Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
277 FXCA62 TJSJ 172027 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 427 PM 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 Tomorrow. Following this 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. && .SHORT TERM...Tonight through Wednesday... Showers and isolated thunderstorms will persist into the night before clearing out over land, with eastern Puerto Rico having the chance of passing showers into the morning hours. A surface induced trough north-northeast of the area and high pressure across the central Atlantic will maintain an easterly to southeasterly flow throughout the short term, combining with moist air from arriving tropical waves to continue the trend of hot days with showers including convection during the afternoon hours. In the mid to upper- levels low pressure begins to establish across the western Atlantic as a surface low builds. The proximity of this upper trough throughout the next few days will enhance rainfall in combination with the surface features moving in from the eastern Caribbean. On Tuesday, a tropical wave is forecast to arrive late in the day from the southeast brining instability across the lower-levels as it matches up with the divergent side of the upper-level trough just west of the region with the axis over Hispaniola, resulting in an increase of moisture across the lower levels 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. Each day the heaviest rainfall is forecast across the interior to western sections of Puerto Rico due to local effects and diurnal heating, urban flooding is possible with the heaviest rainfall. And yet again, daytime high temperatures are forecast to be in the upper 80s to low 90s especially along the north and west coastal areas. This along with the humid conditions and southeasterly winds will again favor maximum heat indices to exceed 100 degrees tomorrow. Please refer to the latest Non- Precipitable Weather Message(NPWSJU) issued by WFO SJU PR for additional information on the expected heat impacts. && .LONG TERM...Thursday through Monday... /from previous discussion/ 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... (18Z TAFS) Brief periods of MVFR conditions will remain possible at JSJ and JBQ in TSRA/SHRA through at least 17/22z. Aft 17/22z VFR conditions will prevail at all TAF sites. Winds are 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... VI... AM...None. && $$ CAM/YZR/RC