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Area Forecast Discussion
Issued by NWS San Juan, PR
Issued by NWS San Juan, PR
125 FXCA62 TJSJ 232036 AFDSJU Area Forecast Discussion National Weather Service San Juan PR 436 PM AST Sun Jun 23 2024 .SYNOPSIS... Fast moving showers will continue to move over the local islands during the overnight hours. An approaching tropical wave will increase shower and thunderstorm activity starting early on Monday through the evening hours. A mass of drier air accompanied by a a dense plume of Saharan dust is expected to move over the local area from Tuesday onward. && .SHORT TERM...Tonight through Tuesday... Variable weather was observed over the local islands during the morning hours. By mid-morning the San Juan streamer formed, leaving moderate precipitation over the San Juan metro area. As the day progressed, strong showers and thunderstorms began to develop over the interior towards western and northwestern Puerto Rico. Doppler radar estimated rainfall accumulations between 2.5 to 3.0 inches. A Special Weather Statement (SPS) and Flood Advisory (FLS) was issued for the mentioned sectors. The rainfall activity started to dissipate by mid-afternoon, leaving only few showers over the eastern municipalities of Puerto Rico. The U.S. Virgin Islands were under a fair weather pattern for most of the day. Today, daytime temperatures were seen in the upper 80s to mid 90s along most of the coastal and urban areas. Heat indices were over the 107 degrees Fahrenheit across the northern, western, and southern portions of Puerto Rico, Culebra, Vieques and St. Croix. A Heat Advisory was in effect for these areas. Surface winds will veer from the southeast today as the surface high pressure located over the central moves toward the eastern Atlantic. A tropical wave is forecast to move over the local islands by Monday, increasing moisture and the potential for stronger showers and thunderstorms over the area. By Tuesday, a drier airmass will filter into the region as the tropical wave exits the islands. Moderate to high concentrations of Saharan dust are expected to move into the area as well, promoting hazy skies through at least Thursday. A warm trend is anticipated by the middle of the week with daytime temperatures reaching the low to mid 90s along the coastal and urban portions of Puerto Rico and the U.S. Virgin Islands. Therefore, heat advisories or excessive heat warning are possible during this period. .LONG TERM...Wednesday through Sunday... ./FROM PREV DISCUSSION...Issued at 425 AM AST Sun Jun 23 2024/ The NASA Dust Aerosol Optical Thickness model shows a dense Saharan Air Layer (SAL) lingering over the local islands through Thursday. In addition, there will be a mid-to upper-level ridge across the northeastern Caribbean, resulting in drier conditions aloft and inducing a very stable weather pattern with hazy skies, sunny mornings, and some afternoon showers across the western sections of Puerto Rico. Instability will slightly increase from late Thursday and Friday as an induced low-level trade wind perturbation streams across the local area. Moisture content will increase once again and isolated thunderstorms are expected to develop over portions of the waters and across the islands. Later in the period, as the SAL exits the region, the islands will be influenced by the easterly winds resulting from the interaction between a surface high pressure over the central Atlantic and an induced surface trough just northwest of the region over the Western Atlantic. This wind pattern will drag tropical moisture into the islands. According to global model guidance, PWAT values will fluctuate from 1.70 to 2.10 inches, just above climatology for the region. That moisture, along with diurnal heating, will promote the development of afternoon showers and isolated thunderstorms each day across the western interior sections of Puerto Rico. Winds will return from the southeast by Saturday into Sunday, increasing the potential to observe heat indices near 108 to 112F across the coastal and urban areas of the islands. && .AVIATION... (18Z TAFs) Mainly VFR conditions are expected to prevail across all terminals through the forecast period. SHRA/TSRA en route from the SE Caribbean should increase across the USVI terminals aft 24/12z due to an approaching tropical wave. Similar conditions should spread across the eastern PR terminals by 24/15z. Sfc winds from the ESE up to 15 kts, bcmg light and variable overnight. && .MARINE... A surface high pressure over the central Atlantic will promote light to moderate easterly winds through tonight. Winds are expected to veer from the east southeast by Monday morning. Trade wind showers will move across the regional waters from time to time. The next tropical wave will approach the local islands from late tonight through Monday. Increasing winds and showers with thunderstorms are expected through Monday night across the local waters. Hazy skies are expected from Tuesday through Thursday due to Saharan dust. && .BEACH FORECAST... There is a low to moderate risk of rip currents across PR and the USVI. Showers and isolated thunderstorms are possible across the coastal areas of the USVI and eastern PR during the morning hours, and across the west/northwest coast of PR during the afternoon. The afternoon thunderstorms could produce frequent lightning and gusty winds. This weather pattern is expected to continue for the next few days with the arrival of a series of tropical waves. && .SJU WATCHES/WARNINGS/ADVISORIES... PR...None. VI...None. AM...None. && $$ SHORT TERM...LIS LONG TERM....GRS AVIATION...DSR PUBLIC DESK...MRR