Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
026 FXCA62 TJSJ 180845 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 445 AM AST Tue Jun 18 2024 .SYNOPSIS... A broad surface high pressure extends southwest into the central Atlantic while an elongated area of low pressure extends across Hispaniola into the eastern Caribbean. A tropical wave with axis now south of Puerto Rico continues to cross the region. This will aid in promoting light east southeast winds today. High pressure is forecast to build north and east of region thereafter and promote an increase in the easterly winds. The upper trough and area of low pressure continues to become amplified west of the region through Wednesday. This along with the tropical wave will maintain moist and unstable conditions through mid week resulting in periods of developing showers and isolated thunderstorms each day. A slight to moderate concentration of Saharan dust is forecast to spread across the area Wednesday into Thursday. && .SHORT TERM...Today through Thursday... Isolated to scattered showers were observed all night into the early morning across the eastern sections of Puerto Rico and the U.S. Virgin Islands. According to radar estimates, rainfall accumulations were between 1 to 2 inches over some Humacao, Yabucoa, Ceiba, and Fajardo coastal areas. Several pockets of showers also affected most of the local waters, causing localized higher seas and gusty winds. The islands registered minimum temperatures in the upper 70s to lower 80s across the coastal areas and in the mid to upper 70s across the interior sections. A deteriorated weather pattern will persist at least until late Wednesday across the region. At the surface, the islands will be mostly influenced by the interaction between a broad surface high pressure over the Central Atlantic and an induced surface trough just northwest, resulting in a light southeast wind flow over the region. This wind flow will bring plenty of tropical moisture, with rainfall amounts exceeding 2 inches from the upcoming tropical wave. In the mid-to-upper levels, a trough will continue to deepen over Hispaniola, placing the islands under the divergent side of this TUTT. This instability will enhance colder temperatures at 500 MB and higher RH values at 750-500 MB. According to the Glvez-Davison Index (GDI), a good indicator of thunderstorm activity, there is a high chance for isolated to scattered thunderstorm activity across the interior. Ample deep-layer moisture will persist through Wednesday as the tropical wave moves over the region. The available moisture, saturated soils, and instability aloft will trigger early afternoon showers, affecting the interior and the San Juan Metro area. On Thursday, as the induced surface trough moves westward, building surface high pressure over the North-Central Atlantic will tighten the pressure gradient and create backing winds. A wet pattern will continue due to abundant moisture over the CWA, but the focus of the showers will shift more to the interior and western sections due to the easterly winds across the area. Due to the significant rainfall observed over the past two days, soils are already saturated, leading to urban and small-stream floods in some areas across the islands. Therefore, it`s crucial for residents and visitors to be aware of these conditions for safety and operational reasons and to check weather conditions during the day for possible advisories or warnings. .LONG TERM...Friday through Tuesday... Conditions are so far forecast to improve by Friday with increasing stability aloft, as the previously mentioned upper trough west of the area weakens and high pressure builds across the area from the central and eastern Caribbean. Low level moisture transport will also erode as the easterly winds increase resulting in less frequent passing late evening and early morning shower activity. The potential for afternoon shower development and isolated thunderstorms will continue on Friday but mainly due to local effects and good daytime heating. Shower activity should be more localized and focused over the interior and west section of Puerto Rico steered by the moderate trade winds. Overall improving condtions is expected elsewhere on Friday and into Saturday with plenty of sunshine and near seasonal temperatures expected. For the rest of the following weekend prevailing easterly winds are forecast with periods of passing early morning showers affecting the windward side of the islands. This will be followed by afternoon convection limited to mainly parts of the central interior and west sections of Puerto Rico. Plenty of sunshine and mostly pleasant summertime weather is expected elsewhere although a weakly induced easterly perturbation will cross the region late Saturday into Sunday followed by the next topical wave so far forecast to enter the eastern Caribbean late Sunday then cross the region Monday through Tuesday. That said expect lesser showers and thunderstorm activity across during the early part of the period with the best potential for increasing instability and convection so far forecast for the latter part of the period on Monday and Tuesday due to the approaching tropical wave and associated moisture field. && .AVIATION... (06z) VCTS and -RA will persist through 18/15Z, followed by SHRA & TSRA, which are forecast from 18/16Z for TJSJ, TJPS, and TJBQ due to strong showers across the region. These conditions will result in MVFR and brief IFR conditions across the major TAF sites of the island. Lower ceilings and limited VIS are expected over the mountains as well. Winds will continue light and VRB, increasing at 18/15Z from the E-SE at 12 knots with gusty winds near the showers. && .MARINE... Surface high pressure over the Atlantic and a tropical wave crossing the region will promote light east to southeast winds today, then becoming more easterly and increasing to moderate to locally fresh by Wednesday and through the rest of the week. Passing showers and isolated thunderstorms will continue to affect the coastal waters and local passages resulting in localized hazardous marine conditions. && .BEACH AND RIVER FORECAST... The risk of rip currents is low for all beaches today through Wednesday morning, however life threatening rip currents may still occur especially near groins, jetties, reefs, and piers. Local rivers and streams are at normal levels however expected afternoon showers and thunderstorm development across the interior and west sections of Puerto Rico may produce locally heavy rains at the headwaters and this may cause sudden surges of water along rivers and streams across theses areas. Please monitor the weather conditions throughout the day and always take time to look upstream. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...LIS LONG TERM....RAM AVIATION...LIS