pressure in the Central Atlantic, the islands will experience backing winds, which will become more variable, especially in the afternoon. This factor will be determinant in the movement of the showers, leaving the islands with an elevated flood threat for today across most of the interior sections; in the rest of the areas, the islands might experience minor flooding with ponding of water in roadways and poor drainage areas. In terms of temperatures, warmer minimum temperatures and the southeasterly wind flow will provided another warm morning where heat indices could reach 112 degrees.Given these conditions, for today, yes again, an Excessive Heat Advisory is in effect for all the northern, eastern, and western coastal areas of Puerto Rico, including Culebra, Vieques, and the U.S. Virgin Islands. By Tuesday, the surface winds will come again from the southeast, leaving the islands under a variable pattern for the morning hours until the next tropical wave shows up by late Tuesday into Wednesday. Instability will be present due to the divergent side of the upper-level trough just west of the region with the axis over Hispaniola, resulting in enough moisture in 700-500 MB as suggested by the RH in the model guidance and colder temperatures in the 500 MB. The Glvez-Davison Index (GDI) shows potential for daily isolated to scattered thunderstorm activity. Although all the variables favor a wet pattern, the NASA Goddard Earth Observing System Model V5 shows a decent pulse of Saharan Air Dust across the region early Wednesday, resulting in hazy skies and muggy conditions across the region. Given the expected conditions, residents should prepare for a variable weather pattern for Tuesday morning, followed by afternoon and evening showers across the eastern sections of Puerto Rico, Culebra, Vieques, and the U.S. Virgin Islands in the early morning hours of Wednesday when the tropical wave arrives in the CWA. .LONG TERM...Thursday through Monday... Moist and unstable weather conditions are expected to continue through at least the early part of the period as a deepening upper- level trough with the axis over Hispaniola west of the region,will linger and provide good ventilation and a divergent pattern aloft. This along with good tropical moisture convergence produced by the prevailing and moderate east to southeast and a passing tropical wave, will maintain good potential for shower and thunderstorms development at least through Friday and moisture trailing the wave will combine with the instability aloft and good daytime heating to maintain moderate to high potential for afternoon and early evening convection. Based on the most recent guidance and Glvez-Davison Index (GDI)product, the potential for isolated to scattered thunderstorm will continue through Friday. Overnight and afternoon showers will be likely with a limited flood threats across portions of the islands each day. Hazy skies will continue at least through Thursday due to some lingering Saharan Dust with improving conditions by Friday and into the weekend. Over the following weekend and through Monday, a broad surface high pressure will again spread across the Central Atlantic and Eastern Atlantic result in increasing east to southeast trade winds and variable weather conditions. Under the expected low level wind flow, plenty of surface moisture will continue to stream across the islands and coastal waters, thus maintaining the potential for passing showers overnight and afternoon convection each day. This expected activity should be focused particularly across the central interior and west sections of Puerto Rico,while the U.S. Virgin Island can expect afternoon showers mainly on the west-end and downwind of the islands with fewer shower activity elsewhere from time to time. A similar and slightly above normal temperature pattern is expected to continue across the islands at least through the early part of the long term. Model guidance remains persistent in maintaining overall hot and humid conditions although the easterly are expected to increase during the latter part of the period. Residents and visitors are urged to stay informed of possible advisories or warnings due to excessive heat conditions. && .AVIATION... (06Z TAFS) VFR conditions will persist for all the morning hours across all the TAF sites. By 17/17Z, brief MVFR conditions are forecast across TJSJ, TJBQ, and TJPS due to t-storm activity developing across the interior and the San Juan Metro area. This activity will affect VIS and ceilings across the mountain area as well. Winds will become more light from the E-SE, and more VRB after 17/22z. && .MARINE... Surface high pressure over the Atlantic and the proximity of an induced trough across the region will promote light to moderate east to southeast winds today through Tuesday. Passing showers and isolated thunderstorms will continue to affect the coastal waters and local passages resulting in localized hazardous marine conditions especially over the offshore Atlantic and Caribbean waters and between eastern Puerto Rico and the U.S. Virgin Islands during the early morning hours and on the west coastal areas Turin the afternoon hours. The easterly winds are forecast to increase to moderate to locally fresh by late Wednesday and through the rest of the work week. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for PRZ001-002-005-008-010-012-013. Excessive Heat Watch from Tuesday morning through Tuesday afternoon for PRZ001-002-005-008-010-012-013. VI...Heat Advisory from 10 AM this morning to 5 PM AST this afternoon for VIZ001-002. Excessive Heat Watch from Tuesday morning through Tuesday afternoon for VIZ001-002. AM...None. && $$ SHORT TERM...LIS LONG TERM....RAM AVIATION...LIS